AQA GCSE Chemistry Paper 2 Summary

this is a summary video for aqa gcse chemistry paper 2. it gives you an overview of everything that could come up on the paper and it will allow you to do a last-minute cram on the night before the exam if you're taking gcse combined science then you can watch out for the green headings at the top of the screen which tell you whenever there's triple science content that you'll need to skip or you can use the timestamps in the description below the first topic in the second paper of aqa gcc chemistry is the rate and extent of chemical change the rate of a chemical reaction is its speed how fast it's going and we can measure this in two ways either how quickly the reaction is using up the reactants or how quickly it is making products there are two calculations that you can use to work this out but they're essentially the same thing an amount of one of the chemicals divided by the time that it takes to either make it or use it up either one of those amounts can be a mass in grams or a volume in centimeters cubed if you're taking higher tier you should also be able to express rate in moles per second but whichever one of these you're working with you're still using the same calculation an amount divided by a time to take an example if we had a reaction that produced 20 centimeters cubed of gas in two seconds then i would do 20 centimeters cubed divided by 2 and that would give me 10 centimeters cubed per second which is the rate it's worth remembering that even if you forget how to calculate this rate as long as the exam board have given you the units which they often do then the calculation is actually wrapped up inside it the word per means divided by so if the units are centimeters cubed per second you need to take centimeters cubed and divide them by seconds you need to be able to plot raw data onto a graph and interpret graphs that the exam board have given you on a rate graph the gradient or the steepness of that graph can be used to tell you the rate of the reaction so in this instance the reaction represented by the purple line has got a faster rate than the blue reaction because the line has a steeper gradient we can also say that for both of these reactions the rate is constant because the gradient isn't changing on a curved graph like this one the rate is changing and we can make a qualitative statement about how at the start of the reaction the rate is faster because the graph has a steeper gradient and then gradually the rate slows down by calculating a numerical value for the gradient we can calculate the rate so we're actually still using the same formula that we used before but even if you forget that you know from gcse maths that to work out the gradient you divide the change in the y-axis by the change in the x-axis so here we divide 5 grams by 10 seconds to get a rate of 0.5 grams per second for a curved graph like this we can still use the start and the end of the graph to work out an overall or mean rate so here the graph increases by 6 grams in 10 seconds giving us a rate of 0.6 grams per second you can also draw tangents to calculate the rate at one particular time stamp a tangent is a line that only touches the graph in one place to be as precise as possible make sure that your tangent is as long as you can draw it and ideally use a transparent ruler or put your ruler on top of the curve so that you can see the size of the gap on either side because we want it to be the same size on both sides so here at 4 seconds the tangent rises from 2 to 9 grams which is a change of 7 grams and that takes 10 seconds so the rate is 0.7 grams per second but if we draw another tangent at 7 seconds where the graph is flatter and we know the rate is slower we can see that here the mass only changes by about 2 grams in 10 seconds so therefore the rate is 0.2 grams per second now we've talked about how to calculate a rate we need to think about why it might change and to explain that we need to use collision theory which is the idea that chemical reactions only happen when the reacting particles collide with each other so they bang into each other and when they have sufficient energy and we call this minimum amount of energy the activation energy there are five different ways that you can speed up a rate of reaction and these are by increasing the pressure increasing the concentration increasing the surface area increasing the temperature or by adding a catalyst you can increase the pressure in two ways one is by increasing the number of particles in a container so if you imagine blowing up a balloon or pumping up a bike tire where you're physically adding more particles and the other is by making the container smaller because the pressure is caused by the particles bouncing off the walls of the container they're in if the particles are in a smaller box or if there are more particles in the same box then it's much more likely that the particles will collide with each other and therefore a reaction can happen so we would say that increasing the pressure will increase the rate because the particles will collide more frequently and frequently is such an important word because the important thing is not just that they collide more but that they do more in the same amount of time so it's really important for all of these rate questions that you're talking about particles colliding more frequently or more often or more times per second concentration is very similar to pressure but it applies to solutions rather than gases overall though it's the same idea when we have a higher concentration there are more particles in the same space and therefore they're more likely to collide so they collide more frequently and therefore we have a higher rate of reaction talking about surface area can sometimes confuse a few people the thing you need to understand is that as you cut something into smaller pieces you increase the overall surface area that can be tricky to get your head around because you're thinking well it's a smaller piece so surely the surface area is smaller but we're talking about all of the pieces combined together it might be easier if you talk in terms of the surface area to volume ratio because for the same amount of stuff we now have a bigger surface area this might be easier to visualize in 2d if this green block represents a piece of magnesium sat in some acid the acid can only react with the outside rim this 16 centimeter perimeter but if we cut each side into two then we expose all these central parts that can now react as well even though each one of those pieces individually is smaller than the original square when we put the four of them together they still have the same area but a larger perimeter so we can increase the rate of chemical reactions by cutting metal into smaller pieces or grinding up marble chips to make powder again this will increase the frequency of collisions increasing the temperature increases the rate of reaction in two different ways and they're both linked to the fact that the particles have more energy firstly increasing the temperature will increase the frequency of collisions the particles have more thermal energy from being heated and that's transferred to kinetic energy which means that they move faster if you think about walking down a corridor versus running down it if you're moving at a higher speed it's much more likely that you'll collide with somebody else and the same is true of the particles because they're moving faster they collide more frequently but also we can think about that second part of collision theory as well as colliding the particles need to have a minimum amount of energy called the activation energy if all of the particles in a reaction have more energy then it stands to reason that more of them will reach that minimum threshold finally to speed up the reaction we can add a catalyst which is a chemical that speeds up the rate of reaction without being used up or changed itself it's important that you understand that catalysts are chemicals we sometimes see in exams people writing about heat acting as a catalyst and that's not true this has to be a chemical that we're adding enzymes are an example of a biological catalyst which just means a catalyst made by a living organism the way that catalysts work is that they provide an alternative pathway for the reaction to happen which has a lower activation energy to visualize this imagine that you're trying to visit a friend who lives on the other side of a mountain for you to get up and over the mountain takes a certain amount of energy and that represents the activation energy now there isn't really much we can do to make the mountain take less energy to climb but if there was an alternative route where you could walk around the bottom of the mountain that might take less energy and this is exactly what a catalyst does it's not that it changes the amount of energy to get over the mountain it just provides an alternative route that needs less energy in unit 5 you learned how to draw and interpret these energy profile diagrams for reactions so this is an example of an exothermic reaction where the reactants have more energy than the products and we can draw an arrow representing the activation energy the amount of energy required for this reaction to start so if this is my diagram without a catalyst then when i add a catalyst the reactants and products will have the same amount of energy but in between them we won't need as big of an activation energy so the very top of that energy profile will be lower and if i was going to label the activation energy i'd label it here the first required practical that comes up in aqa gcc chemistry paper 2 investigates what happens to the rate of reaction when we alter the concentration this is a two-part investigation you need to be able to describe both how you would do this using a method where you collect gas and also how you would do it with a method that involves turbidity the first of these could be any chemical reaction that releases gas but the most normal ones to do would be either adding magnesium or marble chips to some acid for each one of these we need to be measuring the amount of gas that's produced in a certain amount of time there are two ways you can do this either you can collect gas over water by using an upturned measuring cylinder full of water so as the gas is produced it pushes the water out and you can read off the measuring cylinder how much gas has been made or you can use a gas syringe which is a little bit easier but also they tend to be quite expensive and lots of schools don't have them available for use whichever one of these methods you describe it's vital that you mention that you're taking readings at regular time intervals say every 5 seconds or every 10 seconds as if you don't do this you won't be able to calculate the rate as with all of these investigations you also need to be able to describe how you keep other variables controlled in order to make the experiment valid so in this instance we have to change concentration because that's the point of the required practical so i'd be wanting to control things like the volume of the solutions i'm using and the mass of whatever it is that i'm adding for the second part of the required practical you measure a reaction which becomes turbid meaning cloudy every school i know of uses the reaction between hydrochloric acid and sodium thiosulfate for this but whichever reaction you do there will be a solid precipitate formed so if you're given a chemical symbol equation you want to look for the product which has a state symbol which says s next to it this is what makes the reaction become turbid the classic way to monitor this is by timing how long it takes for a cross on a piece of paper to disappear the faster it disappears the higher the rate of reaction this is a perfect opportunity to talk about how subjective human judgment is and how it would be a better experiment if we used a light sensor hooked up to a computer in some chemical reactions it's possible for the products to react together to make the original reactants and we call these reversible reactions and you can recognize them because rather than a normal arrow between them we have this funny double-headed reversible reaction arrow it's possible to change the direction of the reversible reaction by changing the conditions you should also know that if a reversible reaction is exothermic in one direction then it will be endothermic in the opposite direction and there is a named example of this if you heat up some hydrated copper sulfate which is blue you can make a white powder called anhydrous copper sulfate and that's an endothermic process you have to heat it in order for it to happen if you take some anhydrous copper sulfate and you add some water to it then you can turn it back into the hydrated copper sulfate and this is an exothermic process it will get hot on its own when a reversible reaction happens in a closed system it will eventually reach a point called equilibrium and this means that the forward and backward reactions are happening at the same rate or the same speed as each other when this happens the concentration of the reactants and products will stop changing be careful though this doesn't mean that the reactants and products are present at the same concentrations as each other just that their concentrations have stopped changing so if i was going to look at a graph of a reversible reaction reaching equilibrium i could identify equilibrium as the point where the graph completely flattens out if you're taking higher tier then you need to be able to use lush teles principle in order to explain how we can move the position of equilibrium by changing the reaction conditions le chatelier's principle tells us that if a system is already at equilibrium and a change is made to any of the conditions then the system will respond to counteract that change that basically means whatever i try to do the system will try to undo so if you add a reactant the system will remove it and if you heat a reaction the system will try to cool it often with these questions the exam board won't actually give you the names of chemicals because they don't want to confuse you with reactions you haven't heard of so they quite often just refer to a blue copper compound or a pink cobalt compound so in this reaction a blue copper compound reacts with hydrochloric acid to make a green copper compound and water and this is a reversible reaction if i add more hydrochloric acid i know that le chatelier's principle has told me that the system will shift to counteract that change and get rid of the hydrochloric acid the only thing that the system can actually do is make either the forward reaction or the reverse reaction happen faster so whichever reaction removes hydrochloric acid that's the one that will go faster that's the one that we say will be favored so in this instance the forward reaction is favored that means that the equilibrium shifts to the right in other words i'm making more of the products therefore there'll be more of the green copper compound formed and my solution will start to look greener rather than bluer the second reaction condition that we can change is pressure pressure remember is caused by particles colliding with the walls of the container that they're in so the more particles there are in a container the higher the pressure will be if we increase the pressure this will favor whichever side of the reaction has fewer molecules on it to answer a question about pressure and equilibrium i need to look at the chemical symbol formula and identify which side of the equation has more gas molecules on it in this example i have three moles of gas in my reactants but only two moles in my products so the reactant side is a high pressure side and the product side is a low pressure side if i decrease the pressure le chatelier's principle tells me the system will shift to counteract that change and increase the pressure so it's going to move