hi welcome to this video on energetics for Seaside chemistry these are typical exam questions you should be able to distinguish between exothermic and endothermic reactions to our energy profile diagrams to illustrate X of endothermic and exothermic change and calculate energy changes on experiments or from experimental data now in a chemical reaction there's always going to be some sort of energy change in the system this is either going to be energy given out to the surroundings or energy taken in from the surroundings and that's usually in the form of heat in other instances for example when you see fire it's going to be in the form of light but in the context of energetics resist the chemistry we're going to be looking at how heat is given out or absorbed you should recall that in a chemical reaction what you're going to have is Bonds in the reactants being broken and bonds in the products being formed now when you have those existing Bonds in the reactants being broken energy is required you need to take in energy absorb energy to break the reactant bonds when bonds are formed in the products energy is released so you give off energy now a chemical reaction that produces heat which causes the reaction mixture and its surroundings to get hotter is an exothermic reaction in these reactions energy is released to the surroundings for example neutralization reactions respiration in cells and burning fossil fuels so in an exothermic reaction energy is released to the surroundings because the energy that is released when forming Bonds in the products is greater than the energy absorbed to break bonds so if you have more energy being given off when bonds are formed in the products then you have energy being taken in when bonds are broken in the reactants obviously there's going to be a net giving off of energy that is the characteristic of an exothermic reaction and that extra energy is released to the surroundings which causes the temperature of the surroundings to increase conversely a reaction that absorbs heat causing the reaction mixture and its surroundings to get colder is an exothermic reaction and in these reactions energy is absorbed from the surroundings overall endothermic reactions include transport synthesis and thermal decomposition so the energy absorbs the brake Bonds in endothermic reactions is greater than the energy released when forming ones it's the very opposite of the exothermal opath you have more energy being needed to break bonds than energy released and formal bonds overall you're going to have energy being taken in being absorbed and that extra energy that's needed when that's needed to be absorbed is taken from the surroundings which causes the temperature to decrease it's important to note however that for both exothermic and endothermic reactions while we said oh yes an exothermal Productions energy is released and then endothermic reference energy is absorbed that is in terms of an overall basis energy is released and absorbed in each of the reactions it's just not one whether it's released or absorbed is higher than the other either you're releasing more or you're absorbing more when you're releasing more energy than you absorb that's going to be an exothermic reaction when you're releasing less energy than you also or if you're absorbing more energy than you release that's going to be an endothermic reaction now there's this term called enthalpy which is basically the energy content of a substance enthalpy is the energy content of our substance and it's represented by the letter H the thing with enthalpy is that you can't measure it directly whereas you can say oh temperature let's say this thermometer and say oh it's 37 degrees Celsius you can't measure enthalpy directly it only has meaning when there's a change in a given system now the enthalpy of a reaction is equal to the enthalpy of the products minus the enthalpy of the reactants and we're going to see how that falls in shortly so remember that we said for an exothermic reaction energy is released so as you can see here you have the reactants using the products and energy is released now by the law of conservation of energy since energy is neither created nor destroyed and that means that the total energy is the same on either side of the arrow so the reactants the total energy of the reactance has to be equal to the total energy of the products and that extra energy you give off therefore the products have a lower energy than the reactants in an exothermic reaction the enthalpy of the products is less than the enthalpy of the reactants as you just said because the enthalpy is the energy content of a substance and because of that Delta H is just negative because as we said the enthalpy of the reaction is a product is enthalpy of the products minus the enthalpy of the reactants and at the enthalpy of the products is smaller then you're doing smaller number minus a bigger number this is one thing if you are negative therefore Delta H is negative now since the enthalpy change is a change in a substance energy content and we just said that the reaction loses energy or energy is given off the enthalpy change is negative so it makes sense both ways when you think about the products and the reactants and when you think about the energy content or the overall reaction losing energy and endothermic reaction on the other hand can be described as reactants plus energy using products and because of the same energy another created are destroyed