hello this is dr henna asil and today we're discussing the first chapter in unit 2 of international a s edict cell chemistry and this is about chemical energetics so let's remind ourselves what are exothermic and endothermic reactions you should know that exothermic reaction is a reaction that gives out heat now the reaction that gives out heat will give out heat to the surrounding solution so the temperature of the surrounding solution will increase and the reaction will become hotter you should realize that burning or combustion of any fuel is an exothermic reaction now endothermic reactions is a reaction that takes in heat now it takes in heat from what it takes in heat from the surrounding solution so that means that the surrounding solution will get colder or the temperature will decrease so if he says that he did a reaction and the temperature went up what type of reaction is this this is an exothermic if the reaction went down it is an endothermic reaction and then we can draw what we call a reaction profile diagram and that indicates that the reactants at the beginning have a certain amount of energy and then we said when they react if this is an exothermic we said exothermic means the reactants will lose energy or we lose heat or we give out heat and that means that the products will have less energy than the reactants now the difference in energy between the reactants and the products this is what we call the delta h or enthalpy change or heat change for exothermic reactions the products have less energy than the reactants and that means the delta h will have a negative value and then we said where is the activation energy the activation energy is the minimum amount of energy needed to start the reaction it is the difference in energy between the reactants and the top of that hill that we have that is where you have the intermediate or what we call an activated complex so the difference in energy between the reactants and the top that is the activation energy now we include the activation energy if he's asking for a reaction profile but if he's asking for an energy level diagram we just show where reactants are products are and the difference between them is the delta h remember that if you're writing this kind of diagram for a specific equation do not write the word reactants products you write what actually is reacting whatever hydrogen plus chlorine or whatever and whatever the product is supposed to be you actually write what that product is now for an endothermic reaction we said endothermic means that the reactants will take in heat and that means that the product will have more energy than the reactants the difference in energy between reactants and products is a delta h or enthalpy change for the endothermic reaction delta h is positive again the activation energy is the difference in energy between the reactants and the top and if we're writing an energy level diagram you can just write the delta h without showing the energy of activation okay now for the international as or for the as level we're now going to talk about the delta h different types of enthalpy changes under standard conditions so when you have a delta h with this is a theta a small o with a cross in the middle that's called the theta so this represents enthalpy change under standard conditions now what are standard conditions you should know that standard conditions are pressure is one atmosphere or 100 kilo pascal or 100 000 pesca that is the standard condition now the temperature is the 298 degree kelvin which is the 25 degrees celsius and concentration if we're talking about a solution the standard condition for the concentration would be one mole per decimeter cubed if we say that we're talking about a substance in its standard state the standard state means the physical state under the standard conditions that means what is its state at 25 degrees centigrade and one atmosphere which is the normal room temperature and pressure now there are different types of enthalpy changes and you are required to memorize the definition or at least to understand so that you can write it what each definition means well we have something called standard enthalpy change of reaction now if we say we're talking about the standard enthalpy change of reaction it is the enthalpy change again do you remember what enthalpy change means it means heat change or delta h the enthalpy change that takes place when the amount of reactants shown in an equation react to give products under standard conditions reactants and products in their standard state please be careful with the definition when we are defining any of these standard enthalpy change of something it is the enthalpy change that takes place when something happens under standard conditions reactants and products in their standard state so if you're talking about the enthalpy change of reaction well it is the enthalpy change that takes place when that reaction happens under standard conditions with reactants and products in their standard state what if we're talking about standard enthalpy change of formation let's look at the definition it is the enthalpy change that takes place when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions what does that mean for example if i'm trying to make methane methane is ch4 under standard conditions that means under uh temperature of 25 degree centigrade and pressure of one atmosphere that's the normal conditions of the laboratory