Transcript for:
Chemistry Key Concepts Overview

hello lovies in this video we're going to be going through the content that you need for your Ed EXL gcsc chemistry paper 2 and this is to a grade nine standard so if you're doing separate signs all of those bits were marked if you're doing combined signs you can just ignore those bits or you can use the time stamps in the description down below to jump to the bits that you need to to go with this video Charlotte has made predicted papers I will walk through the predicted papers to show you how to layer all the things all the answers the style of answers that the exam would be expecting to [Music] [Applause] see here we're going to have a look at a vital part of chemistry the periodic table we're going to use this so much throughout the course so periodic table why is it called a periodic table it's because similar properties occur at regular intervals or periodically now let's have a look at this periodic table then we should know that the columns are called groups and the periods are the rows so we start with period one which is our first row then the next one is our period two period 3 four and so on now we need to know that all of the different groups have elements with similar properties within it now all of the elements within a group have the same number of electrons in their Al to Shell this is such useful information so for example all of the elements in group one have just one electron in that outer shell all of the elements in group two have two electrons in that outer shell and so on another great fact to know is that elements in the same period all have the same number of electron shells so hydrogen and helium in Period one just have one shell all of the ones in Period two have two shells and so on the group one metals or the alkaline metals as they're sometimes referred to have a few Key Properties the first one is that they all have one electron in their outer shell and the second property we need to know is that they can react with oxygen chlorine and water now we need to know how does the reactivity of the group one elements change down the group it's important we know that the reactivity increases Down group one why why does the reactivity of group one Metals increase down the group we need to know that the atoms increase in size as you go down the group there are also more shells and this means that the outer shell is further away from the nucleus this means that the electrostatic attraction between the positive nucleus and the outer shell electron is weaker now this means it's easier to lose that one outer electron making those elements at the bottom more reactive let's have a look at how the group one metals react with water we need to know the general reaction is that metal plus water makes a metal hydroxide plus hydrogen now an example of this of a group one metal reacting with water is the reaction of sodium with water to make sodium hydroxide and hydrogen the alkaline metals get their name from the fact that when they react with water they form this metal hydroxide side which forms an alkalized solution with a pH greater than seven it's good to know that heat is also given off in these reactions sometimes and this can even sometimes be seen with a flame in these reactions so what is observed when alkaline metals react with water let's think about our equation our general equation here we're going to observe fizzing due to the formation of hydrogen gas and we also going to observe our solid metal disappearing over time and this is because the metal gets used up the halogens are found in group seven of the periodic table Group Seven Elements all have seven electrons in their outer shell they're all non-metals and they all consist of molecules made of pairs of atoms we call these diatomic molecules so they exist as for instance F2 cl2 br2 Etc now we need to know how does the reactivity of the group s elements change down the group we need to know that the reactivity decreases Down group seven so as well as knowing that the reactivity decreases down the group we also need to know why so we need to know the atoms increase in size as they go down the group we need to know that there are more shells and so the outer shell is further away from the nucleus and therefore the electrostatic attraction between the nucleus and the alter shell is actually weaker this means it's going to be harder to gain an electron to fill that alter shell which in turn makes it less reactive as we go down group seven the melting and the boiling point of the halogens increases now this explains why the state changes from a gas to a liquid to a solid as we go down this group but why does the melting and boiling point increase we need to know these three points the molecules increase in size so the intermolecular forces become stronger and therefore more energy is required to overcome the forces of attraction and that is why the melting and boiling point increases so here we have a table that summarizes all of the properties about the group seven elements that we need to know so the state of from temperature goes from a gas to a liquid for Bromine and a solid for iodine we can see that as we go down the group the relative molecular mass increases as does the melting point and boiling point however the reactivity decreases down the group we also need to know about the appearance of group seven elements we need to know that at room temperature Florine is a gas that is yellow in color we need to know that chlorine at room temperature again is a yellow green gas however if we were to dissolve chlorine in water it would appear a pale green solution groine now at room temperature is a red brown liquid however if we put that in water it would appear orange and finally iodine is a gray solid at room temperature however in solution it's actually dark brown in color when the group seven elements react with metals they produce salts for instance if we had sodium plus chlorine we end up making the salt sodium chloride the halogens can also react with non-metals such as hydrogen now when halogens react with hydrogen they form hydrogen halites for example if we had hydrogen plus chlorine we end up making hydrogen chloride so H2 Plus cl2 makes 2 HCL hydrogen halides are gases at room temperature and they dissolve in water to form acidic solutions for example hydrogen chloride would dissolve in water to form hydrochloric acid now let's look at displacement reactions for the group seven elements so we know that reactivity decreases Down group seven now the more reactive group seven elements can take the place of less reactive group seven elements in a compound this is called a displacement reaction so if we have chlorine reacting with potassium bromide we will see that the chlorine which is more reactive will displace the bromine giving us potassium chloride and bromine however if we reacted chlorine with potassium fluoride because chlorine is less reactive than Florine we actually have no reaction take place because the chlorine cannot displace the more reactive Florine this table here summarizes the reactions and the observations that we need to know so if we reacted pottassium chloride with bromine or iodine you can see that we get no reaction each time that's because the chlorine is already more reactive than the bromine or the iodine so no displacement takes place however when we react potassium bromide with chlorine the chlorine will displace the bromide ions now as a result we'll end up forming potassium chloride and bromine now that bromine will be in solution so we will observe an orange color now if we reacted potassium bromide with iodine we would see no reaction take place again and the reason for that is because bromine is more reactive than iodine finally here if we have potassium iodide reacting with chlorine because the chlorine is more reactive than the iodine we get a displacement reaction take place we end up forming potassium chloride and iodine and because this is in solution a brown color is observed now the exact same thing happens with bromine reacting with potassium iodide again the bromine will displace the less reactive iodide ions and iodine will be formed and again we will observe that brown color of the iodine in solution to test for chlorine gas we want to put damp blue litmus paper into the gas and if chlorine was present the litmus paper will be bleached and it will turn white balanced displacement reactions can be written in terms of ions so we look at this balanced displacement reaction here where cl2 reacts with 2 KBR to make 2 K and br2 what we can do is we can write this in terms of its ions now to do this take anything which is aquous and ionically bonded such as KBR and KCl so remember ionically bonded things have Metals with non-metals and split them into their respective ions like we have here now by doing this we can now look to both sides of the reaction and anything that's come up identical on the left and the right we can cancel out so here I can see that I have two k pluses on each side so I'm just going to cancel these out now I'm going to cancel them cancel them now these ions that we've cancelled we call spectator ions now what we're left with here as you can see is our ionic equation now to work this out in terms of Redux reactions we can split this into two half equations you've got a cl2 which goes to 2 CL minus for that to happen to go from no charge in the cl2 to a negative charge of 2 minus in the two CL minuses we need to add two electrons and due to the fact that we know oil rig oxidation is loss of electrons reduction is gain I can see that the chlorine the cl2 has been reduced likewise we can do the same with the bromines we've got the two BR minus we know it becomes br2 to balance the charges of this we need to add two electrons to the right hand side that means that the BR minus has lost those electrons it's split up to make two things and that there because oxidation is loss of electrons means we've ox the BR minus let's look at group zero or the noble gases now this is the column on the periodic table that is found furthest to the right hand side so the key facts we need to know about the group zero elements are that they have a full outer shell of electrons they are unreactive and they do not form molecules they are monatomic they exist as neon or helium not as molecules so why are the group zero elements unreactive this is because they have full outershell of electrons so the next fact we need to know about the noble gases is how does the boiling point of the group zero elements change down the group we need to know that the boiling point increases down the group we need to know that the noble gases or the group zero gases have three particular properties we need to know that they have low densities they are inert or unreactive and they're not flammable so there's two particular noble gases that we need to know a little bit more about for GCSE we need to know that helium is less dense than air and non-flammable and we need to apply this to the fact that it gets used in balloons as I'm sure you've had in your home and birthday parties and also airships as well Argon is the other noble gas that we need to know about now this is actually denser than air unlike helium and it's inert still because it's denser than air we're not going to put it in a balloon it would just sink down to the ground but instead we use it as a shield gas when welding pieces of metal together now the fact that it's denser than air means it