Hess Law of Constant Heat Summation – Chemical Thermodynamics – Chemistry Class 12
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Hess Law of Constant Heat Summation – Chemical Thermodynamics – Chemistry Class 12


Do subscribe to Ekeeda Channel and press bell icon to get updates about latest engineering HSC and IIT JEE main and advanced videos Hello Friends now we are going to do the topic of Hess law of constant heat submission and its application so it is very simple to understand that previous lecture we have studied what is the enthalpy of formation and as well as we have also calculated that how it would be calculated or for ejection so it is very much simple to understand has law right now so now let us learn what is Hess law of constant heat submission so there are the statements of it so let us understand the statement of it so it says that the change in enthalpy for the reaction is same whether the reaction takes place in one or a series of step so what does this explains it say that the enthalpy change for reactions that is suppose if reaction is taking place from suppose A to C and now it if it takes place in a series of steps also suppose for a conversion of the series of step that is from A to C that would be able to be or again B to C so therefore in this case the series of step if it would in Word that would be same as that of the the heat of reaction for a to C so it is very much simple to understand what is it has wrong but there are also different kind of its definition and what to say let us talk about that also it says that the enthalpy change for a chemical reaction is same regardless of the path by which the reaction occurs that means it depends it doesn’t depends on the path it only depends on the final and the initial state like that of institute function it does not depends on the path of it and it will depend on what is the initial state and what is the final state basically what would be the reactant and what will be the product and based on that for whom the change in enthalpy so it measures that and that is nothing but that is applicable under Hess law so let us understand this concept that what does the Hess law was trying to convey when it comes to the heat of a reaction or enthalpy of a reaction so for that we have an example so that I could tell you in a much more easy way which is this that we have discuss earlier also suppose if we have a reaction that is a to see when a is being converted into C so this process the change in enthalpy that we have observed or the change in the heat of friction or the standard enthalpy of the reaction suppose it is fall to be Delta H 1 so but this reaction can be returned in series of steps also suppose if the reaction takes place in two steps that is from A to B it’s friends or the change in enthalpy that we could observe is Delta h2 well again suppose if another step is been carried out from B to C and in this case the change in enthalpy is been found to be h3 so if we sum up this two things so let us see what we could get so these two things so this B which is present on the right hand side as less this way which is present on the left hand side suppose they will cancel out each of them and the overall net acceleration that we could get is that is a is been converted into C but this is the similar reaction that we have got above and in this case the energy that was being with the change in enthalpy that we have observed is nothing but the Delta H 1 so this clearly indicates that the Delta H 1 that we will get from A to C it will be sum up of Delta H 2 plus Delta H thing because it attached to was the change in enthalpy when a was converted to B but again the B if it converts to C then the change is been found to be detach 3 so therefore this the sum up of this H 2 and H we will nothing will be thus overall change in enthalpy from A to C that we have observe here so this implies or this states the first condition of the Hess law that we have studied then we have studied two statements so the first statement was related to this one so the second statement that we have got to know because this first statement it was saying that if it depends on the number of series of steps suppose and if any attend converts from A to C then it can be it people also calculate the series of step and we go to add that so that was the statement one of Hess law but the another statement that was dependent it was regarding the path and it was independent of the path as we could see here it would be much easier to understand suppose the reaction that we are making in consideration is a to see so from A to C the energy or the change in enthalpy that we have observed is Delta s 1 but when it is undergoing a series of steps and and now it supposes changes the path suppose this is the path a that we that you observe here so in this case suppose if a is been converted in to be an intermediate and in which and attached to is been spent or at FX will be energy that change that we observe here in terms of enthalpy and again supposing from B to C it comes and it gives an enthalpy change of H 3 so the thing is if it follows the path 1 it comes from A to C and suppose if it follows path – that is from A to B and again paths 3 from B to C ultimately the reactant a has to reach the product C so whether it takes place in a single step or whether it takes place in a multiple strand by following different paths for the thing is it does fit the when it comes to the change in enthalpy of a particular reaction