the equilibrium towards the higher pressure side which here is my reactants so here i would say that the backward reaction is favored because there are more molecules on the left i need to actually point this out and explicitly tell the exam board that i know that this is why the equilibrium is going to shift so the equilibrium will shift to the left and therefore the yield of sulfur trioxide will be lower we said at the start that if we heat up a reaction at equilibrium the system will shift to try to cool it back down again and the way it does this is by favoring the endothermic reaction it's very likely in a question such as this that you may be told that the forward reaction is endothermic but not told anything about the reverse reaction you are expected to know that for a reversible reaction one way will be exothermic and the other way will be endothermic so in this instance the forward reaction is going to be favored because that is what will cool my reaction back down and i need to tell the exam board that i know that this is because the forward reaction is endothermic therefore the equilibrium shifts to the right and therefore what i will see or observe is that my mixture will turn white the second topic in aqa gcc chemistry paper 2 is organic chemistry which starts off talking all about crude oil so you need to know that crude oil is a finite resource which means that it's going to run out and it's found in rocks and made from the remains of ancient biomass which is mainly plankton that was buried in sediment so basically we're talking about lots of sea creatures that died and then mud and sand fell on top of them and turned into rocks over millions of years under a lot of pressure and meanwhile they turned into fossil fuels like crude oil it's a mixture of hydrocarbons so compounds are made of hydrogen and carbon only absolutely nothing else and those hydrocarbons are mainly a type called alkanes alkanes are an example of a homologous series which is a group of compounds that have similar chemical properties because they have the same functional group and they also have the same general formula if you're taking combined science then you need to know about alkanes and alkenes and if you're taking triple science you also need to know about alcohols carboxylic acids and esters you should know that the names of the first four alkanes are methane ethane propane and butane and i like to remember this by using the mnemonic most elephants prefer bacon the general formula for alkanes is cnh2n plus 2. so what this means is that if we give you any number of carbons say eight carbons we can then use that for n to work out how many hydrogens there should be so two lots of eight plus two gives me a total of 18 hydrogens these alkanes are all examples of small covalent molecules and that means that between the molecules there are weak intermolecular forces as with all small covalent molecules the larger the molecule is the stronger these forces are this means as alkanes get bigger they have higher boiling points they also have higher viscosity which means sort of stickiness and they become less flammable due to the differences in boiling points the different alkanes that are mixed up together in crude oil can be separated out according to their boiling point by using a process called fractional distillation in fractional distillation a mixture of liquids is added to a fractionating column at the bottom of the fractionating column is a furnace or a heat source which heats them all up to an incredibly high temperature for the fractional distillation of crude oil this tends to be somewhere in the region of 400 degrees c at this temperature almost all of the alkanes are going to evaporate as they travel up the fractionating column there's a temperature gradient in other words the nearer you are to the heat source the hotter it is and as you go higher it gets cooler and cooler as each alkane reaches a height that corresponds to a temperature where it can condense it turns back into a liquid and this allows you to separate the liquids and use them afterwards fractional distillation of crude oil is used to provide fuels and feedstocks for the petrochemical industry which are used to make solvents lubricants polymers detergents and of course fuels in order to release the energy from these fuels they are combusted or burned in oxygen which is a kind of oxidation and this releases carbon dioxide and water as well as a huge amount of energy you need to be able to write balanced symbol equations for the complete combustion of alkanes complete combustion means they're burning insufficient oxygen for all of the carbon and all of the hydrogen to be fully oxidized so we're always going to make carbon dioxide and water if you're not very confident with balancing symbol equations in general then it's worth learning a method that will work just for the complete combustion of alkanes firstly you're going to look at the number of carbons in the alkane here we have propane so it has three carbons now because the symbol equation has to balance if i have three carbons on the left i must have three carbons on the right and the only molecule on the right that contains carbon atoms is carbon dioxide so i need to have three carbon dioxide molecules so i'm going to take this three from the propane and put it as a coefficient in front of the carbon dioxide now i'm going to look at the hydrogens in this propane molecule i have eight hydrogen atoms and i'm going to make water molecules which each contain two hydrogen atoms so i actually need to take that eight and half it so i'm going to put a 4 in front of the water now i need to work out how much oxygen i need to make this balance so i need to add up all of the oxygen atoms on the right hand side remembering of course that there are two oxygen atoms in every carbon dioxide molecule so that's two lots of three so that's six atoms in total and then each water molecule only contains one so that's another four atoms giving me 10 in total now since oxygen goes around as divalent molecules o2 the total number of molecules is going to be half of that so i add up all of the oxygen atoms on the right to make 10 i divide it by 2 to get 5 and that's my number of oxygen molecules now i should probably point out that this method is going to work beautifully provided you have an alkane with an odd number of carbon atoms in it it will still work if you have an alkane that has an even number of carbon atoms but the number of oxygen molecules you get out will not be a whole number so you might need four and a half or five and a half and although when we get to a level chemistry we use half as coefficients all the time at gcse we tend not to we tend to stick to whole numbers so if that's what you've been taught and it's a little bit confusing for you there is quite a simple fix let's take this butane molecule here which has four carbon atoms in it if it completely combusts in oxygen it's still going to make carbon dioxide and water but to avoid that situation where i need a half number of oxygen molecules before i start i'm going to double up so i'm going to start with two butane molecules and this is the same thing that i would do for any situation where the alkane has an even number of carbon atoms so i'm going to look at my four carbon atoms there but then i'm going to remember that i have two of these molecules so in total i now have eight carbon atoms and i'm going to make eight carbon dioxide molecules then likewise i look at these 10 hydrogen atoms but because i have two molecules that makes 20 in total and then i half it so i'm back to 10 so that's 10 water molecules and then again i count up the 16 oxygen atoms in the carbon dioxide and the 10 oxygen atoms in the water which makes 26 in total and if i divide 26 by 2 i get 13 oxygen molecules we've already described that crude oil is a mixture of hydrocarbons of all different sizes and one problem that we encounter is that there's a really high demand for the small alkanes because they make excellent fuels but actually crude oil contains large amounts of the much much larger alkanes which don't have nearly as many uses so we use a process called cracking which is a type of thermal decomposition in other words it uses heat to break down large molecules and what cracking does is it breaks down long alkanes into shorter more useful alkanes and also another group of molecules called alkenes you may be asked to complete symbol equations for cracking reactions but these aren't nearly as intimidating as they look in the first style of question we're literally just counting up the numbers of carbons and hydrogens on the left and right of the equation and making sure that they balance so if i know that there are 25 carbons and 52 hydrogens in my reactant then there must also be 25 carbons and 52 hydrogens in my products so if i know that one of my products contains 10 carbons and 22 hydrogens i can just take those away from the numbers on the left and therefore i can work out that my second product is c15h30 another style of question might tell you what the products are but not how many of them there are so in this instance i need to calculate how many ethene molecules are required to balance the equation so again i've started with 25 carbons and 52 hydrogens and i can subtract the non-aim that's already been listed as a product that leaves me with 16 carbons and i know that there are two in each ethereal molecule so therefore i must have eight ethene molecules there are two types of cracking that you need to be able to describe the reaction conditions for and at gcc neither of them mention pressure which is a really common misconception it might help if you remember that you've probably either done or watched a demo of cracking in the gcc classroom and so there isn't anywhere in that room that you can control the pressure in catalytic cracking we vaporize the alkane and pass it over a hot zeolite catalyst in steam cracking there's still a very high temperature but rather than using a traditional catalyst we use steam we've just said that in cracking we make two types of molecules the alkanes that we've already met and these new alkenes alkenes are more reactive than alkanes and this is because of the double bond that is their functional group each alkene is only going to contain one double bond you shouldn't be replacing all of the carbon-carbon bonds in the molecule with double bonds you just need one of them in order to test for an alkene we add an orange liquid called bromine water which will turn colorless in the presence of a double bond if you're doing combined science you can now skip ahead to chapter eight but if you're doing triple science we need to know a little bit more about alkenes and also some other homologous series alkenes are described as being unsaturated hydrocarbons as compared to alkanes which are saturated hydrocarbons this means in the alkene each carbon atom has not necessarily made as many bonds to hydrogen as it could have done there are two hydrogen missing compared to an alkane and so instead there's a carbon-carbon double bond this is reflected in the general formula which for an alkene is cnh2n the first four alkenes are ethene propine butene and pentene there's no methine because you can't have an alkene with only one carbon because there wouldn't be another carbon for it to form a double bond to as we've already said alkenes are produced by cracking and they can be tested for by using bromine water you also need to know a little bit of information about the reaction of alkenes firstly they're useful for making addition polymers including a lot of important plastics like polythene and polypropylene and polybutane they can be converted back into alkanes by hydrogenation using a nickel catalyst they also react with water with an acid catalyst at 300 degrees c and 60 atmospheric pressures in order to make alcohols and finally they can be burned as fuels to produce carbon dioxide and water but they have a smoky flame so they're less good as fuels than alkanes are the third homologous series you need to know about are the alcohols which have the functional group oh their general formula is cnh2n plus one oh and the first four members of the group are methanol ethanol propanol and butanol as well as being found in alcoholic beverages ethanol is a really important solvent and general reagent in chemical industry depending on how pure you need your ethanol to be it can either be produced by fermenting sugar by yeast in warm and wet conditions or by hydrating ethene using steam alcohols dissolve in water to produce neutral solutions you might expect them to make an alkaline solution because you're thinking about hydroxide ions but this oh group isn't an ion it isn't free to enter solution and raise the ph it remains covalently bonded to the molecule alcohols also burn to produce carbon dioxide and water and they react with sodium to release hydrogen which you could test for with a squeaky pop test although you need to be very very careful because of course the alcohol is also highly flammable alcohols can be oxidized either by microbes or by chemical oxidizing agents like potassium dichromate in order to make carboxylic acids these carboxylic acids are the fourth homologous series that you need to know about and the first four are methanoic acid ethanoic acid propanoic acid and butanoic acid their functional group is a carbon atom double bonded to an oxygen atom and then single bonded to another oxygen atom that in turn is bonded to a hydrogen atom if you're struggling to remember which way around the double bond and the single bond go just bear in mind the oxygen always needs to make the same number of bonds so double bond on the top and single bond on the bottom means that both oxygen atoms are making two bonds carboxylic acids dissolve in water to form weak acids in other words they don't fully ionize this means that they have high ph's compared to strong acids of the same concentration so if you have a one molar solution of hydrochloric acid and a one mole of solution of ethanoic acid the ethanoic acid will have a higher ph although still below seven because it is still an acid carboxylic acids react with carbonates to produce carbon dioxide gas bubbles which you could test for with lime water and they react with alcohols to make esters esters are sweet-smelling volatile substances which means that they evaporate easily so they're used as perfumes and also as flavourings they're made from a condensation reaction between an alcohol and a carboxylic acid and if you know the name of the reactants that they were made from then you can name the ester you take the prefix of the alcohol and put oil on the end and then the prefix of the carboxylic acid and put 08 on the end so this example here is methylpropanoate in the condensation reaction that makes this ester a water molecule is lost you've already briefly met polymers in unit 2 and then here in unit 7 we flesh this out with a little bit more detail alkenes and other similar molecules which contain a carbon-carbon double bond are able to react together to form what we call addition polymers these are named in the usual way by putting poly in front of the name of the monomer so for instance here is chloropropine and so the polymer is called polychloropropyne we can draw the polymer out in full like this or we can display it slightly more succinctly in this manner it's important when you're drawing this polymer that you make sure that the bonds are going out the side of the brackets and that you've remembered to include those brackets and the n to show that this repeating unit is happening again and again and again you should also note that whereas there is a double bond in the monomer in the polymer we don't have that double bond anymore because it's broken and that's what's allowed those two carbon atoms to make bonds with the molecules on either side of them in a condensation polymerization reaction you need two different