acts almost like a blanket and prevents air and therefore oxygen getting to the metal now this p with the fact that it's iner it's going to stop the metal that we're welding together from oxidizing so make sure you know those two examples [Music] [Music] [Music] the rate of a reaction is a measurement of how quickly reactants get used up or products are made to calculate the mean rate of a reaction we can take one of two approaches if we're measuring the quantity of reactant used then we can say that the mean rate of reaction is equal to the quantity of reactant Ed divided by the time it took alternatively if we're looking at the amount of product that gets made then we can say that the mean rate of reaction is equal to the quantity of the product made divided by the time taken now when we say the quantity of reactant or product this can either be measured in terms of grams as mass or in cm cubed as a volume the units for rate of reaction action depends upon how we measure the quantity of reactant or product so the units for rate of reaction will either be grams per second or cm cubed per second there's always the chance as well that they could give you a slightly different unit as well so always keep an eye on the answer line and what they want they could use grams per minute or centimet cubed per minute but we do need to make sure that we know what the question wants graphs can tell us a lot about the rate of a reaction so if we look at a curve of amount of substance against time if the curve is steep then that tells us that we have a high rate of reaction which means that the reaction is happening quickly however if our curve is comparatively more shallow then this tells us that we have a low rate of reaction and this tells us that our reaction is happening slowly if we want to experimentally determine the rate of reaction we have two different methods firstly if we're using the mass we can measure the change in mass of our reaction mixture to do this we would use a balance and we would use a stopwatch the stopwatch is very important because that measures the time taken now we'll use this method if a gas gets produced as the mass of our reaction mixture will eventually decrease with time however if we want to measure the volume of gas produced instead then we would use this setup here and we could either use our reaction mixture connected to a syringe or we could connect it to an upturned measuring cylinder we would also always need to have a stopwatch whenever we're working out the rate of anything in science we need a stopwatch to measure our time now this method is perfect if a gas is produced as well this time though we're going to measure the volume of gas produced let's have a look at these experiments in a little bit more detail let's have a look at the method of measuring the change in mass so firstly you put your reaction mixture on a balance you can see here that we have some cotton wall on the top of our conical flask the reason why we use this cotton wall at the top is it will allow gases to escape however it won't allow any other substances to escape so what we're going to see is as this reaction takes place if a gas is produced the mass will decrease we're going to measure the mass of the mixture on the balance and record it at regular time intervals the rate of the reaction can then be determined using rate equals change in mass which we measure using our balance divided by the change in time which we measure using our stopwatch now this particular method because we're using a balance to measure a mass the rate would have the units of grams per second now let's have a look at the method that looks at the volume of gas produced so in this case our reaction mixture is connected either to a gas syringe or to an upside down measuring cylinder as the reaction takes place base if a gas gets produced the apparatus of our measuring cylinder or our gas syringe is going to collect the gas we can record the volume of the gas at regular time intervals using our stopwatch to determine those and then finally we can calculate the rate of reaction using the equation that rate equals the volume of the gas produced divided by the change in time and the units in this case will be cm cubed per second because of course we're measuring of volume in this case we're expected to be able to calculate the rate of reaction using graphs so to work out the mean rate of a reaction between two points in time on a graph firstly we're going to use the graph to find the amount of substance at each time next we're going to find the change in the amount then we're going to find the change in the time and finally we can now calculate the mean rate of the reaction it's going to be equal to the change in the amount divided by the change in time so let's put this into practice then let's work out the mean rate between 0 seconds and 30 seconds for this particular graph here first thing that we would need to do is to have a look at the point at zero and then have a look at 30 seconds so that's just here so now that we've done this let's have a look at what the volume of gas is at each of those points and of course what the time is too we can use this to work out the change in the amount so the change in the amount is going to be 60 minus 0 and the change in time is 30 minus 0 now that we know this we can work out what the mean rate of reaction is we can say that the mean rate is going to be equal to 60 / 30 and that's simplifies down to give us 2 cm cubed per second make sure that you're always careful of the units by looking at what the axes on the graph are in we might also be asked to use the tangent to work out the rate of reaction at a specific point on a graph now to do this if we want it at a specific time say 30 seconds you would need to start off by drawing a tangent to the curve at that specific spefic time required we would then need to find the gradient of the tangent and the gradient would of course be that mean rate so the mean rate is the change in amount over the change in time it's important that you remember that the tangent is just a straight line that just touches the curve at the point you're being asked to find and of course we also need to remember how to calculate the gradient so once you draw that tangent you're going to draw a little triangle as big a triangle as you can and the gradient will be the change in the Y divided by the change in the X but let's put this into practice okay let's have a go at working out the rate of reaction at 30 seconds so in order to do this first off we're going to need to take that tangent see I've used a ruler and use a pencil as well in the exam to just touch the curve at 30 seconds now that I've done this I'm going to draw a little triangle out of this and although it's good to just find a nice easy triangle you need to also make sure that it's big the bigger the more accurate your answer so here I can see that my triangle at the bottom it hits at a volume of gas of 20 cm cubed and at a point that I've chosen at the top of it it's 88 cm cubed this means the change in the amount or that change in y is 88 minus 20 now if we look at the base of that triangle that we've made the change in time or the change in x is 50us 0 so now we can go straight in and put this into our calculator so we can see that the rate at 30 seconds is equal to 68 ided by 50 and that ends up giving us if we put that into the calculator 1.38 cm cubed per second again make sure that you get your units right by looking at the axis of the graph and there we have it that's how we do it different factors can affect the rate of a reaction and there are five main factors that we need to be able to talk about for this exam firstly we need to know about temperature then pressure concentration surface area and catalysts before we look at all those individual factors let's have a think about Collision Theory because we're going to need to describe how those factors affect the rate of reaction in terms of something called Collision Theory now Collision theory is what we use to explain how different factors affect the rate of a reaction Collision theory states that chemical reactions can only occur when reactant particles collide with each other with sufficient energy so there's two key facts there for a reaction action to take place we need a collision and that Collision needs to have sufficient energy now when we say sufficient energy we mean that it has at least the minimum amount of energy needed for a reaction to take place and the name for this amount of energy is the activation energy so let's get started with looking at how temperature affects the rate and we're going to explain this in terms of collision Theory well if we increase the temperature those particles will have more energy and therefore they're going to move more quickly as a result of them moving more quickly we're going to have more frequent collisions and because they're going to have more energy there's going to be a higher proportion of successful collisions now those are collisions which exceed the activation energy therefore because we have more frequent collisions and those collisions also have more energy the rate of the reaction will increase if we look at this graph here of amount of substance against time we can see that when we have a higher temperature the rate is faster we can see that because it's steeper and we make the maximum amount of products quicker than we do at a lower temperature and the graph shows this because it flattens out earlier for a higher temperature than it does for a lower temperature now let's have have a look at pressure if we increase the pressure of a system the particles will become more crowded the result of this if they're more crowded a bit like you're on a busier Street compared to a less busy street is there are more frequent collisions you're banging into everyone more and therefore the rate of reaction will increase again if we look at the graph of amount of substance against time we can see the exact same pattern at higher pressures we've got the steeper curve than the lower pressures and it also reaches that maximum amount of substance faster than at lower pressures and we can see that because it plate at an earlier time than at the lower pressure now let's look at concentration concentration and pressure are very similar except concentration is referring to Solutions whereas pressure is really referring to gases well as you can see in these diagrams if you increase the concentration the particles are more crowded and therefore there are more frequent collisions and the rate of reaction will again increase now this again you can see on our graph of amount of substance Against Time the exact same pattern again for higher concentrations you can see that we have a steeper curve therefore we've got a faster rate of reaction and it plate at an earlier time than for lower concentrations and this means that we've made our maximum amount of substance earlier than we do at lower concentrations now let's look at surface area increasing the surface area would mean taking something which maybe is a lump and grinding it down into a small powder a good analogy here for our everyday life would be if you had like a sugar cube versus some powdered sugar if we put the sugar cube in a drink it's going to dissolve much more slowly than our powdered sugar and the reason for that is that we have a bigger surface area when it's a powder versus a lump now the reason for this is if we increase the surface area like