that is from A to C it will not depend on the path it will just depend on the what was the reactor and what was the product and based on that what would be the change in enthalpy that we would observe so that was he has saw that we have got to know now so this is the thing that we have got to know from the has no statements and but to understand it in a more better way this was a general thing that you or that I have explained but this for a reason suppose if we have such a reaction so it’s not necessary because at how the hence law has been applicable in overall thermo chemistry let us talk about that so suppose this is another example and we have to make a consideration of how the preposition of methylene chloride is Windham so in this case I see a similar like the earlier one as you could see that suppose it is for a path 1 and part 2 suppose the path 1 is directly from A to C similarly in that way suppose if methane is reacting with the glowing molecules and will be all the since I’ve been present in a gaseous state so this versus what I could see is methane it’s been taken with chlorine gas molecules and from which it has been forming in methylene chloride as well as – it’s in but in this formation of methylene chloride what we could see is the enthalpy of the reaction or the change in enthalpy in standard state it is important to be – 2 0 2 point 3 kilo joule so this is the direct reaction or the reaction that has took in a single step so as to form so as to convert the mithya to a methylene chloride but what if it follows multiple steps and it has chooses a different path so what would be the overall heat of the Rachel or what would be the change in enthalpy of the reaction during conversion of ethane to ethylene chloride so for that we have path – and in this case suppose if it follows to step it it is not necessary it will always follow to step in it could be multiple step also but for an instance I provided this thing so suppose if an example which follows two steps so in this case suppose methane is been detected with khloé’ because two moles are flowing now have been rated already used us suppose in this only one mole of chlorine gas is being directed so what will happen is we will get with high chloride along with HCL so during this process basically the energy that is being change or the change in enthalpy that we observe is nothing but minus nine ninety eight point three kilo Joule but in this case the methane methyl torrent that has been obtained it is not the methylene chloride so we have to convert that Minter be thank you write to me think towards so for that similarly the methyl chloride is now reacted with the glory that was because we are using two more so another mole of flowing gas will be used in this case and in that case what will happen is we will get mistletoe right along with its skin and the change in enthalpy that we have got is basically minus one zero four point zero kilo Joule so during this process the thing is which I want to talk about is it is ultimately we will get a conversion of ethane to ethylene but the thing is it is following a multiple step like step one and step two but we have got the product but what would be change in enthalpy will that be similar to that of the early reaction that has took place in a one-step or this will be different so let us check that too so for this thing we’ll do is some kind of simultaneous equation that we have did in algebra also so similar to that of the algebra that we have studied in our lower section that is second section of us would similarly you could solve this also and so the thing that we could do is we could cancel out this thing that is this ch3cl and this is since a which is present on the right hand side and on the left hand side as if we would observe this thing then the answer that we could get is or the overall equation that you could get is nothing but ch4 plus 2 times of cl2 gas and again you obtain that is ch2cl2 plus 2 times of exhale so this is the nature that we will observe that we get if we even though if we follow the two-step process and this would be similar to that of the reaction that has taken place in one step but when we are talking about the change in enthalpy that we will observe so the thing that we have did is we have sum up with these step number 1 and some of these step number 2 and because of which we have got this equation which was like that of path 1 so similar to that we could also arrange this thing for the change in enthalpy so if we add up this minus 98.30 tune with that of minus 1 0 4 0 kilo Joule then the answer would be again minus 2 0 2 point 3 kilo Joule so this implies that we are following hi slow so this reaction even though it has been following in a single step or even though it is following in a multiple step or the thing that is much more essential for us is the reactant is been converted into the product and doing that kind of change it will be similar whether it follows in a single step or multiple streams so that is nothing but the Hess law regardless to the end of the path followed by the system so for that I have a short synopsis that what we have did in this reaction so as to get much more a bedrooms standing that what we have did in Haslam and here it is our first target was to convert the methane and we have to related to much of chlorine gas and we had to obtain ch2cl2 that is methylene chloride with that of two types of HCl so in that process basically when the reaction occurs directly in a one-step