both sides of the equation and that means that the products have higher energy than the reactants that's why in an endothermic reaction they enter the enthalpy of the products is greater than that of the reactants which makes Delta H positive or you can also think of it as is here after this gaining energy the enthalpy change is positive so what the most important thing that you should get from that is that in an exothermic reaction Delta H is negative and then an endothermic reaction Delta H is positive for example a combustion of methane on the left is an exothermic reaction the value of delta H is negative and that indicates that 891 kilojoules of energy is lost when one mole of methane is completely burned in oxygen on the right and on the right and endothermic reaction is being observed and the positive 26.5 kilojoules per mole indicates that 26.5 kilojoules of energy is made when one mole of hydrogen iodine iodide is made during the reaction the overall energy absorbed by this reaction in particular represented in the equation would be double this value because two moles of hydrogen iodide is being made and note in a reversible reaction if the forward direction is exothermic the reverse is endothermic and vice versa for example when nitrogen reacts with hydrogen to form ammonia Delta H is negative 46.1 kilojoules per mole so that means that the forward reaction that produces ammonia is exothermic because Delta 8 is negative the reverse reaction is endothermic that means that 46 kilo 46.1 kiloism you're also required to know energy profile diagrams now the energy changes that we've been talking about in these calculations can be represented on the graphs that they do ask you to draw and those graphs are energy profile diagrams they show the enthalpy of the reactants and products a change in the enthalpy the activation energy which is the minimum quantity of energy needed for the reaction to occur and yeah that's that's the energy profile diagram and it's important to note our product the amount of activator energy does not affect the overall energy change and so is not included in the calculation so for the exothermic reaction the enthalpy of the products is less than the envelope of the reactants and that difference in the lines between the products and the reactants that is where you're going to enable your negative Delta a negative Delta H for an exothermic reaction for an endothermic reaction on the right the enthalpy of the products is greater than the reactants so that difference in the line is going to be your positive your activation energy is a line that moves from the reactance line to the very peak of the graph however you're not going to include that little bit of activation energy in the Delta H calculation now you should recall from race of reaction if you've covered that topic and that catalyst is a substance which Alters the rate of a chemical reaction without itself undergoing any permanent chemical change now you have catalysts that speed up reactions and these catalysts provide an alternative pathway which requires less activation energy and the normal pathway again catalysts that speed up reactions say hey here's another pathway for your reaction and I actually require well this pathway actually requires less activation energy and you want to do what's energetically favorable so in reactions that are spelled up by catalysts the alternative path that requires less activation energy is taken although the reaction has a lower activation energy energy values for the reactants and the products remain unchanged so Delta H is on TNG as well as well so you do need to know how to label the energy profile diagram with and without a cutters so foreign obviously the energy content is going to be the energy content needed for the reference to proceed I.E the activation energy is going to be lower that's the case for both EXO and endothermic reactions and this topic also involves calculations so as we said enthalpy is cannot be directly measured but the change in enthalpy can be determined by measuring the heat of reaction which is as you may have guessed the heat change occurring during a reaction that's calculated by measuring the change in temperature additionally to calculate energy changes it may be necessary to know a substance is specific heat capacity this is the quantity of heat required to raise the temperature of a unit mass of a substance by 1 degree Celsius or 1 Kelvin for example the specific heat capacity of water is 4.2 joules per gram per degree celsius which means that it takes four point joules of heat to raise a temperature of one gram of water by 1 degree Celsius now if you know the mass of the substances as well then you can calculate the heat of reaction from this equation the heat of reaction is equal to the mass times the specific heat capacity times the change in temperature so Delta 2 which is the speed of reaction is equal to m c delta T heat of reaction is measured is typically measured in joules mass in grams specific heat capacity in joules per gram per degree celsius and change in temperature obviously in degrees Celsius and to determine the heat of reaction the reaction is carried out in a calorimeter which is an insulated container that prevents heat from being released to the surroundings or gain from the surroundings a simple calorimeter can be made out of a policy so you have or a styrofoam cup so you have your styrofoam chemical reaction takes place inside the Styrofoam cup