we have when we have this powder we increase the number of reactant particles exposed now as a result of that because there's more exposed there's going to be more frequent collisions again and therefore the rate of reaction will increase again here you can see with the graph when we have this greater surface area which actually means smaller particles we have this steeper curve whereas the smaller surface area which would be our lumps has a shallower curve now I also want to mention here and this is our little top tip instead of seeing it referred to a surface area we might see it referred to as the surface area to volume ratio now bigger particles such as our lumps are going to have a smaller surface area to Vol INE ratio than small particles such as powders so make sure you're comfortable with either language another final factor that affects the rate is a catalyst and whereas all these other factors were very similar in how we talked about them catalysts are going to be a little different so catalysts are substances that can speed up the rate of a reaction without actually getting used up they provide an alternative reaction pathway with a lower activation energy remember we said that the activation energy was the minimum amount of energy needed for the reaction to take place so if we lower this activation energy more particles will have this energy so this means that we're going to have more successful collisions and therefore the rate of reaction will increase when a catalyst is present now this graph here we need to be able to recognize and we to be able to sketch really here we've got energy against progress of reaction and you can see here this particular reaction is an exothermic reaction however you could see this for EXO or endothermic reactions the activation energy is the bump that goes up from the reactants reaching a peak before it goes down to the products and that activation energy is the difference between the reactants and that Peak you can see here that the activation energy with the catalyst is lower than the activation energy without a catalyst just like we said we know that Catalyst provide an alternative reaction pathway with a lower activation energy so make sure that you can draw this graph or label one that they give you enzymes are examples of biological catalysts enzymes are really important in Industry as they can be used in some industrial reactions and the example that we learn about in gcsc chemistry is the fermentation of sugars to make alcoholic drinks now the importance of these enzymes is that they allow these industrial reactions to take place at much lower temperatures and pressures than with otherwise be needed now this makes them quicker and much more cost effective than it would have been without them now we need to know specifically that we can find enzymes in the single celled fungus yeast now this is the source of the enzymes in the process of fermentation we need to be familiar with the fact that the following chemical reactions get accompanied by a change in heat energy firstly we have salts dissolving in water then we have displacement reactions then neutralization reactions and precipitation reactions now when these reactions take place in a solution we can use a thermometer to observe these temp temperature changes we'll often see lots of questions at gcsc about experiments like this now when we're talking about energy transfers we've got two different types of chemical reactions there's exothermic reaction and endothermic reactions let's start off by having a look in a bit more detail at exothermic reactions so we need to know what is an exothermic reaction this is a reaction which transfers energy to the surroundings and we see the temperature of the surroundings increase if we break down that word EXO thermic thermic is referring to heat energy just like the word thermometer and EXO is referring to energy exiting and that's one of my top tips as you can see here so we've got some examples of exothermic reactions that we should be familiar with combustion is an example oxid ation reactions and also neutralization as well as just having everyday uses such as hand warmers that we might use in our coats in the winter and also self-heating cans which would be great if we're going out camping and we just want a way to get hot meal in us so for exothermic reactions make sure you know those examples and make sure you understand that definition of an exothermic reaction now let's have a look at endothermic reactions we need to know that endothermic reactions are reactions which take in energy from the surroundings and so the temperature of the surroundings decreases again if we deconstruct that word we've got thermic which is referring to that heat energy and the Endo is referring to the energy entering again little top tip below here to help you remember just like with exothermic we need to know a couple of examples of endothermic reactions so these are a little bit more Technical and less everyday we've got thermal decomposition that's one example and another reaction is the reaction between sodium hydrogen carbonate and citric acid so some everyday examples of an endothermic reaction include some sports injury packs so we can use these if we have an injury and we want to cool down the area we can use one these Sports Injury packs and it would cool down it's important that we know how we can investigate the variables that affect temperature change and this is one of your required practicals in the course as we should know there's several different chemical reactions that can either be exothermic reactions or endothermic reactions I'm not going to go through all of these I'm just going to have a look at the neutralization reaction that takes place between hydrochic acid and sodium hydroxide so if we want to investigate this let's start off by measuring 30 cm cubes of hydrochloric acid and transferring it to a polystyrene cup now that measurement could change dependent on what you're given in the exam however the importance of the polystyrene cup is the fact that poyene is an insulator it's going to prevent heat loss so let's stand this cup in a beaker to stabilize it so that it doesn't fall off and let's use a thermometer to measure the initial temperature of the acid now this is really important if we need to get the temperature change if it's increased or decreased we need to know where it started so let's record that temperature and now let's measure a certain amount of sodium hydroxide here I suggest 5 cm cubed and let's use a measuring cylinder to do this now let's add this to the polist green cup let's place a lid on the cup with the thermometer still in the solution now the reason why we've placed this lid on the cup is going to be to insulate it because when heat energy is given off to the surroundings it could Escape into the air leave the system and then the thermometer won't measure that change so now let's stir using the thermometer this is going to ensure that the heat is evenly distributed through this solution now let's wait and let's wait until we reach a maximum temperature so keep taking measurements at regular intervals and you'll notice eventually we reach a maximum and note that down now let's repeat this experiment and find the mean maximum temperature reached once we've done that let's finish off by repeating for different volumes of sodium hydroxide for instance going up in 5 cm cubed interval remember the whole point of this is we're investigating the variables that affect temperature change so in this case we're going to investigate the effect that different volumes of sodium hydroxide is going to have on that temperature change we need to understand that when we carry out an investigation like this we need to know the independent variable the dependent variable and the control variable I like to remember that the independent variable is what I change so it begins with an i it's what I change and we have changed the volume of sodium hydroxide so that's our independent variable now the dependent variable depends on what you did and it's the thing that you measure in this case we know that we're measuring the maximum temperature of the solution so that is going to be our dependent variable for this particular experiment the control variable is what we keep the same each time and in this case we've kept the volume of the hydrochloric acid the same so that is our control variable now these questions we always need to be able to identify what sources of error there were and in this case the biggest source of error is going to be due to unwanted heat transfer and this is why we did certain things in this experiment like placed it in a polyyne cup which is an inator and placed a plastic lid on the top again an insulator to prevent and minimize those heat losses to the surroundings finally we generally want to have a think about what do we learn from the Practical and what we would have observed is that up to a point as we increased the volume of sodium hydroxide the temperature change would have increased and we should understand that the bigger the temperature change the more energy has been absorbed or released depending on what your investigation was about there we have it okay that is your investigation into temperature changes that you need to know about we need to make sure that we're really familiar with reaction profiles what they look like and what they represent so reaction profiles can be used to show the relative energies of reactants and products the overall energy change of a reaction and the activation energy if we look at this exothermic reaction we can see that the reactants have more energy than the products and energy is released as we go from our reactant to our products our activation energy is represented by the difference in energy between the reactants and the peak of this curve now endothermic reactions on the other hand we know take in energy and so here we can see that the reactants have less energy than the products and that's because energy gets absorbed to give us the energy of the products the activation energy again is represented from the difference in energy between the reactants and the peak of this curve what is activation energy we need to know that activation energy is the minimum amount of energy that particles must have to react during chemical reactions energy gets taken in to break the bonds in the reactants and Energy's released to form the bonds in the products therefore we can work out the overall energy change of reaction to do that we just need to find the sum of the energy needed to break all the bonds in the reactants and subtract the sum of the energy released when forming all of the bonds in the products as a result of this we can say that during exothermic reactions the energy released when forming new bonds is greater than the energy needed to break existing bonds and this of course explains why energy released to the surroundings during an exothermic reaction action well at the same time we can say for an endothermic reaction The energy needed to break the existing bonds is greater than the energy released when forming new bonds and that's why during endothermic reactions they take in energy from the surroundings now we might find that we need to do some calculations here let's look at this example so methane combusts in oxygen to form carbon dioxide and water the reaction is shown below calculate the energy change for the reaction so notice here that all of the bonds are shown If