then we could observe that minus two zero two point three kilo Joule of energy is big that is it has been deported from the system or basically we could say that the change in enthalpy that we have fought is minus two zero two point three erosion obviously since it is minus M so we could easily say that it is a exothermic reaction so what happens if this kind of step it follows multiple steps for example the main concern was our methane molecules so if meeting multiples react with one mole of C so in that process what happens is the change in enthalpy that we have been observed so as when the methane and growing Mountain comebacks or to form C 3 C and that is with height or intersection and mean that Puss’s be silly – 98.3 producer of energy is being involved is from the system so the CH CCL again it will be reacted with the chlorine atom of glowing molecules so that we have and so as to form ch2cl2 plus 2 times at C n so in that process basically the enthalpy change that we have got is minus 1 0 4 kilo Joule so this is the second step this is the first step that we have got to no end and this is the second step that is returned over here as you could see and equalize that also so the energy that has been spent is basically 1 0 4 so if we sum up this 2 pi this to heat or we could say the enthalpy then it would be similar to that of minus 2 0 2 point 3 because the both have the same energy level as we could see so this is nothing but has long a very much simple thing to understand that how could a thermo chemistry can be related to has low energy could calculate the different heat of reactions all so for example suppose if we have a heat of reaction of my ch4 to suppose the product ch2cl2 and this process suppose if we know the value of that emotion from ch3cl and co2 which is being converted into ch2cl2 plus root and sexy so we could also analyze what would be the value or what will the change in enthalpy when ch4 that is methane went to x 1 mole of fluorine to form ch3 CN plus X here so this is the application of the has zone means we could estimate what would be the reaction of changing enthalpy of a particular reaction many follows mandamus step as if you know this to value people also assume that what put in there are n thumpy of the overall step and as let’s suppose if we know this value and we know this value is then we could have also avoided what will be the rate in enthalpy when this that is ch 3 CN plus here group that will convert into CH 2 CL and and that process what kind of energy will be born so this is basically application that we have did but let us understand more applications of the henceforth so that we could understand it be better way that and what kind of equations that have been used in solving a particular on calculating the enthalpy of reaction so we have basically and how we are talking about mainly we are talking about application of Hess law condition so the first application is the thermo chemical equation can be added subtracted or multiplied by a number factor like ordinary algebraic equation that I have good you earlier also because algebraic which is that we have to study you know school level can be applied so because it’s very much easy to do this thing and it would be very much easy to calculate the heat of reaction or enthalpy or any kind of change in energy for a particular reaction it is and therefore for calculating that the thing that we have to the operations that we have to do in that processes we can add we can subtract SLS we could multiply by a number factor and we could calculate a particular amendment so talking about another one that is suppose the enthalpy of a reaction which is cannot be determined experimentally can be calculated by s or so that is what I have explained you earlier also suppose if we if we don’t know the value of and suppose we cannot calculate what will be the value of when methane is mutated in chlorine and what will be the energy change that we probe so suppose with this we cannot calculate it so by doing this reference by Queen LG bra like we have did when that we have talked about earlier also we could do that thing and we could easily calculate what will be the enthalpy of that personal reaction it’s very simple to understand and talking about the last one that is because of hezron there are few ethyl P that we could calculate so now what we are going to do now let us learn what is the third application of that so the third application says that it can be used to calculate the enthalpy of reaction which do not occur directly so we and so the reaction that do not occur die people also calculate that what would be the change in enthalpy of that particular reaction and it’s really SLS we missed on that earlier also and the enthalpy of formation of any comfort from its element can also be calculated by using the Hess law as well as the enthalpy of combustion is also me can be calculated by using Hess law so this all are the application that we have did and based on that we have studied that how important does a does he has law of constant he’ll Submission plays a very vital role in understanding a particular chemical reaction so this was it and very easy concept so thank you friends for watching this video I hope you have understood this has not many you much more efficiently and you share this with your friends too and share this rid of the friends and your own fuckin subscribe because channel thank you so much

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