and that reduces the amount of energy that is exchange with surroundings the general method for determining Heats of reaction involves measuring a fixed volume of solution or water to be used in a measurable cylinder pouring it into the calorimeter and measuring the initial temperature using a thermometer you then mix the reactants in the calorimeter and measure the mix the money the minimum or maximum temperature reach in the reaction then you calculate the temperature change and use MC Delta to each calculate the heat of reaction importantly there are three assumptions that are made in calculating the heat of reaction they have they have asked you to give DC assumptions before you need to know them one is that the density of a dilute aqueous solution is the same of as that of pure water that is one gram per centimeter Cube two the specific heat capacity of a dilute aqueous solution is the same as that of pure water I.E 4.2 joules per gram per degree Celsius and three a negligible amount of heat is lost to or absorbed from the surroundings during Direction Theta solution so when a solid liquid or gas dissolves in a solvent there's usually a change in enthalpy and the heater Solution by definition which is something you do need to know is the heat change occurring when one mole of solute dissolves in a volume of solvent such that further dilution by the solvent produces no further heat change now when your study involves in the solvent you're going to have bonds between the particles of the solute breaking which causes energy to be absorbed from the surroundings for ionic compounds the ionic bonds break and those covalent compounds the interim molecular forces break now the intermolecular force is between solvent and solvent molecules also great which causes energy to be absorbed and finally attractions are formed between the ions the ions or the molecules of the solute and the molecules of the solvent and that's cause validation which causes energy to be released this is hard to say that that's where the energy absorption comes in to play and that's where the energy being released now if more energy is released during solidation then is absorbed to break the bonds in the solute and solvent and the reaction is exothermic as we said earlier because that would mean that more energy is released and absorbed and of course the opposite for endothermic reactions so calculating the heater solution you need to know the initial temperature and the exact quantity of solute dissolved so that you can calculate the number of moles for example dissolving 15.15 grams of potassium nitrate in 100 centimeters cube of distilled water results in a temperature decrease of 10.2 degrees Celsius calculate the heat of solution of potassium nitrate so we're going to calculate the number of moles of potassium nitrate first now the molar mass of potassium nitrate is going to be 101 grams per mole and moles is equal to mass over the molar mass which is 15.15 over 101 which gives you 0.15 moles the volume of water we're told is 100 centimeters fuel the mass of water is going to be 100 grams because of the Assumption because we're assuming that the density of this dilute aqueous solution is the same as that of pure water so that means which is one gram per centimeter Cube so if you have 100 centimeters cube of water that means you're going to have 100 grams of water by that assumption therefore the final muscle solution is going to be the 100 grams of water plus the 15.15 grams of potassium nitrate which is going to give you 115.15 grams as your final Mass and we calculated the number of moles so we can therefore calculate the heat of reaction Q is equal to MC well Delta Q is equal to NC delta T so the 100 and the 115.15 grams times the specific heat capacity of water which we're assuming is the same as a specificity capacity of this dilute aqueous solution times 10.2 which we're given in the question temperature decrease is going to give us 4933 4933 joules which we're going to convert kilojoules which is 4.933 kilojoules now it's important to note that this calculation that we have right here is 4.923 kilojoules that is the heat absorbed in dissolving 0.15 moles of potassium nitrate so now we have to calculate the heat absorbed in dissolving one more and you can do a simple cast multiplication for that is 0.15 moles produces a heat change of 4.93 kilojoules then one more produces three point Sorry 32.9 kilos yes multiplication divide 4.9 CC by 0.15 and the heater solution is hence 32.9 kilojoules per mole and what's very important is this plus sign and we know that it's positive because we were told in the question that it was a temperature decrease and we know that the temperature decrease means that energy overall was absorbed which means that it's an endothermic reaction it's important when you're writing your final answer to take note of what it tells you in the question because if it says that you know the question results in a temperature increase when you're doing your calculations you're not going to be taking nothing into account unless you put the negative in there just make sure that's in your final answer you have positive if it's endothermic or negative so the other type of calculation that you can be asked to do is heat of neutralization we know what a neutralization reference is acid plus base is a salt plus water and the neutralization between a strong acid and a strong Alkali is exothermic now the heat of neutralization the definition