instead you were just given a balanced equation you would need to write this out just like I have here and if there's any balancing numbers like a big two in front of the oxygen and make sure you write those down twice just like I have here notice also on the right hand side we've got this little table which has the bond energies of all of the different bonds that we're going to see in this question well of course we know that to calculate the overall energy change of a reaction we need to work out the sum of the energy needed to break the bonds in the reactants and then we need to take away the sum of the energy released when forming Bonds in the products so let's have a look at how we calculate this let's start by just looking at the reactants so starting off here I can see that in the methane I have four CH bonds so looking at the table I can see that each bond is 412 K per mole so I'm going to do 4 * 412 now I can see I have two o double bond O's and so again looking at the table I want to do 2 * 498 now let's put that into the calculator to simplify so that ends up equaling 2,644 K per mole so now let us have a look at the products so here I can see I have two c double bond O's in the carbon dioxide so from our table here I can see I need to do 2 * 743 now onto the waters I can see that I have 1 2 3 4 o bonds so here I'm doing 4 * 463 now if I pop that into the calculator now I end up getting the energy released when forming Bonds in the products and that is 3,338 K per mole so let's have a look here then I know that to get the overall energy change of the reaction I need to take the energy from those reactants and take away the products so breaking minus making is how I always think about it so 2,600 44us 3338 gives me - 694 K per mole and that is how we would do one of these questions in the exam so let's summarize here what we know about breaking bonds and making bonds so when breaking bonds we know that heat energy is taken in we know that the temperature of the surroundings will decrease and we know that this is an endothermic reaction forming Bonds on the other hand we know that heat energy is given out we know that there's going to be an increase in the temperature of the surroundings and we know that this is an exothermic reaction [Music] [Music] [Music] [Music] food oil is a finite resource which is found in rocks and is composed of the remains of an ancient biomass that was buried in the mud kudor ends up being a mixture of a large number of different compounds most of which which are hydrocarbons a hydrocarbon this is a really important definition we need to know is a molecule made up of carbon and hydrogen only make sure you remember that word only because that's critical to get in your marks so two examples of hydrocarbons we need to learn about are alkanes and alkenes a word we're going to see described a lot throughout this topic is homologous series so we're going to learn about alkanes alkenes alcohols and carboxilic acids and all of these are examples of homologous series now homologous series all have the following properties in common they all have the same general formula the molecular formula of successive compounds differs by ch2 they have similar chemical properties and they have a gradual change in their physical properties crude oil is a mixture of different hydrocarbons or different lengths we can separate crude oil into fractions each of these contain molecules with a similar number of carbon atoms and we can do this using a process called fractional distillation now this process is really important because the different fractions can be processed to produce fuels and feed stock for the petrochemical industry so this fractural distillation of crude oil produces many fuels that get used in Modern Life for example petrol and diesel oil and these fractions can also be used to produce materials that are useful for Modern Life such as solvents polymers to make Plastics detergents and lubricants now let us look at this really important process of fractional distillation now the crude door is going to pass into a furnace when it's in the furnace of course it gets heated up and it's going to evaporate it's vaporizing by the furnace now this vapor is going to pass on into this fractionating column now in this fractionating column because it's being heated from below the bottom is hotter than the top now this vapor is going to rise up the column and because it's cooler at the top the vapor is going to cool down as it rises this separates out these different fractions the boiling point of the substance is at the top is lower than at the bottom so when the boiling point of the different hydrocarbon compounds get reached as it travels up this fractionating column the hydrocarbons will condense once this happens these different fractions are going to be collected as you can see from the diagram the liquid petroleum gases will be collected at the top because they're going to have the lowest boiling point so they get collected up the top whereas things like the bitman and the heavy fuel oil will be collected towards the bottom of this column we need to make sure that we know the order of the fractions and what they're used for so we need to know at the top of the fractionating column we're making gases then petrol kerosene diesel oil fuel oil and then bin the gases are used for domestic Heating and cooking a lot of homes have gas cured ovens petrol is used as a fueling c as I'm sure you're amiliar with kerosene is a fuel in aircraft diesel oil is a fuel for some cars and also for some trains fuel oil is used for the fueling large ships and also the fuel in some power stations and finally bitchman this is used for Road surfaces and also for roofs it's important that we know the properties of hydrocarbons with relation to the size of the molecule so we need to know that as the molecules get larger the boiling points increase the viscosity also increases but the flammability decreases this applies to both the alkanes and the alkenes so we should be really familiar with these properties the combustion of fuels is a major source of the pollutants within the atmosphere we need to know that fuels may contain carbon hydrogen and also sulfur now when we combust these fuels we can release the following gases into the atmosphere so we might release carbon dioxide water vapor carbon monoxide sulfur dioxide oxides of nitrogen and particulates let's have a look in a bit more detail at the process of complete combustion now complete combustion occurs when a hydrocarbon fuel is combusted in an excess of oxygen and the general equation for complete combustion is fuel plus oxygen makes carbon dioxide and water now an example of this is methane fuel plus oxygen makes carbon dioxide and water we need to make sure that we can write the balanced symbol equation for these equations too so make sure you're familiar with the formula of the alkanes make sure you know the formula of oxygen carbon dioxide and water and you're happy balancing these equations now the great thing about complete combustion is that we release the maximum amount of energy downsides though are that we release carbon dioxide which is a greenhouse gas and water now however let's have a look at incomplete combustion now incomplete combustion occurs when a hydrocarbon fuel is combusted this time in a limited supply of oxygen now this releases much less energy then complete combustion and instead of releasing carbon dioxide you're going to find that carbon monoxide and or partic carbon are going to be produced now whether you produce carbon monoxide and carbon or just carbon monoxide or just carbon or some other combination is going to be dependent on how much oxygen we have available so let's have a look here at the incomplete combustion of ethane in oxygen so here we've got ethane plus oxygen and here we're saying we'll make carbon monoxide and water now again we need to be able to balance this so make sure you know that the formula for carbon monoxide is co and the formula for particular carbon is just C so we need to know that carbon monoxide is a poisonous gas and what makes it so dangerous is it combined to he hemoglobin in the blood and prevent oxygen from binding now this will lead to death now the problem with carbon monoxide which makes it particularly dangerous is that it is a colorless and odorless gas which means it's an invisible silent killer so I'm using this as my opportunity to remind you that if you have a little fire or campfire make sure it is well ventilated don't ever put it in a tent or something like that where it would have po ventilation because there can be tragic consequences so next up we've got partic carbon now that is the carbon or the S that can get released during incomplete combustion this can be seen as a black smoke coming off and the problem with this is it can worsen asthma and it can also cause global dimming so make sure you know the dangers of carbon monoxide and carbon and the fact that we make less energy also during incomplete combustion we need to know where each of these atmospheric pollutants are made so carbon dioxide is made during the complete combustion of a carbon containing fuel carbon monoxide is made during the incomplete combustion of a carbon containing fuel sulfur dioxide is Major on the combustion of a fossil fuel that contains sulfate impurities oxides of nitrogen such as nitrogen monoxide and nitrogen dioxide are major on the oxidation of nitrogen in the air inside a vehicle's engine and carbon particulates or soot are major in the incomplete combustion of a carbon containing fuel some fossil fuels contain sulfur impurities now when we comust these fuels we're going to find that that sulfur is also going to get oxidized so we're going to have the reaction take place that says that sulfur plus oxygen makes sulfur dioxide now this sulfur dioxide is going to get released into the atmosphere and what we'll find is that it's going to get further oxidized and it will end up making sulfur trioxide and this is because the sulfur dioxide plus oxygen makes sulfur trioxide now it's this sulfur trioxide that can dissolve in the rain water and when that happens we make dilute sulfuric acid which forms acid rain now acid rain is particularly harmful to aquatic plants and animals because that acidic rain will go into these bodies of water such as lakes and change the pH making it more acidic now this upsets that perfect balance of that ecosystem and it can lead to plants and animals dying another issue with acid rain is it can damage buildings and statues if the buildings and statues are made from rocks such as Limestone which is a carbonate calcium carbonate what will happen is that will react with the acid turning to salt water and carbon dioxide this erosion can lead to damage to these statues and buildings we should know that the atmosphere is made up of Approximately 80% nitrogen and 20% oxygen car engines where combustion is taking place can reach very high temperatures and pressures now this can lead to the nitrogen and the oxygen that are present in the air reacting together to form oxides of nitrogen now in car engines we'll find that the most common oxides of nitrogen that are made are nitrogen monoxide and nitrogen dioxide it is common that you'll see these oxides of nitrogen referred using the general formula nox now these oxides of nitrogen can be dangerous they can cause photochemical smog and breathing difficulties in humans such as asthma so it's important that we minimize