functional groups this could either be a situation where you have two different monomers each of which has a different functional group so for instance you could have a dicarboxylic acid and a diol which is basically an alcohol with an alcohol group on each end or you can have a single monomer like these amino acids which has two different functional groups one on each end the key thing to remember about condensation polymerization is that we always lose a small molecule and it's very often a water molecule you need to be able to discuss the structure of three naturally occurring polymers the first one is dna which has a double helix structure and is made of four complementary nucleotides you won't get the mark if you just say it's made of four complementary bases the base is the bit that here is shown in red which represents the code but actually the whole monomer is the nucleotide and we need to be talking about that secondly you should know that proteins are made up of amino acids like this molecule here which is glycine and finally you should know that starch is a polymer made from glucose unit 8 is the chemical analysis unit which starts out talking about pure substances a pure substance in chemistry is a single element or compound not mixed together with anything else that's different from everyday life where we might talk about something like pure orange juice because it hasn't had anything added to it even though it's a mixture of hundreds of different compounds pure substances melt and boil at specific temperatures so you can use a data book value to figure out whether a substance is pure for instance we know that pure water boils at exactly 100 degrees so if i have some water and it boils at a temperature that's not 100 degrees well then it's not pure formulations are mixtures that have been designed as a useful product they contain really carefully measured quantities of all of the ingredients examples of formulations include fuels cleaning agents paints medicines alloys fertilizers and foods another way that you can identify whether a substance is pure and which different substances it contains if it's not pure is by using chromatography there are several different types of chromatography but in gcc you're most likely to have met paper chromatography which forms the basis of the required practical all types of chromatography are used to separate out mixtures and then to analyze what is in them they all make use of a stationary phase which remains still and a mobile phase which moves through the stationary phase transferring the sample in paper chromatography the stationary phase which doesn't move will be the piece of chromatography paper and the mobile phase will be a solvent and you're most likely to have used water and that water moves through the chromatography paper and it will transfer your samples which tend to be inks or dyes or something the substances are going to be separated out based on how well they are retained by the stationary phase so basically how good is the stationary phase at holding onto them in paper chromatography this corresponds to how soluble the substance is in the solvent a very soluble substance will travel with the solvent for a long distance whereas a less soluble substance will be deposited on paper after a shorter distance and something that's not soluble at all just won't even move off the start line chromatography is a comparative method we analyze substances by comparing them to chemical standards known substances for which we already know the results this is a bit like fingerprinting you can't just look at a fingerprint left at a crime scene and know who it belongs to you need a database of the fingerprints of everybody who'd been in that area and then you can find out whose fingerprint it was by comparing them rather than just eyeballing a chromatogram and saying well this spot seems very similar to this standard we can calculate something called an rf value or a retention factor value this is a number between 0 and 1 which tells us more precisely whether two samples are behaving in the same way it's particularly good for if we don't want to include a sample of the standard on our chromatogram we just want to look up a number in a book and say well is it the same the rf value is calculated by dividing the distance moved by the sample by the distance moved by the solvent and of course the distance moved by the solvent is the maximum distance that the sample could have traveled because it's being carried by the solvent you can't just have your color jumping ahead of the solvent front since substances have different solubility in different solvents they will also have different rf values and different solvents so repeating the chromatogram with different solvents might allow us to separate out substances that have the same solubility in one solvent imagine you have a dot on a paper chromatogram made with water that actually contains two different inks but then when you run the same chromatogram using ethanol they might separate out a pure compound only contains one substance so even if you put it in a whole range of different solvents you won't split it up into different multiple spots paper chromatography is one of your named required practicals and therefore you need to be able to write a method to explain how it should be carried out so to start with you draw a line on which you're going to put your samples and as you know it's really important that that line is drawn in pencil not pen to stop it from running and interfering with your data you're going to place the sample of the standards and also the sample you're trying to investigate on that line and then you're going to put it so that it's just sitting into a beaker of solvent and it's really important that that solvent is below the line because if the solvent comes above the line then rather than being carried up the chromatography paper by capillary action your sample would just bleed out into the solvent and you wouldn't get any results you may want to put a lid on top of the beaker to prevent the solvent from evaporating as we've said on that start line as well as your sample you're going to put some standards that you can compare it to and then at the end you're going to measure the distance travelled by everything and use these numbers to calculate an rf value there are four gas tests that you need to be able to describe to test for oxygen we take a splint that's on fire and blow it out so it's just glowing and if you add that into the gas and the splint relights then that tells you that it's oxygen to test for hydrogen we also need a lit splint because we're going to ignite the gas in other words set fire to it and if it is hydrogen then it will burn with a squeaky pop definitely hydrogen to test for chlorine we take some litmus paper and it's important that it's damp litmus paper and then the gas will bleach it and turn it white and then finally to test for carbon dioxide we take the gas and we bubble it through lime water which is a solution of calcium hydroxide and if it is carbon dioxide then the lime water will turn white because it will form a white calcium carbonate precipitate now again if you're doing combined science you can skip ahead to chapter nine but if you're doing triple science gcse chemistry there are a few more bits we need to cover firstly you should be able to describe flame tests which can be used to identify cations in ionic compounds you need to take a nichrome wire and rinse this in dilute hydrochloric acid this means if there are already any metal ions on the wire those will react with the acid and be removed then you place that damp wire into a solid sample of the metal compound or into a solution that contains it then you hold this in the clear part of a bunsen burner roaring plane the characteristic colours that you're expected to know are that lithium has a crimson flame sodium has a yellow flame potassium has a lilac or pale purple flame calcium has a brick red or orange red flame and copper has a bluey green flame as seen in this photograph here now as pretty as flame tests are and as much as i like doing them they are quite subjective it's down to you as a human to say well exactly what shade of red is that and also there's another problem because if you have two different cations present then usually the brighter color will obscure the paler one so for instance if you have lithium and potassium in the same sample you're not going to see the pale lilac flame when there's the bright crimson one so instead it's often better to use an instrumental method because usually these are more accurate rapid and sensitive so flame emission spectroscopy is quite a lot like a flame test but using an instrument instead and what we do is we put the sample into a flame and the light that comes out is passed through a spectroscope the output that you get is a line spectrum and the different lines indicate different elements and the size of the line gives you a clue as to how much of it is there the other way that you can test for cations is by adding sodium hydroxide to produce a precipitate which basically means a solid being formed out of the solution if you have aluminium magnesium or calcium ions then when you add sodium hydroxide this forms a white precipitate and you can't distinguish between the three of them if you carry on adding excess sodium hydroxide so adding too much then the precipitate that forms from aluminium ions will then dissolve so you can then eliminate that one the transition metals also form precipitates in response to sodium hydroxide and these have got characteristic colours the three that you need to know about are copper two plus ions which will produce a blue precipitate iron ii plus ions which have a dark green precipitate and iron three plus ions which have a brown precipitate you need to be able to describe the tests for three different anions carbonates react with acids to release bubbles of carbon dioxide gas you can then pass this gas through some lime water and if it turns cloudy you know that it's carbon dioxide and that you had a carbonate halides form precipitates with silver nitrate remember halides come from group seven so we're talking chlorides bromides iodides firstly you need to treat the solution with some nitric acid to remove any carbonates or sulfates because these would give you a false positive then you add your silver nitrate if chlorides are present you'll make a silver chloride precipitate which is white if bromides are present you'll make silver bromide which is cream and if iodide ions are present you'll make silver iodide which is yellow finally we test for sulfates by adding barium chloride solution before we add the barium chloride we need to use dilute hydrochloric acid to remove any carbonates as again this would give us a false positive as part of the required practical you need to be able to describe how you would use these tests or a combination of them to identify what a particular substance is you may need to use flame tests hydroxide tests and the anion tests the next topic is the chemistry of the earth's atmosphere which starts off looking at the modern atmosphere so you need to know that for the past 200 million years the atmosphere has consisted of about 80 nitrogen about 20 oxygen and small amounts of other gases including carbon dioxide water vapor and noble gases like argon next we need to think about the earth's early atmosphere earth's atmosphere first formed about 4.6 billion years ago and that means we can't be exactly certain what happened because there isn't a huge amount of evidence now your keyword here is evidence we don't want to just be saying oh we don't know because nobody was around to see it that won't get you the mark we do know that initially there was intense volcanic activity and that volcanic activity spewed out lots of carbon dioxide and water vapor and it made the earth's early atmosphere quite similar to that of mars and venus today now obviously because there were loads of volcanoes it was really really hot so even though there was lots of water in the atmosphere there wasn't any on the ground so there weren't any oceans there also may have been small amounts of nitrogen and ammonia and methane and that's important because they were key components in the formation of the first amino acids without which life could not have formed as time went on the earth cooled down and then oceans formed so carbon dioxide started to dissolve in those oceans and then plankton tiny sea creatures used that carbon dioxide dissolved in the oceans to make calcium carbonate shells if you've ever seen the white cliffs of dover they're made of a stone called limestone and that's made from the shells of dead sea creatures so that's where lots of this carbon dioxide is locked up in sedimentary rocks like limestone also when they died some of those sea creatures were turned into fossil fuels so a lot of the carbon is also locked up in coal and oil and gas over time these processes led the level of carbon dioxide in the atmosphere to decrease quite dramatically now about 2.7 billion years ago algae started photosynthesizing algae are part of the same kingdom as plants they have chloroplasts and cell walls but they also include things like seaweeds and microscopic single-celled photosynthetic organisms these little green cells started photosynthesizing and as you know this makes glucose but it also makes a lot of oxygen and so levels of oxygen started to rise and then about a billion years later the first higher plants evolved and oxygen levels eventually got high enough for animals to evolve too next we get onto greenhouse gases and first let's establish this has nothing to do with the ozone layer the ozone layer is not mentioned at all in gcc chemistry so just park it on one side and forget that it exists you should know that greenhouse gases have something to do with global warming but we need a little bit more detail here firstly the three named greenhouse gases in your specification are carbon dioxide methane and water vapor these are molecules that will vibrate in response to radiation and they're responsible for the earth warming up having a little bit of them is actually a good thing because it keeps our temperature stable and high enough to sustain life but unfortunately we now have too much of them and so climate change and global warming are happening these greenhouse gases form a sort of blanket around the earth and if you imagine that blanket has holes in it and shortwave radiation like visible light uv gamma and x-rays can get through the holes because they have a short wavelength however when that radiation reaches earth the earth absorbs it and when it re-emits it and gives it back out it does so with a longer wavelength so that radiation being emitted is infrared radiation now that infrared radiation is too big to easily pass back through the greenhouse gas blanket so although some of it will get back through some of it will be trapped and instead it warms up the atmosphere and warms up earth and these rising temperatures are leading to lots of serious consequences now we need to be specific when we're talking about these so firstly as the ice caps melt this causes sea levels to rise this can lead to flooding and other extreme weather events but it's not enough to just say extreme weather events we need to actually name them so talking about droughts and hurricanes polar habitats are being lost and with them many species are going extinct make sure that you're talking about polar or arctic habitats not just saying habitats in terms of things that humans do that are making the situation worse there are six key ways that we're increasing the levels of greenhouse gases in the earth's atmosphere firstly every time we burn fossil fuels this releases the carbon that's stored in them increasing carbon dioxide also cutting down trees because that way we're reducing photosynthesis which removes carbon dioxide from the atmosphere and also because those