is the heat change occurring when one mole of water is produced in a reactional acid and an alkali now when you're calculating that TW elve you need the initial temperature of both Solutions average them and use that value as your initial temperature and you also need the number of moles of water for example a volume of 50 centimeters cubed of sodium hydroxide of concentrations is it 1.0 moles per DM Cube and temperature 26.0 degrees Celsius is added to 50 centimeters field of hydrochloric acid of concentration one point or more per dmq and temperature 27.0 degrees Celsius after mixing the maximum temperature of the solution is 33.2 degrees Celsius determine the heat of neutralization for the reaction it's always useful to write out your reaction equations so that you know your moral coefficients so you're going to have sodium hydroxide reactive hydrochloric acid to give you sodium chloride and water the number of moles of sodium hydroxide made is going to be 0.05 moles now we're told that it is 50 centimeters filled of sodium hydroxide solution of concentration 1.0 motor dmq so for moles we're going to get 0.5 0.05 I'm sorry and we know that the mole ratio between NaOH here and water is one to one so therefore we know that 0.05 moles of water is produced alternatively you could calculate the moles of HCL and do the same calculation as long as you're able to calculate the number of moles of water which is what you need then you're good to go now the volume of solution is going to be it was 50 centimeters cubed of each solution and there's something to give you 100 centimeters Cube and by the density assumption the mass of the solution is hence 100 grams we average the 26.0 and the 27.0 so you get 26.5 degrees Celsius as our initial temperature where it's called as our final was 33.2 so the temperature change is going to be 6.7 degrees Celsius now the heat evolved in forming 0.05 moles of water is going to be 100 grams times the 4.2 joules per gram per degree celsius times the 6.7 which gives you 2840 joules the heat evolved in formula one mole of each tool then is going to be 2.814 divided by 0.05 which gives you 56.28 kilojoules and again heat of neutralization the reaction is exothermic so we're going to have a negative in front of our final value in kilojoules per mole is if you don't have the negative then it's wrong because you're suggesting that the heat of neutralization is endothermic remember to include your science in front of your values now if you do a heat of neutralization reaction between a strong acid and a strong Alkali it's always going to be around that value around 56 or so that's because strong acids and strong alkalis are fully ionized in aqueous solution the common reaction is going to be the hydroxide we have the hydrogen ion forming water that reaction has an enthalpy change of negative 56.8 kilojoules per mole the equation for the neutralization reaction between sodium hydroxide and sulfuric acid is as follows in this reaction you can see that two moles of water is produced but the heat of neutralization is still going to be about 56 negative 56.3 kilojoules per mole however the overall heat change of the reaction is going to be double that because as we said the heat of neutralization is for one mole of water being produced and since two moles of water are produced in this reaction is going to be double that you can also determine the heat of neutralization from the results of a thermometric titration difficult what that is you should head back to acid spaces and salts but in that method we are going to take the initial temperature of the acid and The Alkali you're going to measure them and then the point of intersection between the two lines on that graph of best fit is going to be the maximum temperature exams questions the ones from earlier which well let's do the structured question first distinguish between the terms endothermic and exothermic generally when chemical reactions take place existing bonds are broken and new bonds are formed classify a bond making and bond breaking as either endothermic or exothermic prophecies easy formats so distinguish distinguishing between endothermic and exothermic during an X so oops during an exothermic reaction more energy is released during Bond forming than is absorbed when bond breaking and therefore heat is released therefore the temperature of the surroundings and the rectanglementary increase and the Very opposite for endothermic more energy is absorbed to Great bonds than is released to form bonds and therefore heat is absorbed which means that the temperature of the surroundings are the rapid mixture decrease that is more than enough for the two marks so and of course bond breaking is endothermic and bond making is exothermic that's like that which is the following best of that which of the following diagrams best illustrates the course of an exothermic reaction remember that we said for an exothermic reaction the enthalpy of the products is less than the enthalpy of the reactants and the only one there is going to be a so the answer is a the diagram with a figure below is a diagram of the enthalpy change right exothermic reaction showing the effect of a catalyst on the reaction pathway identify the parts filled with a b c and d so we know that a here is going to be our activation energy but that's also B so the difference is that a is the activation energy without the catalysis and V is activation energy with a calculus C we know is a change in