the presence of these oxides of nitrogen in the atmosphere this table here shows a summary of the problem with the different atmospheric pollutants carbon monoxide is a toxic or poisonous gas and it is colorless and odorless which makes it hard to detect sulfur dioxide causes acid rain oxides of nitrogen cause respiratory problems in humans like aggravating their asthma and it also causes photochemical smog whereas particulates which is that carbon particulates or S they cause health problems in humans and also Global Demming we need to make sure that we know the advantages and disadvantages of using hydrogen rather than petrol as a fueling caruse so this table summarizes everything that we're going to need to know so hydrogen is not a non-renewable fossil fuel like petrol is straight away that's great because we're not using out finite resources however if we think about the disadvantages we need to make that hydrogen in the first place and often it is made using nonrenewable fossil fuels for instance it comes from methane which comes from fossil fuels likewise another method that we could use to get the hydrogen is the process of electrolysis of water now the downside with this is that is also going to use a large amount of energy and we have to ask ourselves where are we getting that energy from although the water is not coming from fossil fuels maybe the energy that we're using to electrolyze it and make that hydrogen is actually still coming from fossil fuels so we have all the same consequences now let's look at another Advantage hydrogen oxygen fuel cells in cars only produce water as a byproduct however the combustion of petrol is going to produce carbon dioxide carbon in the form of soot and also the toxic carbon monoxide so that's a great advantage of the hydro fuel cells they are only making water as a byproduct however the final disadvantage to talk about of these hydrogen for your cells is that there's fewer hydrogen filling stations compared to petrol that means if you own a hydrogen fueled car how are you going to fill it do you know where your nearest hydrogen filling station is probably not whereas we already have the infrastructure for petrol fueled cars we know where petrol stations are they're very very regular on the roads and so that makes petrol easier than hydrogen during fractional distillation the mixture of hydrocarbons in the crude oil are separated but some of these hydrocarbons end up being much more useful than others the shorter chain hydrocarbons are often used as fuel and as a result there's really a great demand for these shorter chain hydrocarbons when we compare them to the longer chain hydrocarbons this means that we end up having too much of the longer chain hydrocarbons and not enough of the shorter chain ones to meet demand we can solve this problem using cracking cracking is a process which we use to break up longer hydrocarbons into smaller more useful hydrocarbons so here we've got a little general formula here and this is very very loose if because we can see it in lots of different formats all that matters is that we have a longer hydrocarbon which gets broken down to a shorter alkane and an alken and what you're going to see here is this also needs to balance so long as these rules are applied and it balances then it is a valid equation for cracking now we need to know that the longer hydrocarbon has lower demand and is less useful and the whole point of why we're cracking it is because we can make a shorter alkane which can be used as a high demand Fuel and an Aline which we can use to make polymers and we can also use it as a starting material to make other chemicals so with cracking we can use one of two methods we can use catalytic cracking using this method we vaporize the hydrocarbons and then the vapors get passed over a hot Catalyst we can also use steam crack C in when we do steam cracking the hydrocarbons get mixed with steam at high temperatures for the past 200 million years the composition of the atmosphere of the Earth has been roughly the same we have about 80% nitrogen about 20% oxygen and small proportions of other gases including carbon dioxide water vapor and the noble gases as well we need to know about how the Earth Earth's atmosphere has evolved so we need to know what the Earth's early atmosphere was like in the first billion years of the earth so we need to know that initially we had a lot of volcanic activity and as these volcanoes erupted they released gases which ended up forming this early atmosphere nitrogen was released by these volcano and it ended up building up in the atmosphere water Vapor was released by these volcanoes as well but it ended up condensing and formed the oceans the early atmosphere of the Earth was mostly made up of carbon dioxide and there was little to no oxygen present we think that back in those first billion years as well as all these other gases there were also small amounts of methane and ammonia present in the early atmosphere now as the oceans formed when that water vapor condensed the carbon dioxide that was present in the atmosphere ended up dissolving in the water and carbonates formed producing sediments under the ocean now this reduced the amount of carbon dioxide in the atmosphere bringing it down and we're now on the way to forming our modern atmosphere over time the carbon dioxide levels came down and the oxygen levels increased now we need to understand why the oxygen levels increased we should understand that plants and algae began to evolve and as they evolved and grew they produced oxygen by photosynthesis now the algae first starting producing oxygen as far back as 2.7 billion years ago so a really really long time ago the algae were their first and over time plants evolved and as the plants started to grow even more oxygen was being put out into the atmosphere because even more photosynthesis could happen now this continued until eventually the oxygen levels had got so high that we could support the evolution of animals which have course need a lot of oxygen to respire to grow and to move So eventually it ended up coming to this 20% of oxygen in the atmosphere which we currently have now now that process of photosynthesis of the algae and the plants also have the knock on effect of decreasing the proportion of carbon dioxide in the atmosphere so this is one of the main ways in which carbon dioxide levels went down over time other ways that the proportion of carbon dioxide decreased was due to the formation of sedimentary rocks such as Limestone and also the formation of fossil fuels such as coal crude oil and natural gas to test for oxygen gas we want to hold a glowing splint inside a test tube containing a gas you want to test now by glowing splint we mean that it was lit but then we blew it out and it's still glowing now if oxygen is present then that glowing splint will relight greenhouse gases are gases that are actually really important to have in our atmosphere without the presence of greenhouse gases the Earth would actually be too cold to support life however the problem arises when there's too many greenhouse gases present when we have too many greenhouse gases in the atmosphere and they're too abundant what will happen is the temperature of the earth will be increased too much now we need to know the names of three greenhouse gases we need to know carbon dioxide methane and water vapor we need to be able to explain how the greenhouse effect works and this is exactly the same reason why in a greenhouse it gets hot on a sunny day or if you're in a car and all the windows are up again it gets really hot in there on a hot day so let's understand why this greenhouse effect happens so we need to know that electromagnetic radiation from the Sun radiates out and passes through the Earth's atmosphere now this electromagnetic radiation that's coming from the Sun includes a range of wavelength including some shorter wavelength and some longer wavelength now the Earth is going to absorb most of this radiation that comes through the atmosphere and the Earth is going to get warmed up now this warmer Earth is going to radiate a longer wavelength infrared radiation now some of this radiation can go back into space that some of this infrared radiation will get absorbed by the greenhouse gases in the atmosphere now it's this process here that increases the temperature of the atmosphere human activities have a direct effect on the amount of some of the greenhouse gases that get released into the atmosphere so if we first think about carbon dioxide the amount of carbon dioxide in the atmosphere can get increased by several human activities these include the burning of fossil fuels in vehicles and factories and deforestation now let's think about the amount of methane in the atmosphere this is increased by cattle farming and Patty fields or rice farming so we need to be able to answer the question are humans causing climate change most scientists believe that the human activities causing the surface temperature of the earth to increase now the evidence that they've used to form these beliefs have come from peer reviewed evidence however climate change is very complex so scientists can't be certain of this despite all these peer reviews we need to consider the fact that maybe we're oversimplifying some of the models that we're using to come to these conclusions we need to consider that people's opinions and speculations may cause biases and also so we might have certain biases from certain companies that might commission the research we need to always think about where this information has come from who has paid the money for this research and if they actually have an ulterior motive to come to a certain conclusion we also need to think about the quality of the evidence and also some uncertainties that might arise from measurements that have been taken and other factors as well we need to know what is meant by climate now climate refers to the average temperature and weather Cycles over a long period of time climate is different to weather which is talking about the temperature and the weather now so the climate of Earth's always actually been changing this isn't a new thing the climate of Earth has cycled between periods of extreme cold which are the ice ages that have happened in the past had also warm periods as well now these used to just be naturally occurring phenomenon they could have been caused by things such as volcanic eruptions and changes that are occurring on the sun however when we talk about global warming this is different to climate change when we refer to global warming we're referring to the warming of the climate that has occurred due to human activities over the last 200 years so we're just looking at recent times where we're talking about global warming this graph here is a very important one and it shows the relationship between the change in carbon dioxide concentration over the last few hundred years and the temperature here you can see that as the carbon dioxide concentration has increased the general trend is that the temperature has also increased we can see here that there's a strong correlation between the Earth's average temperature and the concentration of carbon dioxide in the atmosphere an increase in the average global temperature is a major cause of climate change we need to know some of the effects of global warming so these include glaciers melting droughts sea levels rising and also the loss of habitats [Music] [Music] [Music] sometimes we might need to