trees are a store of carbon and also digging up peat bogs peat bogs form in a similar way to fossil fuels but over a shorter time scale and again they're a carbon sink people use peat as fuel or for compost and so digging up those peat bogs has an impact on carbon dioxide in the atmosphere now you might also have heard because it's generally kind of hilarious that farming cows is massively increasing the amount of methane because cows basically fart out methane also rice farming because you have to grow rice semi underwater and there are bacteria that live in that water in their roots and release methane and also in landfill so rubbish dumps unfortunately it's really difficult to model exactly what's going to happen because this is a really complex system so we tend to come up with models that are quite simplified and miss things out or gloss over them and then also speculate and guess and this can be really problematic and sometimes media coverage can be biased and only tell part of the story so we need to look for evidence wherever we can you should know that the carbon footprint is the total amount of greenhouse gases emitted over the life cycle of a product service or event and this could be reduced in a number of ways like buying resources more locally so you don't release greenhouse gases while transporting them in a plane or by using less energy during manufacture so that you need to burn fewer fossil fuels to generate the electricity you also need to know apart from global warming about some of the other problems associated with using fuels there are various pollutants released when fuels burn and as you know when you burn a hydrocarbon you release carbon dioxide and water and carbon dioxide leads to global warming but if you don't have enough oxygen for complete combustion then instead of making carbon dioxide you make carbon monoxide or even carbon particulates which are basically soot carbon monoxide is a toxic gas which stops your red blood cells from working properly and carbon particulates cause a phenomenon called global dimming which literally means the sky is getting darker and less light is getting to earth also almost all fossil fuels have small amounts of sulfur in them as impurities so when you burn the fossil fuel you also burn the sulfur and make sulphur dioxide and sulfur dioxide along with carbon dioxide and nitrous oxides which can be produced when you burn things at a high enough temperature for super unreactive nitrogen from the atmosphere to react with oxygen or contribute to acid rain as well as making acid rain sulphur dioxide and oxides of nitrogen can cause respiratory problems like asthma humans use the earth's resources to provide warmth shelter food and transport natural resources supplemented by agriculture or farming provide food timber clothing and fuels in addition we can supplement these resources with synthetic resources which are those that have been made from chemicals so for instance cotton is a natural fiber that grows on a cotton plant but we now also have fibers like nylon which is made from chemicals you should be able to identify whether a resource is finite and is running out faster than we can replace it so for instance fossil fuels or whether it's renewable like wood and you should also know that sustainable development is development that meets the needs of current generations without compromising the ability of future generations to meet their own needs in other words using resources in a way that isn't going to mean our grandchildren don't have them to use potable water is water that is safe for us to drink but it's not the same thing as pure water which has nothing else dissolved in it potable water needs to have low levels of salts but it probably does still have some salts in it and it should also be free of pathogenic microorganisms but it may have other things dissolved in it like fluoride ions for dental care or chlorine as a disinfectant in order to make potable water or drinking water from rivers and lakes we start by choosing an appropriate source then filtering it to remove solid matter and sterilizing it using chlorine ozone or ultraviolet light we can also make potable water from sea water by a process called desalination which means removing the salt and there are two ways this can be achieved either we can distill it by heating it up until it evaporates leaving the salt behind or we can use reverse osmosis where it's pushed through a membrane both of these methods are very energy intensive and this makes them expensive which is why countries that have access to sources of fresh water tend not to use them potable water can be made from waste water including sewage agricultural waste and industrial waste in order to make potable water from sewage we first need to undergo screening and grit removal to remove large objects then sedimentation to split it apart into sewage sludge which is semi-solid and effluent which is the liquid and then these are treated with anaerobic and aerobic biological treatment where bacteria digest the sewage and leave us with potable water industrial wastewater may also need to have harmful chemicals removed it's likely that these could come up in the context of an evaluation question where you're asked which method would be easier so you want to think about the number of steps and the amount of energy involved if you're sitting higher tier you need to be able to describe two novel methods of metal extraction which are increasingly being used to extract copper because the supply is running out but demand is increasing so the price is going through the roof these can both be used on low-grade ores which contain less than one percent of copper in phytomining plants called hyper accumulators are grown on the low-grade ore and they absorb the copper into their leaves and stems and then the plants are burned leaving ash the copper can then be extracted from the copper compounds in the ash by using displacement or electrolysis in bioleaching rather than using plants we're using bacteria both of these methods have the advantage that they involve very low energy phytomining is also carbon neutral and bioleaching has an advantage over traditional chemical leaching methods in that it doesn't involve nasty chemicals like cyanide however they tend to be quite slow and it may still be necessary to remove lots of rock companies write life cycle assessments in order to assess the environmental impact of a product they're making by thinking about the extraction of the raw materials the manufacture the use and operation the distribution and finally the disposal some parts of this can be done quite easily and objectively like how much water is going to be used but other things are harder for instance it can be hard to know exactly how much carbon dioxide will be released as a result of this product being made because some of that might come down to the amount of electricity that's used when it's being used and we might not know that in advance in addition it's often the company that's manufacturing a product that is writing the lifecycle assessment so they're sometimes not objective and can be misused to reach predetermined conclusions reducing our use of raw materials reusing manufactured products and recycling products to make new ones can all help to deal with the growing scarcity of resources growing concerns about our energy usage and limited space in landfill you should be able to describe how glass is recycled by color sorting crushing melting and reforming and how metal is recycled by melting it and recasting the amount of effort required in recycling is linked to how pure the final product needs to be and you may also need to evaluate whether recycling is worthwhile in a certain situation corrosion is the breakdown of materials by chemical reactions with other substances in their environment like water and oxygen if we're specifically talking about iron and talking about it being broken down by water and oxygen together then we can refer to this as rusting we can protect metals from corrosion by painting them greasing them or electroplating them and in fact aluminium forms its own protective layer of aluminium oxide which protects it from further corrosion because it's so hard we can also protect iron by galvanizing it which means adding a small amount of a more reactive metal usually zinc and then the oxygen in the atmosphere will react with that zinc instead of with the iron and this is a type of sacrificial protection in addition to knowing from unit 2 that alloys are mixtures of metals and that they're harder than pure metals for unit 10 we need to know the names and uses of some specific alloys so there's bronze which is made from copper and tin alloyed together which is used to make coins and brass which is made from copper and zinc and this is used for the pins of plugs we can talk about how gold is often used as an alloy in jewelry to make it slightly harder and you need to know that pure gold is referred to as 24 karat gold and you can then use other carat ratings to work out the percentage purity of a type of gold there are three kinds of steel that you need to know about all of which are basically iron with small amounts of other things added so there's high carbon steel which is strong but brittle and this is used for making cutting tools and low carbon steel which is softer and more easily shaped so it's used for making things like car bodies then we have stainless steels which have small amounts of chromium and nickel and these are resistant to acids so they're really good for making cutlery finally aluminium alloys are very useful because they're low density so they're used for making aircraft ceramics are non-metallic solids made from a raw material heated to a high temperature and there are two types that you need to know about clay ceramics and glass are both waterproof and brittle but clay ceramics are opaque whereas glass is usually transparent soda lime glass is used for everyday uses like windows and drinking glasses and is made from sand sodium carbonate and limestone but it doesn't have a very high melting point so for lab use and also some cooking uses you may need borosilicate glass or hard glass which is made from sand and boron trioxide polymers are very long chains of repeating units called monomers and those monomers aren't individual atoms they're actually small covalent molecules that have formed further covalent bonds between them to make these very long chains in between the chains there are weak intermolecular forces but because the polymer chains are so large those weak intermolecular forces are comparatively strong and so most polymers are solid at room temperature we can name a polymer based on the name of its monomer so for instance if you take ethene and you polymerize it you make polyethyne or if you take propine and polymerize it you make polypropy the physical properties of a polymer such as its density and its melting point will depend on the monomer that it's made out of but also the reaction conditions that we use to make it so the temperature and the pressure and the catalyst for instance we can compare two types of polyethene low density polyethylene and high density polyethy low density polyethyne is made under very high pressure at a high temperature with a trace amount of oxygen as a catalyst and it has an amorphous structure where it doesn't stack nicely together whereas if we use a much lower pressure and a lower temperature and a z-glanata catalyst that we can produce high-density polyethylene which has a crystalline structure where the strands pack tightly together meaning that it has a much higher density completely separately from that you need to be able to distinguish between thermo-setting and thermo-softening polymers then most setting polymers have crosslinks which are just a fancy name for covalent bonds between the chains and this means that the chains can't slide past each other and so they don't melt when they're heated they actually char and eventually burn whereas in a thermo-softening polymer there are only weak intermolecular forces between the layers so these can slide over each other and they will melt a composite material is made by combining two or more materials you can still tell them apart as they don't blend into one another they're still distinct and this use of two materials allows composites to have useful combinations of properties like being both strong and lightweight or being able to resist being squashed and stretched the two materials making up a composite are called the reinforcement and the matrix or binder the reinforcement provides the heavy duty structure while the matrix sticks it all together a natural example of composite material is wood which contains soft cellulose combined with a harder substance called lignin their lignin fibers form the reinforcement and they're surrounded by the cellulose matrix for a synthetic example we can look at steel reinforced concrete concrete is itself actually a composite because it's made from cement sand and aggregate or small stones and concrete has good compressive strength so it resists being squashed but if you try to bend it at all it can shatter and the steel reinforcement has good tensile strength and prevents this from happening the harbour process is a really important industrial chemical process used to manufacture ammonia for making fertilizers it's made out of nitrogen which is extracted from the air and hydrogen which is made by reacting methane with steam the nitrogen and hydrogen are passed over an iron catalyst at 450 degrees c and 200 atmospheric pressures and you need to be able to explain why these reaction conditions are used it all links to equilibrium because this is a reversible reaction using a high pressure increases both the yield of ammonia and the rate of reaction but if we push the pressure much higher it will be both expensive and also even more dangerous using a high temperature decreases the yield so means we would make less ammonia so we want the temperature to be as low as possible but having a low temperature will decrease the rate of reaction and make the process very slow so this is why we don't do it at room temperature we still use a reasonably high temperature and also why we need the iron catalyst once the ammonia is produced the mixture of gases is cooled so that the ammonia will liquefy while the nitrogen and hydrogen remain as gases and the ammonia is removed the leftover hydrogen and nitrogen are then recycled background into the reactor to react again a large proportion of the ammonia made using the harbour process goes into mpk fertilizers these are an example of a formulation added to plants in order to increase their yield mpk fertilizers contain salts that have nitrogen phosphorus and potassium in them as these three elements are vital for healthy plant growth ammonia can be used to manufacture ammonium salts but also nitric acid can be used as another source of nitrogen potassium chloride potassium sulfate and phosphate rock can all be mined from the ground but obviously phosphate rock can't be used directly so something called the order process is used to convert this into phosphoric acid which can then be used as a source of phosphorus that's it for aqa gcc chemistry or combined science paper 2. i hope that you found this a useful summary of all the content that's likely to come up in your exam good luck and don't forget to like and subscribe