methane is a gas now if i'm talking about standard enthalpy change of formation that means i'm talking about the enthalpy change that needs to make one mole of methane from its constituent elements so what are the constituent elements of methane carbon and hydrogen in their standard state so i have to start with carbon in its standard state it's a solid hydrogen is a gas so and i'm trying to make one mole so remember that when i balance this equation it has to give one mole of methane we do not uh use multiples so this would be the equation for the standard enthalpy change of formation of methane what about this one now this equation is the formation of one mole of ethanol so this would represent the enthalpy change that takes place when one mole of ethanol is formed from its constituent elements now the standard seed of ethanol is a liquid now what elements do we use to make ethanol we use carbon and hydrogen and oxygen the state of carbon has to be a solid hydrogen is a gas oxygen is a gas and i balance it so that i give one mole of ethanol i don't multiply through by two i don't say four carbon plus six hydrogen plus one oxygen to give two c2h5oh no i want the reaction to give one mole of the compound in its standard state so ethanol as a liquid formed from its constituent elements in their standard states carbon solid hydrogen gas oxygen gas under standard conditions do you remember what the standard conditions were pressure is one atmosphere or 100 kilo pascal temperature to 98 degree kelvin or 25 degrees celsius these are what we call the standard conditions now keep in mind that the standard enthalpy change of formation for any element is zero that's because i don't form carbon for example i don't form hydrogen i form a compound so this is the enthalpy change that takes place when one mole of a compound but if i have an element the delta h of formation for any element is z another kind of enthalpy change that we need to know is the standard enthalpy change of combustion you should know that this is the enthalpy change that takes place when one mole of a substance is burnt in excess oxygen under standard conditions reactants and products being in their standard states for example if i have carbon plus oxygen to give carbon dioxide this will be the equation for the standard enthalpy change of combustion of carbon right this is standard enthalpy change of combustion of carbon the enthalpy change that takes place when one mole of a substance so one mole of carbon is burned in excess oxygen under standard conditions or reactants and products in their standard state so carbon is a solid oxygen is a gas carbon dioxide is a gas now remember that you cannot burn water you cannot burn oxygen so remember that the enthalpy change of combustion for water is zero enthalpy change of combustion for oxygen is zero so if i write this reaction hydrogen plus oxygen to give water this would be the reaction for delta h of what well here this is a reaction in which you have burning of hydrogen to give water so this is actually the reaction for the enthalpy change of combustion of hydrogen but it's also something that forms one mole of water so the delta h for this reaction would be also the delta h of formation of water so you can have the same equation giving you so if he gives you the delta h for that reaction hydrogen plus half oxygen to give water that is the delta h of combustion of hydrogen and it is also the delta h of formation of what another kind of enthalpy change is standard enthalpy change of neutralization remember what was the reaction that we call neutralization reaction of acid with base to give salt plus water so the standard enthalpy change of neutralization is the enthalpy change that takes place when one mole of water is formed by the reaction of an acid with an alkali under standard conditions so for example if i have sodium hydroxide plus hydrochloric acid sodium hydroxide is an alkali plus hydrochloric acid to give the salt sodium chloride plus water but then when we wrote the ionic equation for this reaction we ended up with h plus plus or h minus to give h2o this was the ionic equation for any uh neutralization reaction so we say that the standard enthalpy change of neutralization is the enthalpy change that takes place when one mole of water is full by reaction of an acid with an alkali under standard conditions another kind of enthalpy change is standard enthalpy change of atomization now what is atomization atomization first of all this is the enthalpy change that takes place when one mole of gaseous atoms pay attention to the gaseous it has to be gaseous atoms formed from its elements under standard conditions so for example if i have iodine iodine is i2 under standard conditions that means 25 degrees celsius and one atmosphere under normal laboratory conditions it's a salt so that is its standard condition if i'm talking about standard enthalpy change of atomization that means first i have to change the iodine to a gas and break it up into atoms so instead of having i2 which is a diatomic molecule i'm going to break this molecule to form atoms remember that would mean that you need this amount of energy to break the bond between the two i iodine and iodine atoms to form uh individual atoms in their gaseous state also if i have cl2 as a gas i need to break it up of course the standard condition of cl2 is a gas now standard enthalpy change of atomization would be the energy needed to