identify some metal ions in a solid substance to do this we can use a flame test so the method for this is we want to dip a clean wire Loop into a solid sample that we're trying to test and then once we've done that we're going to pick up the loop and we're going to put it in a blue flame on a buns and burner now what we're going to do is observe the flame color because what we're going to expect to see here is the flame will change color if certain ions are present now these are the results that we need to know so it's only these five different metal ions that you need to know their flame colors so lithium the li+ ion will give you a Crimson flame sodium na+ gives us a yellow flame potassium k+ gives us a lilac flame calcium or ca2+ it gives us an orange red flame and finally copper which is cu2+ gives us a green flame now occasionally a problem will pop up when trying to identify metal ions so there's a good chance that some samples might have a mixture of different ions within them and as you know with paint and so on if you mix together different colors then you can't see the original colors anymore and so this can mask certain flame colors imagine if we had like an orange red calcium ion as well as a crimson lithium ion the orange red and the Crimson they're going to hide each other and we won't necessarily be able to identify them both so we need to know that that's one of the main problems that can arise during a flame test when we have metalions in a solution instead we can't use a flame test instead we're going to use this sodium hydroxide test so to test for methyl ions and solutions we add sodium hydroxide to our solution that we testing and then if a certain methal is present we will see a precipitate form and the color of that precipitate will enable us to identify it now here there are six different metal ions in solution that you need to be able to identify here so as you can see here we've got aluminium and when that reacts with sodium hydroxide we form aluminium hydroxide which is a white precipitate now here you can see this ionic equation and all of these reactions have ionic equations I don't want you to freak out that there's so many different ones instead I want to quickly show you the pattern that we see here all of them involve that metal Ion with its relevant charge and then you plus on as many hydroxide ions so oh minus ions as you had that charge so aluminium is aluminium 3+ and then we add three o minus and then you form Al and in Brackets o and again with that three notice all of the rest of these is exactly the same so actually you don't need to learn six different reactions here just learn the pattern again here we've got calcium 2 plus plus 2 oh minus makes calcium O2 so just spot that pattern and also notice that the state symbols tell us we have an aquous plus an aquous makes that solid for the precipitate calcium also reacts with sodium hydroxide to form a white precipitate this time of calcium hydroxide magnesium also reacts with sodium hydroxide to form a white precipitate this time of magnesium hydroxide now copper 2 and that Roman numerals tells us the charge of the copper ion is going to react with sodium hydroxide to form a blue copper hydroxide solution ion 2+ reacts with sodium hydroxide to form the green Ion 2 hydroxide solution and ion 3 will react with sodium hydroxide to form the brown ion 3 hydroxide now it's very important that you go away and you learn these different colors that are formed as well as these ionic equations now you would have noticed that aluminium calcium and magnesium all react with sodium hydroxide to form a white precipitate now that's going to cause issues when we're trying to do these tests to identify What ions we have present how will we know if it's aluminium calcium or magnesium well there's one key additional fact here that's going to help us if we add more sodium hydroxide so that we have an excess of sodium h hydroxide only the aluminium hydroxide precipitate will dissolve again so that's one way that we could positively identify aluminium hydroxide if we add a little sodium hydroxide we get a white precipitate form and then if we add more we'll notice that it will disappear and dissolve back into the solution whereas calcium hydroxide and magnesium hydroxide won't ammonium ions will also re with hydroxide ions but the test is a little bit different to test for ammonium ions we're going to first add it to dilute sodium hydroxide then we're going to warm up the mixture and then as you can see in this ionic equation the ammonium ions will react with hydroxide ions to make ammonia and water now it's that ammonia that makes this important so now we can test for the ammonia remember that the the test for ammonia is if we take some damp red litus paper and dip it into the gas it will turn blue and that tells us that originally ammonium ions were present to test for ammonia gas we put some damp red litmus paper into the gas and if ammonia is present The Damp red litmus paper will turn blue as you can see in this diagram to test for carbonate ions we want to react the carbonates with a dilute acid now we should know that when carbonates react with acid we form carbon dioxide gas now this gas we can bubble through lime water and if the carbonate was present and carbon dioxide was formed the lime water will turn milky or cloudy in color and this confirms the presence of carbon dioxide and therefore a carbonate in that original solution we need to know how to test the halide ions and in this exam we only need to know about the C minus the BR minus and the I minus ions to test for these ions we have to First add nitric acid and then add silver nitrate to our solution now if those halide ions are present we're going to get a different precipitate formed dependent on the halide if chloride ions were present they would react with silver nitrate to form silver chloride this is a white precipitate the ionic equation for this is ag+ aquous which is silver ions plus CL minus those chloride ions which are aquous and they go on to form agcl solid which is the precipitate now if bromide ions were present they would react to form silver bromide which is a cream precipitate the ionic equation is almost exactly the same we just replace the CL minus with BR minus and agcl becomes agbr and then for the iodide ions iodide will react with silver nitrate to form silver iodide which is a yellow precipitate and the ionic equation here is just ag+ aquous plus IUS aquous makes AGI solid but again with those ionic equations don't think that they're three separate equations that makes things so complicated just notice how similar they are and notice the pattern that you can see forming there and just learn that and in addition to that when looking at these observations notice as you go down the group from chloride to bromide to iodide the color is getting kind of grimier it's going from white to creen to Yellow so just remember that Trend in the colors as well well as it will help you get them in the right order to test for sulfate ions in solution we want to add a barium chloride solution and dilute hydrochloric acid now if the sulfate ions are present then a white precipitate will form the ionic equation for this is ba2 plus aquous plus those sulfate ions which are s so42 minus aquous will form barium Sul at baso4 solid now it's that barium sulfate that gives us that white precipitate chemical tests can be really really helpful however elements and compounds can also be detected and identified using instrumental methods and there's a lot of advantages of using instrumental methods compared with these chemical tests one is that they're very accurate imagine the test for Hali d you've got a white precipitate or a cream precipitate or a yellow precipitate it could be very reasonable to assume you could misjudge those colors however an instrumental method wouldn't do that instrumental methods are also very sensitive they can detect even very very small amounts which perhaps our chemical tests couldn't pick up on they also very rapid we just pop them in the machine and they can immediately identify what we have and those are three advantages that we need to know flame photometers can be used for flame testing as well the data that they get can be used to identify metal ions that are present but they can also work out the concentration of metal ions that are in those Solutions so not only does it tell us that they're present like the standard flame test colors we can also get those concentrations so what photometers do their machines which are going to split the colored light from a vaporized sample and produce something called an emission spectrum well this diagram here is an example of an emission spectrum you can see that we have this black band in the background and then we have a series of different colored bands or in exam they might just show up as white bands but there is this distinct pattern and every different metal ion is going to have its own unique spectrum they will all be different and that's how we can identify what we have so to determine concentrations of metal ions using a flame photometer what we want to do is take readings at different known concentrations of the metal so let's say we take it at 0 moles per decim cubed 0.1 0.2 0.3 and so on and you can see that we've got all those distinct points here where we have done that we've taking a different concentration and then we've got our reading we've got a different concentration and then we've got our reading for the flame photometer now we use all of these different results to plot this curve and we call this a calibration curve and that's really important because now we could take a reading of an unknown sample we don't know what's in it and we can get the flame photometer reading let's imagine this is our reading here we read it along and then we go along until we meet that line and when we meet the line we will then use a ruler of course to then get a concentration of metal ions and that's exactly what we've done here so with thanks to that calibration curve we can find the concentration of metal ions of an unknown sample most of the hydrocarbons found within crude oil are alkanes the homologous series of alkanes has the general formula CN h2n + 2 so make sure you remember that we need to know the names of the first four members of the alkane homologous Series so these are methane this one has one carbon then we have ethane which has two carbons then we have propane which has three carbons and finally butane which has four carbons we only need to know these first four and notice that for the alkanes all of the names end in a and the beginning is the same too and this carries for all of the different homologous series that we will see so we've got a little top tip of how to remember these if you remember the phrase monkeys eat peanut butter then you will remember the correct order of the meath eth prob and but and therefore you'll be able to write down the correct name for the number of carbons that you need so now let's look at this sum so here we've got methane which has the formula CH4 ethane has the formula c2h6 all of these formulas are obeying that rule of cnh h2n +2 propane is C3 h8 and butane is C4 h10 alkenes are hydrocarbons that have a double carbon carbon Bond within them the homologous series of alkenes have the general formula of CN h2n now alkenes are saturated molecules this means that they have a carbon carbon double bond which in turn means that they have two fewer hydrogen