Alpha enthalpy Delta h and D is the products so a energy attribution energy without calculus B Anthony with C enthalpy change and D the products when heat is given after the surroundings during a chemical reaction it is because bond breaking a releases energy as well as Bond making B requires less energy than is released when new bonds are formed C requires more energy than its release when new bonds are formed or D is an endothermic process whereas Bond making is exothermic so we're talking about bond breaking here and why heat is given off to the surroundings and we know that bond breaking is an endothermic reaction whereas Bond making is an exothermic one however we're talking about when heat is given off so therefore we need to meet the distinction that when heat is given off I.E and exothermic reaction bond breaking requires less energy than is released when new bonds are formed I.E overall energy is released so the answer for that is going to be D what D is saying is correct bond breaking is an endothermic process and bond making is an exothermic one however it's even more important to note that the question says when heat is given after the surroundings so you can't say it is d and now an exam question this is from January 2018. when a reaction is accompanied by energy changes it can be categorized as either endothermic or exothermic distinguish between those terms we saw that question already so we know generally when chemical reactions take place resistant bonds are broken and new bonds are formed classified as endothermic and exothermic as we said earlier Bond making is EXO and bond breaking is endo in an experiment when 12.0 grams of potassium nitrate is dissolved in 100 centimeters cup of water the temperature drops by 4.20 degrees Celsius calculate the number of moles of potassium nitrate used in the experiment given that the rmm the relative molecular the relative molecular mass of potassium nitin is 101. so easy one Mark we know that moles is equal to the Mass over the molar mass and networking that as iron here so that is going to be the 12th 0.0 grams divided by the 101 grams per mole 12 divided by 101 parameters gives me what 0.119 I believe that it would be perfect fine to put 0.12 moles easy one mark how to make the heat change of the reaction so we know that the heat turns is going to be m C delta T they already give us this assumption and this assumption so well we already given the values for those assumptions so we know that our Mass is going to be that 12.0 plus 100 centimeters cubed of water now I just want to make sure because there is a little bit of ambiguity here but for this you should use 112 the 100 centimeters cube of water plus well the 100 grams of water because of what is one gram per centimeter Cube plus the 12.0 grams of potassium nitrate some say that you don't use the 12 grams of potassium nitrate you just say 100 grams of water but what CXC has in their subject report is that it should be 112 grams I'm reading from the subject report now I'll put it on screen in this question this is assume a specifically capacity the solution is the same as specific positive water this assumption is acceptable for dilute Solutions and it is to be noted that M which is the mass of the solution is the mass of water plus the mass of salt 100 grams plus 100 plus 12 grams is 112 grams in some instances where the mass of the salt is much less than the mass of the water the mass of solution can be assumed to be equal to the mass of water the accepted answer is the heat change is equal to 12 grams times the 4.2 joules per gram per degree celsius times 4.2 degrees Celsius because that's what we're told temperature change is which is going to be 9 1976 joules and hence calculate the enthalpy change for one mole of potassium nitrate now if we knew that the enthalpy change for 0.12 moles as we calculated earlier was 1976. joules and the enthalpy change for one mole is going to be 1976 joules and let's just converters equal joules so we have our answer so 1.976 kilojoules over 0.12 moles will give us per my ability to calculate I hope 16.5 kilojoules per mole and then as we always do we note that the temperature drops by 4.2 or degrees Celsius therefore it's positive so we can leave it by that but just to make sure let's put our positive there so the next question asks us to list two pieces of apparatus necessary to conduct the experiment in a school lab and state how each piece is used so as we looked at earlier you're going to have our styrofoam or polystyrene cup insulating container or a large meter and of course our thermometer for measuring heat change easy formats and finally draw a labeled energy profile diagram to represent the enthalpy change for the reaction on your diagram indicate the sign of Delta e is for the reaction so let's draw up our axes for this question here we're going to have the energy content or the enthalpy and here we're going to have the course well let me see the progress of the reaction and this is an endothermic reaction so that we so we know the enthalpy of our reactants is going to be less than the enthalpy of the products so our line the little dotted line across here so here we have our reactants and here our products the activation energy is going to be the entirety of this here under Delta h is going to be positive don't forget that Delta similar there just like that well John is a ruler and in legible handwriting three marks for those are the 15 months for this question