 Heading all of you right down. So this portion of this chapter is, you know, easier than the previous one. We don't have, you know, conditions like we did in Tamil and English. But yes, importance, importance of this chapter, of this portion, if you see, it is equally important. Whether you're gonna write, you know, neat or JEE, this portion is very important. We don't have, you know, this chapter is, this part is not that big. Okay, we can finish this early. But the thing is they definitely ask question from this. So obviously, if you've seen the, you know, importance and advantage of this portion, if you are very good at it, then it's used because there's a number of things you need to study over here. And then definitely since questions you get from this point also, you can solve those questions. Sometimes what happens, they won't even touch thermodynamics part. They'll ask question from this only, thermochemistry. Okay, so don't consider this as, you know, not that important. Okay, it is easier to score marks in this particular portion. Okay, so this is it, okay. Okay, so write down, write down in this chapter or in this part, we will be studying, we will be studying the energy change in a chemical system, the energy change between a chemical system and surroundings. When, oh, let it be like this one, we do surroundings. We can have any kind of chemical reaction. We can have a fridge or we can have any normal chemical reaction. So whatever kind of reaction is there, how the energy change is taking place between system and surroundings that we are going to study, right? It is also named as, this chapter is also named as chemical energetics, chemical energetics. And this is based on, this is based on float. That is first law of thermodynamics. Okay, energy is conserved. So there are only two possibilities in a chemical reaction. Like either the energy will get absorbed or the energy will get released, mostly. However, delta at zero is also possible, but very rare, okay? Mostly we have two conditions where the energy is getting evolved, where the energy is getting absorbed, right? Consumes, okay? So whenever we have energy evolve or energy consumes, we can represent in a chemical reaction, right? For example, you see, if I write down one reaction here, we have two SO2 gas plus O2 gas in this two SO3 gas and we'll get plus 694 kilojoule of energy. So what do you mean by this, right? Here, the energy that we have, an energy of this, the difference in energy of this minus this is minus of 9694 kilojoule. Whenever you write plus sign over here, right? It means the heat is evolved or absorbed. Tell me, heat is evolved or absorbed, evolved, right? So this plus sign means that delta H is negative. If you write down delta H for this reaction, that would be minus 694 kilojoule, okay? If you write negative sign here, then delta H would be positive energy is getting consumed. Like for example, you see, we have a reaction N2 gas plus O2 gas plus O2 gas. It gives 2NO minus 180 kilojoule of energy, roughly I'm writing it down. So delta H would be what? Delta H is positive here, okay? Endothermic reaction, okay? So obviously we have heat involved in this reactant product. So what would be the heat of a reaction? Copy this down first, then heading right down. No, it's not nice. The contrast is too high. I knew that humans text this, oh no. The contrast is too high. You'll get a strain in your eyes, okay? So I talk heat of reaction. We also call it as enthalpy of reaction. So basically here we are going to study about the different types of enthalpy of reactions. Like we can have enthalpy of combustion, enthalpy of formation, enthalpy of neutralization, enthalpy of solution, many things we have, okay? So basically these are the definitions, right? We have few, few key points that you need to keep in mind, but it is basically definitions and based on that you have to do some addition or subtraction in order to get the desired result as per the question. How do we do that? We'll discuss all those things. So right on first of all, heat of reaction. It is defined as the amount of heat. It is the amount of heat evolved or absorbed, evolved or absorbed both, right? Amount of heat evolved or absorbed when quantities of substance are right on like this, when the given substance reacts completely or we can say when the reaction is over, reacts completely or when the reaction is over. Then enthalpy of reaction is equals to the summation of enthalpy of product minus summation of enthalpy of reacted. We'll write down the sum of the energy of product minus some of the energy of reactant that is the enthalpy of reaction, product minus reacting basically, okay? Next write down factors affecting. This is not that important. Thus you need to have this information that what are factors which affects the heat of the reaction? Like you factors affecting, affecting delta Hr. Synthalpy change R stands for reaction. First one is the physical state, physical state of reactant and product. Physical state of reactant and product. With one example, this you need to know like they may ask you this particular comparison but values you don't have to memorize how it affects that is not important. But if you look at this example, you will understand like suppose I'm taking H2 gas and half of O2 gas. It forms H2O gas and the enthalpy change in this reaction is all these are experimental. This is the value which is given, right? You cannot find out this if certain things are not given in the question, right? This is given delta H is this. Similarly, if it is