change that into chlorine atoms in the gaseous state uh bromine in under its standard condition is a liquid now the energy needed to um form atoms of bromine in the gaseous state that would be the standard enthalpy change of atomization sodium in its standard state is a solid it's a metal the energy needed to change it into sodium gaseous atoms that would be the enthalpy change of atomization okay you should remember that bond energy is the enthalpy change required to break and separate one mole of bonds in the molecules of a gaseous element or compound so if i have hydrogen plus chlorine to give to hcl what do we have we have a bond between h and h we have a bond between cl and cl and we have two hcl bonds that are formed now each of these bonds needs a certain amount of energy to be broken or to be formed this is called the bond energy so the hydrogen hydrogen bond he gives you these numbers you're not required to memorize them so from what he gives me we have found out that to break the hh bond i need to give it 436 kilocalories or kilojoules the to break the clcl bond i need to give it 242 so remember that the uh breaking of bonds in reactants we need to add energy it is an endothermic process now when bonds are formed in the products they release energy so he's saying that each hcl will release 431 i have two hcl bonds that will be formed so that will release 862 kilojoules addition of energy is positive release of energy is negative now to get the overall delta h in order to calculate the overall delta h we have the energy needed to break the bonds in a positive number minus the energy released when the bombs are formed in the products overall here for example the answer is a negative number and that tells me that overall this reaction is exothermic okay why is it exothermic because heat is lost to the environment if it's a negative number that means heat is given out to the attack remember that the bond energy is actually an indication of the strength of the bond so the stronger the bond the more energy it needs to be broken and you should also realize that the shorter the bond the stronger it is so a bond is shorter if it is a double bond for example you should realize that covalent bonds are very strong they will need a lot of energy to be broken we will talk about what is hydrogen bonds but for now just remember that covalent bonds are stronger and need more energy to be broken than hydrogen bonds and these will need more energy to be broken than the van der waals forces the van der waals forces where the weak uh forces between molecules the weak attraction forces between molecules that we were saying for a covalent compound to melt for example i need to break the weak attraction forces between the molecules these are called van der waals forces so these need a very small amount of energy now we said the shorter the bond the stronger it is remember that a double bond between carbons is shorter than a single bond so this it will need more energy to be broken than the single bond uh an hcl bond is stronger than an hi bond this is because when we talk about chlorine bromine iodine the chlorine is a smaller molecule so there is stronger attraction forces uh between the positive nucleus of the atoms and the bonding pair of electrons for hi the iodine is a large atom so there is weaker attraction forces between its nucleus and the bonding pair of electrons so the hcl will need more energy to be broken than the hr so let's take a look at this example he gives me bond energies and he wants me to determine what is the delta h for this reaction so let's look at these reactions nitrogen you should realize that between these two nitrogens we have a triple bond and he's telling me that to break the triple bond i need 941 kilo jute now i have three hh bonds and he's telling me i need 436 uh kilojoules to break these bonds the nh bonds each nh bond needs 391 but how many do we have he's telling me i have two ammonias each ammonia has three nh bonds so actually i have a total of six in h box so to calculate the delta h the energy needed to break the n bond plus the energy needed to break the three hydrogen bonds and then minus the energy that is given off when the six nh bonds are formed the overall is a negative number and that tells me that the reaction is exotherm this is another example please pay attention to what is happening to the bonds so here he has a reaction he's telling me calculate the molar enthalpy change that is the delta h okay we said that bonds in the reactants we need to break them we add energy bonds in the products are formed so they give out energy they release energy so that's a negative so the ones in the reactants are positive the energy from the products is a negative number so let's take a look the energy that i need to give this reaction in order to break the bonds in the reactants well i have four ch bonds and he tells me that each ch bond needs 412 plus i have two oxygen oxygen bond and he says that each of them needs 496 so it is 2 times 496. this overall is the amount of energy that i need to give in a positive number to the reaction in order to break the bonds in the reactants now in the products what am i forming i'm forming two co bonds can you see the c double bond o i have a c double bond on this end and instead of we're looking at the bonds not the atoms so how many c double bond o do i have i have two and he's telling me each one is 743 plus the o h bond each