atoms than an alkane would have if it had the same number of carbon atoms now this table here summarizes the names of the first four alkenes and the formulas and the structures of these we can see that eth is actually the first of these alkenes not methan and that's because we need to have two carbons to have this CC double bond so we have Ethan which has two carbons so it's formula is c2h4 propine has three carbons so the formula is c3h6 see here that we're obeying that general formula of cnh2n buttine has four carbons so has the formula c4h8 and finally penene is C5 h10 now you can see the structures drawn here remember that every carbon can only have four bonds attached and every hydrogen just one bond this will help you when drawing these structures during fractional distillation the mixture of hydrocarbons in the crude oil are separated but some of these hydrocarbons end up being much more useful than others the shorter chain hydrocarbons are often used as fuel and as a result there's really a great demand for these shorter chain hydrocarbons when we compare them to the longer chain hydrocarbons this means that we end up having too much of the longer chain hydrocarbons and not enough of the shorter chain ones to meet demand we can solve this problem using cracking cracking is a process which we used to break up longer hydrocarbons into smaller more useful hydrocarbons so here we've got a little general formula here and this is very very loose because we can see it in lots of different formats all that matters is that we have a longer hydrocarbon which gets broken down to a shorter alkane and an alken and what you're going to see here is this also needs to balance so long as these rules are applied and it balances then it is a valid equation for cracking now we need to know that the longer hydrocarbon has lower demand and is less useful and the whole point of why we're cracking it is because we can make a shorter alkane which can be used as a high demand Fuel and an alkine which we can use to make polymers and we can also use it as a starting material to make other chemicals so with cracking we can use one of two methods we can use catalytic cracking using this method we vaporize the hydrocarbons and then the vapors get passed over a hot Catalyst we can also use steam cracking when we do steam cracking the hydrocarbons get mixed with steam at high temperatures now alkenes are more reactive than alkanes are this means that alkenes can react with bromine water whereas alkanes cannot this is our test for alkenes if we add the alkenes to bromine water we will observe a color change bromine water is normally orange but when it's mixed together with an alken the alken will decolorize the bromine water so the bromine water will now become colorless now here's a little top tip which is really important never describe this decolorized bromine water as clear it is not clear it is colorless you can have a color and still be clear so we never want to use the word clear to describe this we either want to describe it as colorless or decolorized combustions of hydrocarbons release energy during this combustion process hydrocarbons become oxidized we need to know that the general formula for the complete combustion of a hydrocarbon is hydrocarbon plus oxygen makes carbon dioxide plus water let's have a look at an example of this we could be asked to show the combustion of methane which is CH4 in this case we'd write CH4 plus oxygen makes CO2 plus H2O now though we need to balance it we can see that there's four hydrogen's in the reactants but only two in the products so in order to balance these let's double those H2O now that we've done this we can see that we have two oxygens in the CO2 and two oxygen in the water of course the carbons are already balanced so those are fine in total this means that we have four oxygens we can balance this now by adding a two in front of the O2 because 2 o2s gives us four oxygen atoms and so that is our balanced equation for the complete combustion of methane apply that process to whatever hydrocarbon you're given alkenes can be used to make polymers using addition polymerization so a couple of examples of addition polymers are polyethene and polypropene as you can see we just stick poly in front of the name of that alkine and in addition polymerization reaction many small molecules will join together to form a very large molecule that polymer here we can see that reaction taking place so we have Ethan here and it will become the polymer polyethene notice here in this diagram we can see that our double bond is broken open and it is now a single bond with two Open Arms now those open arms would be connected to other repeat units okay so it's to show that this just repeats and repeats and repeats and repeats and this is why we call this a repeat unit now that little n down here represents a large number of molecules so if we had 100 ethenes then that would be a little 100 to show that we have 100 of these repeat units in a row now The small molecules are called monomers and the large molecules that they form are called polymers now we'll notice as well that this repeat unit of an addition polymer has the exact same atoms as the monomer that formed it and that's because that polymer was the only product in these reactions this table here shows some examples of addition polymers here we've got the monomer Ethan if we wanted to show the formation of polyethene all we would need to do is open up that double bond open up its arms put the hes above and below those carbons and then do your brackets around it let's do it for propine now propine has three carbons and just the one double bond so here I am drawing it in this particular way and when we have propene forming polypropene again we just open up that double bond open up the arms and put everything else above and below those carbon and then throw in the brackets as well I hope you can notice a pattern here so let's try one that you've not seen before this is chloroethene so here it has and you're not expected to know this it has two hes on the first carbon and an h and a CL on the other but just notice here we can apply the same principles double bond that's in the middle we just open up the arms and we put two h's above and an h and a CL below those carbons and again throw in the brackets at the end we need to know about the properties and uses of some different polymers so polyethene or polythene as it's sometimes called is cheap and flexible so we often use it for Carrier bags and cling film polypropene is instead strong and flexible and it resists shattering that means it's great to use as plastic bowls or plastic buckets next up we've got polychloro Ethan now this is sometimes called PVC now this material is actually really handy because it can be made hard or flexible depending on what we need it for it's also very tough and it's an electrical insulator for these reasons we can use it for insulation for electrical cables and also for guttering and pipes finally here we've got poly Tetra floro Ethan or PTFE now this is unreactive and it's slippery this makes makes it really great for non-stick pans which you may have in your kitchen condensation polymers are formed using monomers with two different functional groups now these functional groups will end up reacting together and losing a small molecule such as water now this is why we call these reactions condensation reactions so two typical types of molecules that would react together to form condensation polymers include dial and dicarboxylic acid now all that a diol is is a molecule that has two alcohol groups and a dicarboxylic acid is just a molecule that has two carboxylic acid groups as you can see in these diaphragms now we will sometimes see the middle parts of these molecules represented with a simple shape such as a rectangle and the whole point of this is it just simplifies things rather than you at GCSE having to draw these complicated molecules you can just cut out the middle bit and use a shape now dials and dicarboxylic acids can react together to form polyesters and what will happen is the O group of the Dio will react with the O part of the carboxilic acid group and we will form water see what's happening here we are going to remove the water and this in turn will will form this Bond here that is our Esther Bond and here you can see we've removed one molecule of water now this is just one reaction we don't yet have a polymer but this reaction would go on and on and on to form a polymer with lots more alternate dials than carboxylic acid Dio carboxylic acid repeating and this is what our general equation would look like here we have our di if we had n of those we can react it with n of the dicarboxylic acid we would make our polyester and we would make 2 N H2O so that would be our general reaction and that is how we make condensation polymers we need to know some of the problems associated with polymers these include the availability of the starting materials it also includes the fact that they're non-biodegradable or at least most of them are for this reason they're going to last for years in landfill sites taking up valuable space next we have the fact that if they get combusted they're going to release carbon dioxide and this contributes to global warming of of course and toxic gases can also be released which can have very harmful consequences and even cause death finally we need to think about the problem of recycling the polymers we can't just say let's recycle them all we need to sort them all out bit by bit before we can then melt them and reform them into new products now this is labor intensive it's costly not only do you have to pick through every little bit of plastic some things are actually made of several different Plastics and so how do you take these apart so these are the things that we need to think about when dealing with polymers DNA or deoxy ribonucleic acid is a large molecule which is essential for life now DNA contains genetic instructions these are the codes needed for the development and functioning of all living organisms and viruses DNA molecules consist of two polymer chains which are made up four different monomers called nucleotides arranged in a double helix this table here gives us a great summary of the biological polymers that you need to know about we need to know that DNA is a naturally occurring polymer whose monom are the nucleotides and we need to know that DNA provides the instructions needed for every living organism to stay alive next up we've got proteins we need to know that the polymer of proteins or polypeptides have the monomer of amino acids and proteins have many roles including making enzymes and making hormones next up we've got starch the monomer for starch is a sugar and the role of starch is that it stores energy in Plants finally the last naturally occurring polymer that we need to know about is cellulose cellulose you may remember from biology is used to strengthen cell walls in plant cells the monomer for cellulose just like starch is a sugar alcohols contain the functional group O now the homologous series of alcohols can be represented in the form of something like ch3 o or we can draw out its structure now we also need to know the names of the first four members of the alcohol homologous series now this follows the examp same pattern as all of these other homologous series when we have a one carbon alcohol it's called methanol when we have a two carbon alcohol it's called ethanol three carbon is