this is a summary video for aqa gcse chemistry paper 2. it gives you an overview of everything that could come up on the paper and it will allow you to do a last-minute cram on the night before the exam if you&#39;re taking gcse combined science then you can watch out for the green headings at the top of the screen which tell you whenever there&#39;s triple science content that you&#39;ll need to skip or you can use the timestamps in the description below the first topic in the second paper of aqa gcc chemistry is the rate and extent of chemical change the rate of a chemical reaction is its speed how fast it&#39;s going and we can measure this in two ways either how quickly the reaction is using up the reactants or how quickly it is making products there are two calculations that you can use to work this out but they&#39;re essentially the same thing an amount of one of the chemicals divided by the time that it takes to either make it or use it up either one of those amounts can be a mass in grams or a volume in centimeters cubed if you&#39;re taking higher tier you should also be able to express rate in moles per second but whichever one of these you&#39;re working with you&#39;re still using the same calculation an amount divided by a time to take an example if we had a reaction that produced 20 centimeters cubed of gas in two seconds then i would do 20 centimeters cubed divided by 2 and that would give me 10 centimeters cubed per second which is the rate it&#39;s worth remembering that even if you forget how to calculate this rate as long as the exam board have given you the units which they often do then the calculation is actually wrapped up inside it the word per means divided by so if the units are centimeters cubed per second you need to take centimeters cubed and divide them by seconds you need to be able to plot raw data onto a graph and interpret graphs that the exam board have given you on a rate graph the gradient or the steepness of that graph can be used to tell you the rate of the reaction so in this instance the reaction represented by the purple line has got a faster rate than the blue reaction because the line has a steeper gradient we can also say that for both of these reactions the rate is constant because the gradient isn&#39;t changing on a curved graph like this one the rate is changing and we can make a qualitative statement about how at the start of the reaction the rate is faster because the graph has a steeper gradient and then gradually the rate slows down by calculating a numerical value for the gradient we can calculate the rate so we&#39;re actually still using the same formula that we used before but even if you forget that you know from gcse maths that to work out the gradient you divide the change in the y-axis by the change in the x-axis so here we divide 5 grams by 10 seconds to get a rate of 0.5 grams per second for a curved graph like this we can still use the start and the end of the graph to work out an overall or mean rate so here the graph increases by 6 grams in 10 seconds giving us a rate of 0.6 grams per second you can also draw tangents to calculate the rate at one particular time stamp a tangent is a line that only touches the graph in one place to be as precise as possible make sure that your tangent is as long as you can draw it and ideally use a transparent ruler or put your ruler on top of the curve so that you can see the size of the gap on either side because we want it to be the same size on both sides so here at 4 seconds the tangent rises from 2 to 9 grams which is a change of 7 grams and that takes 10 seconds so the rate is 0.7 grams per second but if we draw another tangent at 7 seconds where the graph is flatter and we know the rate is slower we can see that here the mass only changes by about 2 grams in 10 seconds so therefore the rate is 0.2 grams per second now we&#39;ve talked about how to calculate a rate we need to think about why it might change and to explain that we need to use collision theory which is the idea that chemical reactions only happen when the reacting particles collide with each other so they bang into each other and when they have sufficient energy and we call this minimum amount of energy the activation energy there are five different ways that you can speed up a rate of reaction and these are by increasing the pressure increasing the concentration increasing the surface area increasing the temperature or by adding a catalyst you can increase the pressure in two ways one is by increasing the number of particles in a container so if you imagine blowing up a balloon or pumping up a bike tire where you&#39;re physically adding more particles and the other is by making the container smaller because the pressure is caused by the particles bouncing off the walls of the container they&#39;re in if the particles are in a smaller box or if there are more particles in the same box then it&#39;s much more likely that the particles will collide with each other and therefore a reaction can happen so we would say that increasing the pressure will increase the rate because the particles will collide more frequently and frequently is such an important word because the important thing is not just that they collide more but that they do more in the same amount of time so it&#39;s really important for all of these rate questions that you&#39;re talking about particles colliding more frequently or more often or more times per second concentration is very similar to pressure but it applies to solutions rather than gases overall though it&#39;s the same idea when we have a higher concentration there are more particles in the same space and therefore they&#39;re more likely to collide so they collide more frequently and therefore we have a higher rate of reaction talking about surface area can sometimes confuse a few people the thing you need to understand is that as you cut something into smaller pieces you increase the overall surface area that can be tricky to get your head around because you&#39;re thinking well it&#39;s a smaller piece so surely the surface area is smaller but we&#39;re talking about all of the pieces combined together it might be easier if you talk in terms of the surface area to volume ratio because for the same amount of stuff we now have a bigger surface area this might be easier to visualize in 2d if this green block represents a piece of magnesium sat in some acid the acid can only react with the outside rim this 16 centimeter perimeter but if we cut each side into two then we expose all these central parts that can now react as well even though each one of those pieces individually is smaller than the original square when we put the four of them together they still have the same area but a larger perimeter so we can increase the rate of chemical reactions by cutting metal into smaller pieces or grinding up marble chips to make powder again this will increase the frequency of collisions increasing the temperature increases the rate of reaction in two different ways and they&#39;re both linked to the fact that the particles have more energy firstly increasing the temperature will increase the frequency of collisions the particles have more thermal energy from being heated and that&#39;s transferred to kinetic energy which means that they move faster if you think about walking down a corridor versus running down it if you&#39;re moving at a higher speed it&#39;s much more likely that you&#39;ll collide with somebody else and the same is true of the particles because they&#39;re moving faster they collide more frequently but also we can think about that second part of collision theory as well as colliding the particles need to have a minimum amount of energy called the activation energy if all of the particles in a reaction have more energy then it stands to reason that more of them will reach that minimum threshold finally to speed up the reaction we can add a catalyst which is a chemical that speeds up the rate of reaction without being used up or changed itself it&#39;s important that you understand that catalysts are chemicals we sometimes see in exams people writing about heat acting as a catalyst and that&#39;s not true this has to be a chemical that we&#39;re adding enzymes are an example of a biological catalyst which just means a catalyst made by a living organism the way that catalysts work is that they provide an alternative pathway for the reaction to happen which has a lower activation energy to visualize this imagine that you&#39;re trying to visit a friend who lives on the other side of a mountain for you to get up and over the mountain takes a certain amount of energy and that represents the activation energy now there isn&#39;t really much we can do to make the mountain take less energy to climb but if there was an alternative route where you could walk around the bottom of the mountain that might take less energy and this is exactly what a catalyst does it&#39;s not that it changes the amount of energy to get over the mountain it just provides an alternative route that needs less energy in unit 5 you learned how to draw and interpret these energy profile diagrams for reactions so this is an example of an exothermic reaction where the reactants have more energy than the products and we can draw an arrow representing the activation energy the amount of energy required for this reaction to start so if this is my diagram without a catalyst then when i add a catalyst the reactants and products will have the same amount of energy but in between them we won&#39;t need as big of an activation energy so the very top of that energy profile will be lower and if i was going to label the activation energy i&#39;d label it here the first required practical that comes up in aqa gcc chemistry paper 2 investigates what happens to the rate of reaction when we alter the concentration this is a two-part investigation you need to be able to describe both how you would do this using a method where you collect gas and also how you would do it with a method that involves turbidity the first of these could be any chemical reaction that releases gas but the most normal ones to do would be either adding magnesium or marble chips to some acid for each one of these we need to be measuring the amount of gas that&#39;s produced in a certain amount of time there are two ways you can do this either you can collect gas over water by using an upturned measuring cylinder full of water so as the gas is produced it pushes the water out and you can read off the measuring cylinder how much gas has been made or you can use a gas syringe which is a little bit easier but also they tend to be quite expensive and lots of schools don&#39;t have them available for use whichever one of these methods you describe it&#39;s vital that you mention that you&#39;re taking readings at regular time intervals say every 5 seconds or every 10 seconds as if you don&#39;t do this you won&#39;t be able to calculate the rate as with all of these investigations you also need to be able to describe how you keep other variables controlled in order to make the experiment valid so in this instance we have to change concentration because that&#39;s the point of the required practical so i&#39;d be wanting to control things like the volume of the solutions i&#39;m using and the mass of whatever it is that i&#39;m adding for the second part of the required practical you measure a reaction which becomes turbid meaning cloudy every school i know of uses the reaction between hydrochloric acid and sodium thiosulfate for this but whichever reaction you do there will be a solid precipitate formed so if you&#39;re given a chemical symbol equation you want to look for the product which has a state symbol which says s next to it this is what makes the reaction become turbid the classic way to monitor this is by timing how long it takes for a cross on a piece of paper to disappear the faster it disappears the higher the rate of reaction this is a perfect opportunity to talk about how subjective human judgment is and how it would be a better experiment if we used a light sensor hooked up to a computer in some chemical reactions it&#39;s possible for the products to react together to make the original reactants and we call these reversible reactions and you can recognize them because rather than a normal arrow between them we have this funny double-headed reversible reaction arrow it&#39;s possible to change the direction of the reversible reaction by changing the conditions you should also know that if a reversible reaction is exothermic in one direction then it will be endothermic in the opposite direction and there is a named example of this if you heat up some hydrated copper sulfate which is blue you can make a white powder called anhydrous copper sulfate and that&#39;s an endothermic process you have to heat it in order for it to happen if you take some anhydrous copper sulfate and you add some water to it then you can turn it back into the hydrated copper sulfate and this is an exothermic process it will get hot on its own when a reversible reaction happens in a closed system it will eventually reach a point called equilibrium and this means that the forward and backward reactions are happening at the same rate or the same speed as each other when this happens the concentration of the reactants and products will stop changing be careful though this doesn&#39;t mean that the reactants and products are present at the same concentrations as each other just that their concentrations have stopped changing so if i was going to look at a graph of a reversible reaction reaching equilibrium i could identify equilibrium as the point where the graph completely flattens out if you&#39;re taking higher tier then you need to be able to use lush teles principle in order to explain how we can move the position of equilibrium by changing the reaction conditions le chatelier&#39;s principle tells us that if a system is already at equilibrium and a change is made to any of the conditions then the system will respond to counteract that change that basically means whatever i try to do the system will try to undo so if you add a reactant the system will remove it and if you heat a reaction the system will try to cool it often with these questions the exam board won&#39;t actually give you the names of chemicals because they don&#39;t want to confuse you with reactions you haven&#39;t heard of so they quite often just refer to a blue copper compound or a pink cobalt compound so in this reaction a blue copper compound reacts with hydrochloric acid to make a green copper compound and water and this is a reversible reaction if i add more hydrochloric acid i know that le chatelier&#39;s principle has told me that the system will shift to counteract that change and get rid of the hydrochloric acid the only thing that the system can actually do is make either the forward reaction or the reverse reaction happen faster so whichever reaction removes hydrochloric acid that&#39;s the one that will go faster that&#39;s the one that we say will be favored so in this instance the forward reaction is favored that means that the equilibrium shifts to the right in other words i&#39;m making more of the products therefore there&#39;ll be more of the green copper compound formed and my solution will start to look greener rather than bluer the second reaction condition that we can change is pressure pressure remember is caused by particles colliding with the walls of the container that they&#39;re in so the more particles there are in a container the higher the pressure will be if we increase the pressure this will favor whichever side of the reaction has fewer molecules on it to answer a question about pressure and equilibrium i need to look at the chemical symbol formula and identify which side of the equation has more gas molecules on it in this example i have three moles of gas in my reactants but only two moles in my products so the reactant side is a high pressure side and the product side is a low pressure side if i decrease the pressure le chatelier&#39;s principle tells me the system will shift to counteract that change and increase the pressure so it&#39;s going to move the equilibrium towards the higher pressure side which here is my reactants so here i would say that the backward reaction is favored because there are more molecules on the left i need to actually point this out and explicitly tell the exam board that i know that this is why the equilibrium is going to shift so the equilibrium will