o h bond when it forms it will give 4 6 3. now how many o h bonds do i have this tell me i have two water each water has two o h bonds so that's a total of four and that means that this is the amount of energy released now to get delta h you do reactants minus products this comes out as a negative so that is one way of determining delta h when he gives you bond energies and he says calculate delta h but in other cases he will say he did a reaction in a calorimeter like this and he measured initial temperature and final temperature and he's telling me to calculate delta h so in this case i'm going to use these equations so if we do a reaction and he says at the beginning i had a certain temperature at the end i had another temperature so i first of all calculate q q is the amount of energy or quantity of energy now how do we calculate that it is the mass of the solution i'm going to remind you that a solution is mostly water the density of water is one so for a solution the mass is the same as the volume so if he says mass of solution is the same as its volume if i say i have 25 centimeter cubed of a solution that means i have 25 grams of this so m is the mass c is a specific heat capacity this is a constant number so whenever we see c we're going to put 4.18 and the delta t is the temperature change now once we have calculated q then we will need to calculate delta h delta h is the q over the number of moles if you remember from chemical calculations how we calculated the number of moles depending on the information he gives me whether it's mass over molecular mass or concentration times volume and so on and remember delta h will need to have a sign so let's try this question this says find the enthalpy change for this reaction when excess magnesium is added to a hundred centimeter cubed of two mole per decimal cubed copper sulfate solution the temperature rises from 20 to 65. so we said in order to calculate the enthalpy change i need to calculate q first we said how do we calculate q it is m c delta t m is the mass of the solution where is the mass of the solution he said he used 100 centimeter cubed that means 100 grams times c we said c is always 4.18 times the difference in temperatures 65 65-20 that comes out to be a number in joules remember that the number that comes out for q from this equation is in joules usually it's a big number so if we divided by a thousand then the answer is in kilojoules then i need to calculate delta h well delta h is q over n so i need to calculate n now how do i calculate n for this reaction well what information do i have he's talking about copper sulfate solution he's giving me volume and concentration so i use the equation that says number of moles is concentration times volume so that is the two times remember if the volume is in centimeter cubed you need to divide it by a thousand before you put it into this equation so this is 0.2 moles of copper sulfate and that means to get delta h i divide q over n but then the number that comes out must have a sign now should i put the delta h in a positive number or a negative number how do i know we said if it is positive it's endo if it is negative it is exo is this reaction endothermic or exothermic how do we determine that we look at what happened to the temperature he's telling me the temperature rises from 20 to 65 that means the reaction became hotter that means this is an exothermic reaction that means delta h is a negative number so i have to put the minus sign before 94 kilo joule this is another question where he said he did a titration and he did volume of potassium hydroxide is 25 centimeter cubed and he has a starting temperature and then the volume of acid added in the titration is 25 centimeter cubed and the highest temperature reached was this calculate the heat energy or the quantity of heat well he's telling me the equation if he doesn't tell me the equation i have to know that it is m c delta t what is m remember we said m is the volume of the solution which is the same as the mass of the solution now what is the mass of the solution well what did he do he added 25 centimeters of potassium hydroxide to 25 centimeter cubed of acid so the total mass of the solution is 50. the delta t is the difference in temperature so q will be the mass times 4.18 times the change in temperature so that comes out in joules this is another reaction where he says a student uses this apparatus to measure the temperature change when solid sodium hydrogen carbonate is added and he says use a measuring cylinder to add 35 centimeter cubed of one mole per decimeter cubed of citric acid he recorded the initial temperature he added seven grams of sodium hydrogen carbonate and he spared the reaction now the initial temperature is this and the lowest temperature is this show that the heat energy is about 2500 well to calculate q i need m c delta t what is m let's go back what is m m is the mass of the solution what is the mass of the solution well i have 35 centimeter cubed of citric acid that means the m is 35 and the difference in temperature we calculate the difference and that means my m is this number in joules which is about 2500. then he says calculate the enthalpy change for the reaction so how do i get delta h of citric acid again what does he tell me he says i have a certain concentration in certain volume so i multiply that and get the number of moles and then i get delta h is q over n are we going to put a positive