propenol and four carbon is butanol again we can prompt ourselves with our monkeys eat peanut butter now we'll notice here that when we have an alcohol the name ends in anol so make sure that you learn that that is our ending of these names this table here summarizes the names of the first four alcohols their molecular formula and their structure and we can also see some of their uses so let's run through these methanol can be used as chemical feed stock and it's also added to Industrial ethanol to stop people drinking it because it's toxic ethanol on the other hand is the alcohol used in alcoholic drinks it's also used as a fuel F and as a solvent now propenol and butanol can also be used as both fuels and solvents we need to know a series of different reactions that alcohols can undertake the first one is combustion reactions we need to know that an alcohol plus oxygen makes carbon dioxide and water so in this case we could look at ethanol plus oxygen makes carbon dioxide and water complete combustion occurs when we have an excess of oxygen if we have a limited supply of oxygen then incomplete combustion will take place instead during incomplete combustion we still make our water but carbon and or carbon monoxide will get made instead of that carbon dioxide we need to know that shter alcohols will combust more easily and this makes the more efficient fuels our shorter alcohols will will combust with a blue flame and will release lots of energy the next reaction that we need to know for alcohols is that if an alcohol reacts with sodium we will make sodium Alco oxide and hydrogen so if we look at an example here if we had ethanol plus sodium we'd make sodium ethoxide and hydrogen now when we add sodium to an alcohol we will observe bubbles of gas forming or we might call it effervescent and that's due to the hydrogen gas that gets formed in that reaction alcohols get oxidized by combustion however we also need to know that they can also be oxidized using an oxidizing agent alcohols will react with oxidizing agents to form carboxilic acids an example of an oxidizing agent that we use in chemistry is potassium dichromate so this is our general equation here alcohol plus an oxidizing agent makes a carboxylic acid and water if we look at a specific example if we had ethanol plus an oxidizing agent we would make ethanoic acid plus water looking at the balanced equation here you can see that when it comes to writing an oxidizing agent we just use a square brackets o that's important that we know that that represents our oxidizing agent we also need to know about the solubility of alcohols short chain alcohols such as methanol ethanol and propenol are all soluble in water however as the length of the alcohol chain increases alcohols become less soluble so we need to make sure that we know this trend finally we need to know that we can make ethanol by fermenting glucose ethanol of course is an alcohol it's found in alcoholic drinks like wine and beer and it can also be used as a fuel for vehicles for these reasons it's something that we often want to make industrially we need to know that fermentation uses yeast and anerobic conditions so without oxygen to make ethanol from glucose so the overall equation here is that glucose will be fermented to make ethanol plus carbon dioxide now we need to know that we need the following conditions to carry out fermentation to make ethanol we need sugar or glucose dissolved in water we need to have some yeast we also need anerobic conditions so we need an absence of oxygen and we need temperatures between 25 and 35° C now the reason why these temperatures are so important is because yeast contains enzymes if the temperature was too low and it was very cold then fermentation will happen happen very slowly to the point it might even stop however if the temperatures are too high then we have a different problem the enzymes will denat and this will also stop fermentation so we need to make sure we stay within that sweet spot of 25 to 35° C carboxilic acids contain the functional group Co the homologous series of carboxylic acids can be represented in the form of some something like ch3 Co so with that Co at the end of its formula or using this particular structure which shows a c double bond o and then an O also attached to that carbon we also need to know the names of the first four members of the carboxilic acid homologous Series so the names of the carboxilic acids end with ano acid and they begin with meth eth prob and P just like all the other ones so a one carbon carboxilic acid is called methanoic acid a two carbon carboxilic acid is called ethanoic acid three carbon is propanoic acid and four carbons is butanoic acid this table here perfectly summarizes the first four carboxilic acids together with their molecular formulas and their structures carboxilic acids we know have a pH less than seven when dissolved in water carboxylic acids are weak acids not strong acids and the reason why they're weak acids is because they only partially ionize or dissociate in water now it's important that we know some of the different reactions of carboxilic acids now carboxylic acids actually they're acids aren't they so they will undergo the same reaction as other acids with Metals bases and carbonates so if we had a metal reacting with an acid we'd still make a salt and hydrogen if it was a carboxylic acid if we had a carboxilic acid reacting with a base we would still make a salt and water and if we had a carboxylic acid reacting with a carbonate we'd still make a salt carbon dioxide and water the main difference that we'll see here is that we end up making carboxilate salts when we're using a carboxilic acid for instance if we had sodium reacting with ethanoic acid the salt we would make would be sodium ethanoate finally we need to know that carboxylic acids react with alcohols to form Esters Esters have the functional group Co and the general formula for the formation of an Esther is an alcohol plus a carboxylic acid makes an Esther plus water now the one reaction that they could ask about is that ethanol plus ethanoic acid makes ethal ethanoate plus water now let me show you what that reaction looks like so let's start off by drawing ethanol remember ethanol has two carbons and it has the O functional group so once we' finished drawing that I'm going to draw ethanoic acid and I'm going to draw it so that the carboxilic acid functional group faces that alcohol all will become clear so again ethanolic acid has two carbons see here that the H from the O and the O from the carboxilic acid can react together to form water now that water gets removed in this reaction and we combine these two parts together so this o is going to join together with this cble Bond o and this gives us our ethal ethanoate structure plus the water that got removed in that reaction now Esters like this ethal ethanoate are used as solvents flavorings and perfumes and you'll find that they have a fruity smell now when're making ethanol the fermentation mixture contains yeast and other insoluble substances that we need to remove now we can do this using fractional distillation now in this case we have our mixture of water and ethanol which we're going to heat up now out of these two ethanol evaporates first it's going to move up the fractionating column and into the condenser where it's going to condense and this is due to the fact of course we've got this cold water supply going into the condenser with the aim of pulling that vapor and condensing it now all of that ethanol is now going to drip into our conical flask and because water has a different boiling point to ethanol it will evaporate and condense later in fact water has a higher boiling point than ethanol and that's why it does happen later now in this particular experiment it's important that we know for safety we use an electric heater to heat that mixture of water and ethanol rather than a buns and burner we need to know that the reason for this is the fact that ethanol is very flammable both in its liquid form and also the vapors that will inevitably be given off in this experiment we need to know about nanoparticles well what are nanoparticles nanoparticles are particles which are between 1 and 100 nanometers in size made up of a few hundred atoms we need to know the relative size of a few different particles we need to know that atoms are around the size of 1 * 10- 10 m nanop particles are between 1 * 10 - 9 and 1 * 10 - 7 m fine particles which we may see represented as PM 2.5 are between 1 * 10- 7 and 2.5 * 10- 6 M and finally Co particles which are pm10 are between 2.5 * 10- 6 and 1 * 10- 5 m now we need to know a few uses of Nan particles too we'll see that they can be used in medicine Electronics Cosmetics Sun cream deodorants and catalysts now the main reason why they're used in so many places is because they have a large surface area to volume ratio however as well as being useful for this fact there are also some risks associated with using nanop particles firstly they can be breathed in because they're so small they can enter cells it can catalyze harmful reactions and also there's a chance that toxic substances could bind to them due to their large surface area to volume ratios now let's explore this surface area to volume ratio in a bit more detail let's work out the surface area of 1 cm Cub we know that there are six faces and each of those faces has an area of 1 * 1 so the surface area of this 1 cm cubed is 6 cm SAR now if we work out its volume we do 1 * 1 * 1 which gives us 1 cm cubed so now if we need to get the surface area to volume ratio we simply divide the surface area by the volume so in this case that is 6 / 1 which gives us six now let's do exactly the same for 10 cm cubed six for the six faces Times by 10 * by 10 which is the area of one face gives us 600 cm s for the surface area now the volume will be 10 * 10 * 10 which gives us 1,000 cm cubed now working out the surface area to volume ratio Again by doing surface area divided by volume we get 600 ided 1,000 which gives us 0.6 so what we can see here is that the smaller the particle the bigger the surface area to volume ratio so we can see that if we increase the size of a cube by a factor of 10 we actually decrease the surface area to volume ratio by a factor of 10 too we need to know about two types of Ceramics glass Ceramics and Clay Ceramics clay Ceramics are used to make bricks and pottery these are made by shaping wet clay and then heating it in a furnace this causes Crystal to form and join together now camic is a brittle hard and resistant to corrosion so now let's think about glass Ceramics the main type of glass used is sodal lime glass however sometimes Boris silica glass is used instead they have different uses these different uses come from the fact that Bora silic glass has a much higher melting point than sodar line glass that means that we might want to use Bora silic glass for certain purposes that means it's going to get hot examples of this include laboratory glass wear and oven glasswear whereas sodine glass we just going to use for everyday glass objects like the glasses that we drink out of now soda lime glass we make using a mixture of sand sodium carbonate and Limestone heated together and that gives us this brittle transparent soda lime glass or a silic glass on the other hand is made when we heat sand with Orin trioxide that's what gives us that characteristic High melt Point ouch this is why 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