shift to the left and therefore the yield of sulfur trioxide will be lower we said at the start that if we heat up a reaction at equilibrium the system will shift to try to cool it back down again and the way it does this is by favoring the endothermic reaction it&#39;s very likely in a question such as this that you may be told that the forward reaction is endothermic but not told anything about the reverse reaction you are expected to know that for a reversible reaction one way will be exothermic and the other way will be endothermic so in this instance the forward reaction is going to be favored because that is what will cool my reaction back down and i need to tell the exam board that i know that this is because the forward reaction is endothermic therefore the equilibrium shifts to the right and therefore what i will see or observe is that my mixture will turn white the second topic in aqa gcc chemistry paper 2 is organic chemistry which starts off talking all about crude oil so you need to know that crude oil is a finite resource which means that it&#39;s going to run out and it&#39;s found in rocks and made from the remains of ancient biomass which is mainly plankton that was buried in sediment so basically we&#39;re talking about lots of sea creatures that died and then mud and sand fell on top of them and turned into rocks over millions of years under a lot of pressure and meanwhile they turned into fossil fuels like crude oil it&#39;s a mixture of hydrocarbons so compounds are made of hydrogen and carbon only absolutely nothing else and those hydrocarbons are mainly a type called alkanes alkanes are an example of a homologous series which is a group of compounds that have similar chemical properties because they have the same functional group and they also have the same general formula if you&#39;re taking combined science then you need to know about alkanes and alkenes and if you&#39;re taking triple science you also need to know about alcohols carboxylic acids and esters you should know that the names of the first four alkanes are methane ethane propane and butane and i like to remember this by using the mnemonic most elephants prefer bacon the general formula for alkanes is cnh2n plus 2. so what this means is that if we give you any number of carbons say eight carbons we can then use that for n to work out how many hydrogens there should be so two lots of eight plus two gives me a total of 18 hydrogens these alkanes are all examples of small covalent molecules and that means that between the molecules there are weak intermolecular forces as with all small covalent molecules the larger the molecule is the stronger these forces are this means as alkanes get bigger they have higher boiling points they also have higher viscosity which means sort of stickiness and they become less flammable due to the differences in boiling points the different alkanes that are mixed up together in crude oil can be separated out according to their boiling point by using a process called fractional distillation in fractional distillation a mixture of liquids is added to a fractionating column at the bottom of the fractionating column is a furnace or a heat source which heats them all up to an incredibly high temperature for the fractional distillation of crude oil this tends to be somewhere in the region of 400 degrees c at this temperature almost all of the alkanes are going to evaporate as they travel up the fractionating column there&#39;s a temperature gradient in other words the nearer you are to the heat source the hotter it is and as you go higher it gets cooler and cooler as each alkane reaches a height that corresponds to a temperature where it can condense it turns back into a liquid and this allows you to separate the liquids and use them afterwards fractional distillation of crude oil is used to provide fuels and feedstocks for the petrochemical industry which are used to make solvents lubricants polymers detergents and of course fuels in order to release the energy from these fuels they are combusted or burned in oxygen which is a kind of oxidation and this releases carbon dioxide and water as well as a huge amount of energy you need to be able to write balanced symbol equations for the complete combustion of alkanes complete combustion means they&#39;re burning insufficient oxygen for all of the carbon and all of the hydrogen to be fully oxidized so we&#39;re always going to make carbon dioxide and water if you&#39;re not very confident with balancing symbol equations in general then it&#39;s worth learning a method that will work just for the complete combustion of alkanes firstly you&#39;re going to look at the number of carbons in the alkane here we have propane so it has three carbons now because the symbol equation has to balance if i have three carbons on the left i must have three carbons on the right and the only molecule on the right that contains carbon atoms is carbon dioxide so i need to have three carbon dioxide molecules so i&#39;m going to take this three from the propane and put it as a coefficient in front of the carbon dioxide now i&#39;m going to look at the hydrogens in this propane molecule i have eight hydrogen atoms and i&#39;m going to make water molecules which each contain two hydrogen atoms so i actually need to take that eight and half it so i&#39;m going to put a 4 in front of the water now i need to work out how much oxygen i need to make this balance so i need to add up all of the oxygen atoms on the right hand side remembering of course that there are two oxygen atoms in every carbon dioxide molecule so that&#39;s two lots of three so that&#39;s six atoms in total and then each water molecule only contains one so that&#39;s another four atoms giving me 10 in total now since oxygen goes around as divalent molecules o2 the total number of molecules is going to be half of that so i add up all of the oxygen atoms on the right to make 10 i divide it by 2 to get 5 and that&#39;s my number of oxygen molecules now i should probably point out that this method is going to work beautifully provided you have an alkane with an odd number of carbon atoms in it it will still work if you have an alkane that has an even number of carbon atoms but the number of oxygen molecules you get out will not be a whole number so you might need four and a half or five and a half and although when we get to a level chemistry we use half as coefficients all the time at gcse we tend not to we tend to stick to whole numbers so if that&#39;s what you&#39;ve been taught and it&#39;s a little bit confusing for you there is quite a simple fix let&#39;s take this butane molecule here which has four carbon atoms in it if it completely combusts in oxygen it&#39;s still going to make carbon dioxide and water but to avoid that situation where i need a half number of oxygen molecules before i start i&#39;m going to double up so i&#39;m going to start with two butane molecules and this is the same thing that i would do for any situation where the alkane has an even number of carbon atoms so i&#39;m going to look at my four carbon atoms there but then i&#39;m going to remember that i have two of these molecules so in total i now have eight carbon atoms and i&#39;m going to make eight carbon dioxide molecules then likewise i look at these 10 hydrogen atoms but because i have two molecules that makes 20 in total and then i half it so i&#39;m back to 10 so that&#39;s 10 water molecules and then again i count up the 16 oxygen atoms in the carbon dioxide and the 10 oxygen atoms in the water which makes 26 in total and if i divide 26 by 2 i get 13 oxygen molecules we&#39;ve already described that crude oil is a mixture of hydrocarbons of all different sizes and one problem that we encounter is that there&#39;s a really high demand for the small alkanes because they make excellent fuels but actually crude oil contains large amounts of the much much larger alkanes which don&#39;t have nearly as many uses so we use a process called cracking which is a type of thermal decomposition in other words it uses heat to break down large molecules and what cracking does is it breaks down long alkanes into shorter more useful alkanes and also another group of molecules called alkenes you may be asked to complete symbol equations for cracking reactions but these aren&#39;t nearly as intimidating as they look in the first style of question we&#39;re literally just counting up the numbers of carbons and hydrogens on the left and right of the equation and making sure that they balance so if i know that there are 25 carbons and 52 hydrogens in my reactant then there must also be 25 carbons and 52 hydrogens in my products so if i know that one of my products contains 10 carbons and 22 hydrogens i can just take those away from the numbers on the left and therefore i can work out that my second product is c15h30 another style of question might tell you what the products are but not how many of them there are so in this instance i need to calculate how many ethene molecules are required to balance the equation so again i&#39;ve started with 25 carbons and 52 hydrogens and i can subtract the non-aim that&#39;s already been listed as a product that leaves me with 16 carbons and i know that there are two in each ethereal molecule so therefore i must have eight ethene molecules there are two types of cracking that you need to be able to describe the reaction conditions for and at gcc neither of them mention pressure which is a really common misconception it might help if you remember that you&#39;ve probably either done or watched a demo of cracking in the gcc classroom and so there isn&#39;t anywhere in that room that you can control the pressure in catalytic cracking we vaporize the alkane and pass it over a hot zeolite catalyst in steam cracking there&#39;s still a very high temperature but rather than using a traditional catalyst we use steam we&#39;ve just said that in cracking we make two types of molecules the alkanes that we&#39;ve already met and these new alkenes alkenes are more reactive than alkanes and this is because of the double bond that is their functional group each alkene is only going to contain one double bond you shouldn&#39;t be replacing all of the carbon-carbon bonds in the molecule with double bonds you just need one of them in order to test for an alkene we add an orange liquid called bromine water which will turn colorless in the presence of a double bond if you&#39;re doing combined science you can now skip ahead to chapter eight but if you&#39;re doing triple science we need to know a little bit more about alkenes and also some other homologous series alkenes are described as being unsaturated hydrocarbons as compared to alkanes which are saturated hydrocarbons this means in the alkene each carbon atom has not necessarily made as many bonds to hydrogen as it could have done there are two hydrogen missing compared to an alkane and so instead there&#39;s a carbon-carbon double bond this is reflected in the general formula which for an alkene is cnh2n the first four alkenes are ethene propine butene and pentene there&#39;s no methine because you can&#39;t have an alkene with only one carbon because there wouldn&#39;t be another carbon for it to form a double bond to as we&#39;ve already said alkenes are produced by cracking and they can be tested for by using bromine water you also need to know a little bit of information about the reaction of alkenes firstly they&#39;re useful for making addition polymers including a lot of important plastics like polythene and polypropylene and polybutane they can be converted back into alkanes by hydrogenation using a nickel catalyst they also react with water with an acid catalyst at 300 degrees c and 60 atmospheric pressures in order to make alcohols and finally they can be burned as fuels to produce carbon dioxide and water but they have a smoky flame so they&#39;re less good as fuels than alkanes are the third homologous series you need to know about are the alcohols which have the functional group oh their general formula is cnh2n plus one oh and the first four members of the group are methanol ethanol propanol and butanol as well as being found in alcoholic beverages ethanol is a really important solvent and general reagent in chemical industry depending on how pure you need your ethanol to be it can either be produced by fermenting sugar by yeast in warm and wet conditions or by hydrating ethene using steam alcohols dissolve in water to produce neutral solutions you might expect them to make an alkaline solution because you&#39;re thinking about hydroxide ions but this oh group isn&#39;t an ion it isn&#39;t free to enter solution and raise the ph it remains covalently bonded to the molecule alcohols also burn to produce carbon dioxide and water and they react with sodium to release hydrogen which you could test for with a squeaky pop test although you need to be very very careful because of course the alcohol is also highly flammable alcohols can be oxidized either by microbes or by chemical oxidizing agents like potassium dichromate in order to make carboxylic acids these carboxylic acids are the fourth homologous series that you need to know about and the first four are methanoic acid ethanoic acid propanoic acid and butanoic acid their functional group is a carbon atom double bonded to an oxygen atom and then single bonded to another oxygen atom that in turn is bonded to a hydrogen atom if you&#39;re struggling to remember which way around the double bond and the single bond go just bear in mind the oxygen always needs to make the same number of bonds so double bond on the top and single bond on the bottom means that both oxygen atoms are making two bonds carboxylic acids dissolve in water to form weak acids in other words they don&#39;t fully ionize this means that they have high ph&#39;s compared to strong acids of the same concentration so if you have a one molar solution of hydrochloric acid and a one mole of solution of ethanoic acid the ethanoic acid will have a higher ph although still below seven because it is still an acid carboxylic acids react with carbonates to produce carbon dioxide gas bubbles which you could test for with lime water and they react with alcohols to make esters esters are sweet-smelling volatile substances which means that they evaporate easily so they&#39;re used as perfumes and also as flavourings they&#39;re made from a condensation reaction between an alcohol and a carboxylic acid and if you know the name of the reactants that they were made from then you can name the ester you take the prefix of the alcohol and put oil on the end and then the prefix of the carboxylic acid and put 08 on the end so this example here is methylpropanoate in the condensation reaction that makes this ester a water molecule is lost you&#39;ve already briefly met polymers in unit 2 and then here in unit 7 we flesh this out with a little bit more detail alkenes and other similar molecules which contain a carbon-carbon double bond are able to react together to form what we call addition polymers these are named in the usual way by putting poly in front of the name of the monomer so for instance here is chloropropine and so the polymer is called polychloropropyne we can draw the polymer out in full like this or we can display it slightly more succinctly in this manner it&#39;s important when you&#39;re drawing this polymer that you make sure that the bonds are going out the side of the brackets and that you&#39;ve remembered to include those brackets and the n to show that this repeating unit is happening again and again and again you should also note that whereas there is a double bond in the monomer in the polymer we don&#39;t have that double bond anymore because it&#39;s broken and that&#39;s what&#39;s allowed those two carbon atoms to make bonds with the molecules on either side of them in a condensation polymerization reaction you need two different functional groups this could either be a situation where