gas than this and if you have H2 gas and half of O2 gas equals to H2O liquid, then enthalpy change would be, if it is liquid then it is found out to be 68.32 kilo calorie. So you can see obviously there's a difference in enthalpy change. That's why we say that the physical state of reactant or product affects the enthalpy of the reaction. Copy this down. Now the second factor we have, second factor is the allotropic forms of element. The allotropic forms. Same thing here you see. Carbon if you are taking different allotropes. Suppose for carbon, one of the allotropes you are taking diamond plus O2 gas. This converts into CO2 gas. Plus if you look at the enthalpy change here, delta H is found to be minus 94.3 kilo calorie. And if the carbon if you take graphite with O2 gas forms CO2 gas, delta H for this reaction is minus 97.6 kilo calorie. You see different value we are getting. So allotropic form is also a factor we have here. Apart from this two, we can have another factors like temperature, not always but temperature also you can consider. We can also have quantities of reactant. Of reactant is a factor here. We can also have the condition, okay? The condition of reactant and the reaction actually. Condition at which the reaction is taking place, okay? See temperature is not always a factor. It is a factor when the heat capacity is a function of temperature. If it is not, then temperature is not a factor. If heat capacity is a function of temperature. All these five factors we have which affects the heat of the reaction. COVID, one second guys. Okay, yes. Now, yes. You see next, next we have different types of enthalpy. Write down the types of enthalpies of reaction. I'll show once again, just once again. We have one more thing here. That is thermochemical equation heading right down, thermochemical equation. Not at all important, but I'll tell you what is thermochemical equations. You need to know the term here, right? Thermochemical equation is, which one? Achha, that one. Okay, once again. See, basically, example of temperature is not a factor. See, usually what happens if you look at the actual scenario here, just a second, Anj, once again. If you look at the actual scenario here, right? Then specific heat capacity depends upon temperature for every reaction. There will be some change in specific capacity when you change the temperature. But in the, for solving questions, we don't consider the variations in a specific capacity with respect to temperature. If it is not mentioned in the question, are you getting my point? Right, so if it is not mentioned in the question, then we ignore this relation of specific heat capacity and temperature. Most of the time, the relation would not be given. Like, suppose if I give you one example, suppose delta H you need to find out, which is NCP dt, correct? And if CP relation is given with respect to temperature, like suppose we have two T square plus three T plus 10, this kind of some quadratic equation or some relation of specific heat capacity with temperature is given. Then what you need to do, the CP you need to substitute over here, when you differentiate this to find out the exact value. So we'll assume this, if it is given in the question, otherwise we'll take it as constant, clear? Yeah, and your doubt is harder to, see it's not like that, graphite is more stable. Right, the carbon, the graphite form is more stable than diamond. That's why the natural form of carbon in which it exists in nature is graphite only, it's not diamond. However, at very high pressure, we can convert carbon into diamond. That's possible extremely high pressure, but the natural existing form of carbon is graphite because it is more stable in graphite form. And because of its high stability only, we have that difference in enthalpy. That's why we say every allotropic form has its own stability, has its own energy, right? That's why if the allotropic form will change in the reaction, the enthalpy of the reaction will also get changed. Got it? Thermochemical equation you write down. Write down, these are the equation which represents, these are the equation which represents thermal as well as chemical change. Like whatever reactions we have written so far in this particular chapter, that is thermochemical equation. Suppose I have this reaction A gas, the state must be mentioned, solid liquid gas, right? B liquid, suppose, gives C solid. Randomly, I have written some equation here. So state of each reactant or product must be mentioned plus whatever the enthalpy change is there, plus X kilo zoolophoon. So for example, I'm assuming, this enthalpy change must also be mentioned. So when the chemical change, as well as thermal change is there, mentioned in the equation, it is said to be thermochemical equations. Clear no doubt? Okay. Next you see the heading write down, different types of enthalpies of reaction. Types of enthalpies. Okay. The first type you write down, it is enthalpy of formation, enthalpy of formation. It is represented by delta FH. If it is a standard state, then this would be not over here, standard state, enthalpy of formation, definition write down. Let me just put this in the charge, my laptop once again. Okay, write down the definition here. It is enthalpy change. It is the enthalpy change when one mole of a compound, when one mole of a compound, of a compound is formed from its constituent element, constituent element, which should be in their reference state or standard state. So two things are important here. One is forms from its constituent element. This is