or a negative for that h you should realize that the initial temperature is 21 the lowest temperature is four that means temperature went down that means your final answer must be a positive number okay okay remember that the value of delta h that is calculated using this kind of calculation is usually different from the actual delta h and if he says why this is because usually when we do this experiment there is heat lost to the environment sometimes we have incomplete combustion if we're trying to burn a fuel uh incomplete transfer of heat some of the fuel may evaporate so uh our masses different in mass or something would be uh different and some of the heat is absorbed by the apparatus itself which we call the calorimeter sometimes if we're not measuring them under standard conditions then the equation should not apply and we get a wrong answer okay now another method of getting the delta h would be through hess's law and hesse's law says the total enthalpy change in reaction is independent of the root by which the chemical reaction takes place as long as the initial and final conditions are the same what does that mean i have reactants in a i'm trying to react them to form b do we understand that now how am i going to change a to b well i can use a certain reaction which we call root one and i can use a different method which we're going to call root two and what has his law says is that the total enthalpy change for root one should be the same as the enthalpy change for root two because once you're starting with the same thing and ending with the same thing you should have the same enthalpy change so he will then give you a reaction and he will give you some information and this information is either the delta h of combustion for each of these or the delta h of formation for each of these so here he's telling me i have carbon plus two hydrogen to give methane and he gives me the information that delta h of combustion of carbon that means if i burn carbon in oxygen it will give out 394 kilojoules if i burn one mole of hydrogen it will give out to 86 if i burn one mole of methane it will give 890. now he wants the overall dent h and that means that i have two pathways either i'm going carbon plus two hydrogen to give methane or i'm going from carbon and hydrogen and i'm burning each of these so if i burn carbon it gives carbon dioxide if i burn two hydrogen it will give to water if i burn the methane it gives carbon dioxide plus water so going from carbon plus hydrogen to give carbon dioxide plus water can you see where i'm going carbon plus hydrogen to give carbon dioxide plus what how can i go i can either go with the green arrow carbon plus hydrogen to give methane and then burn the methane to give carbon dioxide plus water or i go directly and burn carbon plus hydrogen to give carbon dioxide and water now the root the green arrow the total delta h following the green arrow should be the same as the delta h following the red arrow that means the delta h of the reaction that he's given me carbon plus hydrogen to give methane the delta h for that reaction plus the amount of delta h combustion of methane should be equal to the burning of carbon plus the burning of two hydrogens can you see what we're doing he said burning of carbon to give carbon dioxide we release -394 and then one mole of hydrogen gives minus 286 i have two moles so going with the with the red arrow the delta h is minus three nine four plus two times minus two eight six but going the other way around carbon plus hydrogen to give methane and then burning methane to give carbon dioxide on water i have delta h plus the minus 890 for the burning of methane so the delta h plus the minus 890 is equal to the ones on the other side now from that equation i can calculate for delta h and from that reaction the delta h is minus 76. do we understand that so if he gives information about the enthalpy change for combustion i draw the arrows going down burning carbon burning hydrogen burning methane going down to form carbon dioxide plus water but if he gives me this kind of reaction and he gives information about the delta h of formation that means he's telling me to form c2h6 it is -85 to form one mole of co2 it's minus 394. to form one mole of water it's 286. what about the delta h of formation of oxygen he did not give me the age of formation of oxygen well we said that the h of formation of any element is zero so i don't have that h of formation for oxygen and delta h of formation means my arrows go up i am forming each of these things from its elements so which elements do i have i have the c2h6 was made from two carbon the six hydrogens was made from three h2 the o2 was made from three and a half o2 so the box down will give you the elements and my arrows go up now where are my pathways well what am i trying to do either i go from the lower box two carbon plus three and half oxygen plus three h2 to form at the end two co2 plus three watt or i go directly to give to co2 plus 31 so the green arrow delta h plus the formations of c2h6 will be equal to the um formations of co2 and three water i have two co2s and i have three waters this is the equation and that means i can solve for delta h please try these questions so that you can understand them a little bit better okay thank you for listening and i hope this will be better and we will be doing uh another video on some questions and answers on this chapter thank you for listening