you have two different monomers each of which has a different functional group so for instance you could have a dicarboxylic acid and a diol which is basically an alcohol with an alcohol group on each end or you can have a single monomer like these amino acids which has two different functional groups one on each end the key thing to remember about condensation polymerization is that we always lose a small molecule and it&#39;s very often a water molecule you need to be able to discuss the structure of three naturally occurring polymers the first one is dna which has a double helix structure and is made of four complementary nucleotides you won&#39;t get the mark if you just say it&#39;s made of four complementary bases the base is the bit that here is shown in red which represents the code but actually the whole monomer is the nucleotide and we need to be talking about that secondly you should know that proteins are made up of amino acids like this molecule here which is glycine and finally you should know that starch is a polymer made from glucose unit 8 is the chemical analysis unit which starts out talking about pure substances a pure substance in chemistry is a single element or compound not mixed together with anything else that&#39;s different from everyday life where we might talk about something like pure orange juice because it hasn&#39;t had anything added to it even though it&#39;s a mixture of hundreds of different compounds pure substances melt and boil at specific temperatures so you can use a data book value to figure out whether a substance is pure for instance we know that pure water boils at exactly 100 degrees so if i have some water and it boils at a temperature that&#39;s not 100 degrees well then it&#39;s not pure formulations are mixtures that have been designed as a useful product they contain really carefully measured quantities of all of the ingredients examples of formulations include fuels cleaning agents paints medicines alloys fertilizers and foods another way that you can identify whether a substance is pure and which different substances it contains if it&#39;s not pure is by using chromatography there are several different types of chromatography but in gcc you&#39;re most likely to have met paper chromatography which forms the basis of the required practical all types of chromatography are used to separate out mixtures and then to analyze what is in them they all make use of a stationary phase which remains still and a mobile phase which moves through the stationary phase transferring the sample in paper chromatography the stationary phase which doesn&#39;t move will be the piece of chromatography paper and the mobile phase will be a solvent and you&#39;re most likely to have used water and that water moves through the chromatography paper and it will transfer your samples which tend to be inks or dyes or something the substances are going to be separated out based on how well they are retained by the stationary phase so basically how good is the stationary phase at holding onto them in paper chromatography this corresponds to how soluble the substance is in the solvent a very soluble substance will travel with the solvent for a long distance whereas a less soluble substance will be deposited on paper after a shorter distance and something that&#39;s not soluble at all just won&#39;t even move off the start line chromatography is a comparative method we analyze substances by comparing them to chemical standards known substances for which we already know the results this is a bit like fingerprinting you can&#39;t just look at a fingerprint left at a crime scene and know who it belongs to you need a database of the fingerprints of everybody who&#39;d been in that area and then you can find out whose fingerprint it was by comparing them rather than just eyeballing a chromatogram and saying well this spot seems very similar to this standard we can calculate something called an rf value or a retention factor value this is a number between 0 and 1 which tells us more precisely whether two samples are behaving in the same way it&#39;s particularly good for if we don&#39;t want to include a sample of the standard on our chromatogram we just want to look up a number in a book and say well is it the same the rf value is calculated by dividing the distance moved by the sample by the distance moved by the solvent and of course the distance moved by the solvent is the maximum distance that the sample could have traveled because it&#39;s being carried by the solvent you can&#39;t just have your color jumping ahead of the solvent front since substances have different solubility in different solvents they will also have different rf values and different solvents so repeating the chromatogram with different solvents might allow us to separate out substances that have the same solubility in one solvent imagine you have a dot on a paper chromatogram made with water that actually contains two different inks but then when you run the same chromatogram using ethanol they might separate out a pure compound only contains one substance so even if you put it in a whole range of different solvents you won&#39;t split it up into different multiple spots paper chromatography is one of your named required practicals and therefore you need to be able to write a method to explain how it should be carried out so to start with you draw a line on which you&#39;re going to put your samples and as you know it&#39;s really important that that line is drawn in pencil not pen to stop it from running and interfering with your data you&#39;re going to place the sample of the standards and also the sample you&#39;re trying to investigate on that line and then you&#39;re going to put it so that it&#39;s just sitting into a beaker of solvent and it&#39;s really important that that solvent is below the line because if the solvent comes above the line then rather than being carried up the chromatography paper by capillary action your sample would just bleed out into the solvent and you wouldn&#39;t get any results you may want to put a lid on top of the beaker to prevent the solvent from evaporating as we&#39;ve said on that start line as well as your sample you&#39;re going to put some standards that you can compare it to and then at the end you&#39;re going to measure the distance travelled by everything and use these numbers to calculate an rf value there are four gas tests that you need to be able to describe to test for oxygen we take a splint that&#39;s on fire and blow it out so it&#39;s just glowing and if you add that into the gas and the splint relights then that tells you that it&#39;s oxygen to test for hydrogen we also need a lit splint because we&#39;re going to ignite the gas in other words set fire to it and if it is hydrogen then it will burn with a squeaky pop definitely hydrogen to test for chlorine we take some litmus paper and it&#39;s important that it&#39;s damp litmus paper and then the gas will bleach it and turn it white and then finally to test for carbon dioxide we take the gas and we bubble it through lime water which is a solution of calcium hydroxide and if it is carbon dioxide then the lime water will turn white because it will form a white calcium carbonate precipitate now again if you&#39;re doing combined science you can skip ahead to chapter nine but if you&#39;re doing triple science gcse chemistry there are a few more bits we need to cover firstly you should be able to describe flame tests which can be used to identify cations in ionic compounds you need to take a nichrome wire and rinse this in dilute hydrochloric acid this means if there are already any metal ions on the wire those will react with the acid and be removed then you place that damp wire into a solid sample of the metal compound or into a solution that contains it then you hold this in the clear part of a bunsen burner roaring plane the characteristic colours that you&#39;re expected to know are that lithium has a crimson flame sodium has a yellow flame potassium has a lilac or pale purple flame calcium has a brick red or orange red flame and copper has a bluey green flame as seen in this photograph here now as pretty as flame tests are and as much as i like doing them they are quite subjective it&#39;s down to you as a human to say well exactly what shade of red is that and also there&#39;s another problem because if you have two different cations present then usually the brighter color will obscure the paler one so for instance if you have lithium and potassium in the same sample you&#39;re not going to see the pale lilac flame when there&#39;s the bright crimson one so instead it&#39;s often better to use an instrumental method because usually these are more accurate rapid and sensitive so flame emission spectroscopy is quite a lot like a flame test but using an instrument instead and what we do is we put the sample into a flame and the light that comes out is passed through a spectroscope the output that you get is a line spectrum and the different lines indicate different elements and the size of the line gives you a clue as to how much of it is there the other way that you can test for cations is by adding sodium hydroxide to produce a precipitate which basically means a solid being formed out of the solution if you have aluminium magnesium or calcium ions then when you add sodium hydroxide this forms a white precipitate and you can&#39;t distinguish between the three of them if you carry on adding excess sodium hydroxide so adding too much then the precipitate that forms from aluminium ions will then dissolve so you can then eliminate that one the transition metals also form precipitates in response to sodium hydroxide and these have got characteristic colours the three that you need to know about are copper two plus ions which will produce a blue precipitate iron ii plus ions which have a dark green precipitate and iron three plus ions which have a brown precipitate you need to be able to describe the tests for three different anions carbonates react with acids to release bubbles of carbon dioxide gas you can then pass this gas through some lime water and if it turns cloudy you know that it&#39;s carbon dioxide and that you had a carbonate halides form precipitates with silver nitrate remember halides come from group seven so we&#39;re talking chlorides bromides iodides firstly you need to treat the solution with some nitric acid to remove any carbonates or sulfates because these would give you a false positive then you add your silver nitrate if chlorides are present you&#39;ll make a silver chloride precipitate which is white if bromides are present you&#39;ll make silver bromide which is cream and if iodide ions are present you&#39;ll make silver iodide which is yellow finally we test for sulfates by adding barium chloride solution before we add the barium chloride we need to use dilute hydrochloric acid to remove any carbonates as again this would give us a false positive as part of the required practical you need to be able to describe how you would use these tests or a combination of them to identify what a particular substance is you may need to use flame tests hydroxide tests and the anion tests the next topic is the chemistry of the earth&#39;s atmosphere which starts off looking at the modern atmosphere so you need to know that for the past 200 million years the atmosphere has consisted of about 80 nitrogen about 20 oxygen and small amounts of other gases including carbon dioxide water vapor and noble gases like argon next we need to think about the earth&#39;s early atmosphere earth&#39;s atmosphere first formed about 4.6 billion years ago and that means we can&#39;t be exactly certain what happened because there isn&#39;t a huge amount of evidence now your keyword here is evidence we don&#39;t want to just be saying oh we don&#39;t know because nobody was around to see it that won&#39;t get you the mark we do know that initially there was intense volcanic activity and that volcanic activity spewed out lots of carbon dioxide and water vapor and it made the earth&#39;s early atmosphere quite similar to that of mars and venus today now obviously because there were loads of volcanoes it was really really hot so even though there was lots of water in the atmosphere there wasn&#39;t any on the ground so there weren&#39;t any oceans there also may have been small amounts of nitrogen and ammonia and methane and that&#39;s important because they were key components in the formation of the first amino acids without which life could not have formed as time went on the earth cooled down and then oceans formed so carbon dioxide started to dissolve in those oceans and then plankton tiny sea creatures used that carbon dioxide dissolved in the oceans to make calcium carbonate shells if you&#39;ve ever seen the white cliffs of dover they&#39;re made of a stone called limestone and that&#39;s made from the shells of dead sea creatures so that&#39;s where lots of this carbon dioxide is locked up in sedimentary rocks like limestone also when they died some of those sea creatures were turned into fossil fuels so a lot of the carbon is also locked up in coal and oil and gas over time these processes led the level of carbon dioxide in the atmosphere to decrease quite dramatically now about 2.7 billion years ago algae started photosynthesizing algae are part of the same kingdom as plants they have chloroplasts and cell walls but they also include things like seaweeds and microscopic single-celled photosynthetic organisms these little green cells started photosynthesizing and as you know this makes glucose but it also makes a lot of oxygen and so levels of oxygen started to rise and then about a billion years later the first higher plants evolved and oxygen levels eventually got high enough for animals to evolve too next we get onto greenhouse gases and first let&#39;s establish this has nothing to do with the ozone layer the ozone layer is not mentioned at all in gcc chemistry so just park it on one side and forget that it exists you should know that greenhouse gases have something to do with global warming but we need a little bit more detail here firstly the three named greenhouse gases in your specification are carbon dioxide methane and water vapor these are molecules that will vibrate in response to radiation and they&#39;re responsible for the earth warming up having a little bit of them is actually a good thing because it keeps our temperature stable and high enough to sustain life but unfortunately we now have too much of them and so climate change and global warming are happening these greenhouse gases form a sort of blanket around the earth and if you imagine that blanket has holes in it and shortwave radiation like visible light uv gamma and x-rays can get through the holes because they have a short wavelength however when that radiation reaches earth the earth absorbs it and when it re-emits it and gives it back out it does so with a longer wavelength so that radiation being emitted is infrared radiation now that infrared radiation is too big to easily pass back through the greenhouse gas blanket so although some of it will get back through some of it will be trapped and instead it warms up the atmosphere and warms up earth and these rising temperatures are leading to lots of serious consequences now we need to be specific when we&#39;re talking about these so firstly as the ice caps melt this causes sea levels to rise this can lead to flooding and other extreme weather events but it&#39;s not enough to just say extreme weather events we need to actually name them so talking about droughts and hurricanes polar habitats are being lost and with them many species are going extinct make sure that you&#39;re talking about polar or arctic habitats not just saying habitats in terms of things that humans do that are making the situation worse there are six key ways that we&#39;re increasing the levels of greenhouse gases in the earth&#39;s atmosphere firstly