important. Another one is one mole should be formed, constituent element and references it. These three things are important here. What do you mean by constituent element? Suppose carbon dioxide is forming. So what are the constituent elements of carbon dioxide? It is carbon and oxygen. This is a constituent element of carbon and oxygen. So these constituent elements must be there in their reference state, the state in which it naturally exists. Like for carbon, the reference state is graphite. It is not diamond. So it must be in graphite. Oxygen O2, it is the gas, the natural state we have, right? So natural state must be there. Plus only one mole of substance must be formed. If one mole is not there, then the enthalpy that we get in this reaction, that enthalpy is not enthalpy of formation. See enthalpy is what? Enthalpy is an extensive property. Enthalpy is an extensive, yes or no? Is extensive. So when we define an extensive property, whether it is enthalpy or any other property, we always define the amount that we are taking. It is an extensive property. So to define an extensive property, we always define the amount because one mole you take, you will have a definite value of energy, 100 mole you take, the value will be more, right? So what is the amount you are taking in order to define the enthalpy of formation? The amount is what? Amount is one mole. Did you get it? All right, we'll finish this enthalpy of formation and then we'll have a break, one second. Let me finish this. Okay, so you see two questions we'll discuss on this. The first one is which of these reactions represents, which of these reactions represents the delta FH of CO2? Delta FH means enthalpy of formation. I'm going back once again. Enthalpy of formation of CO2, done? Yeah, four reactions I'm writing down here. So first one is CH4 gas plus 202 gas gives CO2 gas and H2O liquid. First one is this. The second one, we have CaCO3 solid gives CaO solid plus CO2 gas. Third one is carbonyl and CO2 gas. Third one is carbon graphite plus O2 gas gives CO2 gas. Carbon we have diamond plus O2 gas gives CO2 gas. Which one represents the enthalpy of formation of carbon dioxide? Clear? What is the answer? Tell me. Yeah, obviously the first two you can eliminate easily because these are not the constituent elements. These are compounds over here. Carbon and graphite, we know diamond is not the reference state of carbon. It is the third one. Okay, now based on this, you see one question they asked in JEE. What is that question? I'll tell you. The question is which of these reactions represents the enthalpy of formation? After this, we'll take a break. Which of this reaction, these reactions represent the enthalpy of formation? The first one is half of H2 gas plus half of Br2 gas gives HBr gas. The second one is N2 gas plus 3H2 gas gives 2NH3 gas. Third one is half of H2 gas plus half of I2 gas gives HI gas. And the last one is half of H2 gas plus half of Cl2 gas gives HCl gas. Okay, now you see this. The answer for this question is option D. This is wrong. This is wrong. This is wrong. And this is wrong. Why this is wrong? Because the reference state for bromine you should know it is liquid. It is not gas. And you should know this. You should know this information. The same question for us in JEE, exact. The reference state is liquid, not possible. This, the reference state is fine. But here it is given two moles. It should be what? It should be one mole. Hence, this is also not right. H2 I2. I2, the reference state is solid. It is not gas. This is also not correct. Chlorine, the reference state is gas. And this one is fine. So you should know the reference state of the element. Got it? Once again, I'll go back. This one. Yes. Tell me. Once done. Once again, auto. So you should know these things. Right. What is the reference? Let me give you some example here. Just you can keep in mind. One note you write down after this. Done auto. One note you write down the enthalpy of formation. Delta FH. The enthalpy of formation of substance. In pure or. Standard state. Or a standard state is. Zero. Okay. Or a standard state is zero. And those like for molecules for this delta F of H, like enthalpy of formation is zero. Is we have O2. Gases estate. In that pure formation is zero. We have H2. Gases estate. In that pure formation is zero. And to gaseous estate. In that pure formation is zero. Carbon. Graphite form. Then help you formation is zero. Sometimes they also write on solid solid. You understand graphite only. If diamond is not mentioned. Reference rate is zero. Phosphorus. If you have white phosphorus. Delta FH is zero. Sulphur if it is rhombic. Rhombic sulfur. Delta FH is zero. Tin if you have white tin. Delta FH is zero. Copy this down. For all these elements. The standard state you know that the. Enthalpy of formation would be. Zero. Okay. Now one last thing you see. If you have a reaction. A. Of that state is given a gas. I'm assuming B. Gas I'm assuming gives. See gas and D gas. Suppose we have this reaction. So enthalpy of formation of C and D would be. Delta FH. Obviously for one more is equals to. The enthalpy of. Formation of. C. Plus enthalpy of formation of D. Minus the reactant one enthalpy of formation of A. Plus enthalpy of formation of B. This is the enthalpy of formation. Product minus reactant does you need to do. And it should be one more if it is not one more. Then this won't be the enthalpy of formation. Keep that in mind. Take a break now after the break will resume with this. Some other enthalpy term will see. 635 will resume. Take a break.