every time we burn fossil fuels this releases the carbon that&#39;s stored in them increasing carbon dioxide also cutting down trees because that way we&#39;re reducing photosynthesis which removes carbon dioxide from the atmosphere and also because those trees are a store of carbon and also digging up peat bogs peat bogs form in a similar way to fossil fuels but over a shorter time scale and again they&#39;re a carbon sink people use peat as fuel or for compost and so digging up those peat bogs has an impact on carbon dioxide in the atmosphere now you might also have heard because it&#39;s generally kind of hilarious that farming cows is massively increasing the amount of methane because cows basically fart out methane also rice farming because you have to grow rice semi underwater and there are bacteria that live in that water in their roots and release methane and also in landfill so rubbish dumps unfortunately it&#39;s really difficult to model exactly what&#39;s going to happen because this is a really complex system so we tend to come up with models that are quite simplified and miss things out or gloss over them and then also speculate and guess and this can be really problematic and sometimes media coverage can be biased and only tell part of the story so we need to look for evidence wherever we can you should know that the carbon footprint is the total amount of greenhouse gases emitted over the life cycle of a product service or event and this could be reduced in a number of ways like buying resources more locally so you don&#39;t release greenhouse gases while transporting them in a plane or by using less energy during manufacture so that you need to burn fewer fossil fuels to generate the electricity you also need to know apart from global warming about some of the other problems associated with using fuels there are various pollutants released when fuels burn and as you know when you burn a hydrocarbon you release carbon dioxide and water and carbon dioxide leads to global warming but if you don&#39;t have enough oxygen for complete combustion then instead of making carbon dioxide you make carbon monoxide or even carbon particulates which are basically soot carbon monoxide is a toxic gas which stops your red blood cells from working properly and carbon particulates cause a phenomenon called global dimming which literally means the sky is getting darker and less light is getting to earth also almost all fossil fuels have small amounts of sulfur in them as impurities so when you burn the fossil fuel you also burn the sulfur and make sulphur dioxide and sulfur dioxide along with carbon dioxide and nitrous oxides which can be produced when you burn things at a high enough temperature for super unreactive nitrogen from the atmosphere to react with oxygen or contribute to acid rain as well as making acid rain sulphur dioxide and oxides of nitrogen can cause respiratory problems like asthma humans use the earth&#39;s resources to provide warmth shelter food and transport natural resources supplemented by agriculture or farming provide food timber clothing and fuels in addition we can supplement these resources with synthetic resources which are those that have been made from chemicals so for instance cotton is a natural fiber that grows on a cotton plant but we now also have fibers like nylon which is made from chemicals you should be able to identify whether a resource is finite and is running out faster than we can replace it so for instance fossil fuels or whether it&#39;s renewable like wood and you should also know that sustainable development is development that meets the needs of current generations without compromising the ability of future generations to meet their own needs in other words using resources in a way that isn&#39;t going to mean our grandchildren don&#39;t have them to use potable water is water that is safe for us to drink but it&#39;s not the same thing as pure water which has nothing else dissolved in it potable water needs to have low levels of salts but it probably does still have some salts in it and it should also be free of pathogenic microorganisms but it may have other things dissolved in it like fluoride ions for dental care or chlorine as a disinfectant in order to make potable water or drinking water from rivers and lakes we start by choosing an appropriate source then filtering it to remove solid matter and sterilizing it using chlorine ozone or ultraviolet light we can also make potable water from sea water by a process called desalination which means removing the salt and there are two ways this can be achieved either we can distill it by heating it up until it evaporates leaving the salt behind or we can use reverse osmosis where it&#39;s pushed through a membrane both of these methods are very energy intensive and this makes them expensive which is why countries that have access to sources of fresh water tend not to use them potable water can be made from waste water including sewage agricultural waste and industrial waste in order to make potable water from sewage we first need to undergo screening and grit removal to remove large objects then sedimentation to split it apart into sewage sludge which is semi-solid and effluent which is the liquid and then these are treated with anaerobic and aerobic biological treatment where bacteria digest the sewage and leave us with potable water industrial wastewater may also need to have harmful chemicals removed it&#39;s likely that these could come up in the context of an evaluation question where you&#39;re asked which method would be easier so you want to think about the number of steps and the amount of energy involved if you&#39;re sitting higher tier you need to be able to describe two novel methods of metal extraction which are increasingly being used to extract copper because the supply is running out but demand is increasing so the price is going through the roof these can both be used on low-grade ores which contain less than one percent of copper in phytomining plants called hyper accumulators are grown on the low-grade ore and they absorb the copper into their leaves and stems and then the plants are burned leaving ash the copper can then be extracted from the copper compounds in the ash by using displacement or electrolysis in bioleaching rather than using plants we&#39;re using bacteria both of these methods have the advantage that they involve very low energy phytomining is also carbon neutral and bioleaching has an advantage over traditional chemical leaching methods in that it doesn&#39;t involve nasty chemicals like cyanide however they tend to be quite slow and it may still be necessary to remove lots of rock companies write life cycle assessments in order to assess the environmental impact of a product they&#39;re making by thinking about the extraction of the raw materials the manufacture the use and operation the distribution and finally the disposal some parts of this can be done quite easily and objectively like how much water is going to be used but other things are harder for instance it can be hard to know exactly how much carbon dioxide will be released as a result of this product being made because some of that might come down to the amount of electricity that&#39;s used when it&#39;s being used and we might not know that in advance in addition it&#39;s often the company that&#39;s manufacturing a product that is writing the lifecycle assessment so they&#39;re sometimes not objective and can be misused to reach predetermined conclusions reducing our use of raw materials reusing manufactured products and recycling products to make new ones can all help to deal with the growing scarcity of resources growing concerns about our energy usage and limited space in landfill you should be able to describe how glass is recycled by color sorting crushing melting and reforming and how metal is recycled by melting it and recasting the amount of effort required in recycling is linked to how pure the final product needs to be and you may also need to evaluate whether recycling is worthwhile in a certain situation corrosion is the breakdown of materials by chemical reactions with other substances in their environment like water and oxygen if we&#39;re specifically talking about iron and talking about it being broken down by water and oxygen together then we can refer to this as rusting we can protect metals from corrosion by painting them greasing them or electroplating them and in fact aluminium forms its own protective layer of aluminium oxide which protects it from further corrosion because it&#39;s so hard we can also protect iron by galvanizing it which means adding a small amount of a more reactive metal usually zinc and then the oxygen in the atmosphere will react with that zinc instead of with the iron and this is a type of sacrificial protection in addition to knowing from unit 2 that alloys are mixtures of metals and that they&#39;re harder than pure metals for unit 10 we need to know the names and uses of some specific alloys so there&#39;s bronze which is made from copper and tin alloyed together which is used to make coins and brass which is made from copper and zinc and this is used for the pins of plugs we can talk about how gold is often used as an alloy in jewelry to make it slightly harder and you need to know that pure gold is referred to as 24 karat gold and you can then use other carat ratings to work out the percentage purity of a type of gold there are three kinds of steel that you need to know about all of which are basically iron with small amounts of other things added so there&#39;s high carbon steel which is strong but brittle and this is used for making cutting tools and low carbon steel which is softer and more easily shaped so it&#39;s used for making things like car bodies then we have stainless steels which have small amounts of chromium and nickel and these are resistant to acids so they&#39;re really good for making cutlery finally aluminium alloys are very useful because they&#39;re low density so they&#39;re used for making aircraft ceramics are non-metallic solids made from a raw material heated to a high temperature and there are two types that you need to know about clay ceramics and glass are both waterproof and brittle but clay ceramics are opaque whereas glass is usually transparent soda lime glass is used for everyday uses like windows and drinking glasses and is made from sand sodium carbonate and limestone but it doesn&#39;t have a very high melting point so for lab use and also some cooking uses you may need borosilicate glass or hard glass which is made from sand and boron trioxide polymers are very long chains of repeating units called monomers and those monomers aren&#39;t individual atoms they&#39;re actually small covalent molecules that have formed further covalent bonds between them to make these very long chains in between the chains there are weak intermolecular forces but because the polymer chains are so large those weak intermolecular forces are comparatively strong and so most polymers are solid at room temperature we can name a polymer based on the name of its monomer so for instance if you take ethene and you polymerize it you make polyethyne or if you take propine and polymerize it you make polypropy the physical properties of a polymer such as its density and its melting point will depend on the monomer that it&#39;s made out of but also the reaction conditions that we use to make it so the temperature and the pressure and the catalyst for instance we can compare two types of polyethene low density polyethylene and high density polyethy low density polyethyne is made under very high pressure at a high temperature with a trace amount of oxygen as a catalyst and it has an amorphous structure where it doesn&#39;t stack nicely together whereas if we use a much lower pressure and a lower temperature and a z-glanata catalyst that we can produce high-density polyethylene which has a crystalline structure where the strands pack tightly together meaning that it has a much higher density completely separately from that you need to be able to distinguish between thermo-setting and thermo-softening polymers then most setting polymers have crosslinks which are just a fancy name for covalent bonds between the chains and this means that the chains can&#39;t slide past each other and so they don&#39;t melt when they&#39;re heated they actually char and eventually burn whereas in a thermo-softening polymer there are only weak intermolecular forces between the layers so these can slide over each other and they will melt a composite material is made by combining two or more materials you can still tell them apart as they don&#39;t blend into one another they&#39;re still distinct and this use of two materials allows composites to have useful combinations of properties like being both strong and lightweight or being able to resist being squashed and stretched the two materials making up a composite are called the reinforcement and the matrix or binder the reinforcement provides the heavy duty structure while the matrix sticks it all together a natural example of composite material is wood which contains soft cellulose combined with a harder substance called lignin their lignin fibers form the reinforcement and they&#39;re surrounded by the cellulose matrix for a synthetic example we can look at steel reinforced concrete concrete is itself actually a composite because it&#39;s made from cement sand and aggregate or small stones and concrete has good compressive strength so it resists being squashed but if you try to bend it at all it can shatter and the steel reinforcement has good tensile strength and prevents this from happening the harbour process is a really important industrial chemical process used to manufacture ammonia for making fertilizers it&#39;s made out of nitrogen which is extracted from the air and hydrogen which is made by reacting methane with steam the nitrogen and hydrogen are passed over an iron catalyst at 450 degrees c and 200 atmospheric pressures and you need to be able to explain why these reaction conditions are used it all links to equilibrium because this is a reversible reaction using a high pressure increases both the yield of ammonia and the rate of reaction but if we push the pressure much higher it will be both expensive and also even more dangerous using a high temperature decreases the yield so means we would make less ammonia so we want the temperature to be as low as possible but having a low temperature will decrease the rate of reaction and make the process very slow so this is why we don&#39;t do it at room temperature we still use a reasonably high temperature and also why we need the iron catalyst once the ammonia is produced the mixture of gases is cooled so that the ammonia will liquefy while the nitrogen and hydrogen remain as gases and the ammonia is removed the leftover hydrogen and nitrogen are then recycled background into the reactor to react again a large proportion of the ammonia made using the harbour process goes into mpk fertilizers these are an example of a formulation added to plants in order to increase their yield mpk fertilizers contain salts that have nitrogen phosphorus and potassium in them as these three elements are vital for healthy plant growth ammonia can be used to manufacture ammonium salts but also nitric acid can be used as another source of nitrogen potassium chloride potassium sulfate and phosphate rock can all be mined from the ground but obviously phosphate rock can&#39;t be used directly so something called the order process is used to convert this into phosphoric acid which can then be used as a source of phosphorus that&#39;s it for aqa gcc chemistry or combined science paper 2. i hope that you found this a useful summary of all the content that&#39;s likely to come up in your exam good luck and don&#39;t forget to like and subscribe

Transcript for:AQA GCSE Chemistry Paper 2 Summary

Transcript for:
AQA GCSE Chemistry Paper 2 Summary