 Yeah, hi Aditya Let's wait for another minute or two for others to join in Yeah, hi guys. All those who are there if it would be really nice if you can just put in your name Yeah, so hi everyone. Let's begin. I guess I can see a few of them few of you already joined in and I assume that the rest would be just joining up Yeah, so this is today we look at energy change in chemical reactions basically The chapter which deals with thermodynamics and thermochemistry at such now everything to do with heat energy work and one of the most important chapters For your upcoming board exam now I'll also try and touch base upon a couple of problems may be a problem or two To really show how we can approach them at the same time. How can the answer be presented? I can see that Aditya is already in and a few more I can I can see so let's begin in understanding How this chapter really unfolds if you have any questions in between you can always put the questions up You know on the on the chat and I'll be happy to really address them now The this chapter specifically deals with what are what what really means by Having an energy change in a chemical reaction and how does it affect the other enthalpies or energies? as such of the reactants and products now So, so let's let's go step by step Firstly, let's understand how chemistry and energy is related Primarily, so if you really see that you know the energy as well as Chemistry is related in terms of the work or transfer of heat You know, so energy means that you have some ability to do the work or some ability to change this energy into some other another energy form of energy and one of these forms is heat and heat is this form where If there is a temperature difference between two objects then the energy is exchanged So heat is that form of energy that flows between two objects because of their difference in temperature So, you know temperature is like, you know the potential difference that we see in Electricity so when electrical energy has to flow there has to be a potential difference and when heat has to flow There has to be a temperature difference So heat is just one form of energy as we are as we are seeing this and what are the other forms of energy? Some energy are, you know explicitly visible as light Some of them is electrical as we just mentioned a few minutes ago A few of them can also be visible as mechanical energy where we talk about kinetic and potential energy So whenever there is an energy exchange Whether it is from a chemical energy or into a chemical energy the forms of energy that You know, we can really see is in either light electrical or mechanical forms Now, let's look at a few examples. So if you see, you know, let's when we burn peanuts, you know with You know with sufficient energy either in a cup of water or You know or also So, you know when the peanuts are burned the energy that comes out of it That actually heats up the water in a pan. It means that there was some energy inside peanuts that actually turned or converted into energy which was supplied to water and Once it was supplied to water that its temperature really increased. So burning peanuts is one example. The second example is You know, this is a jelly made up of, you know sugar and when you put this sugar in KClO3, which is a very strong oxidizing agent You'll find that the sugar entirely burns. It turns to char So that's also another form of energy when I'm actually not using any heat or any thermal energy at the same time converting sugar into Carbon or for that matter char. So this is another form of energy that can be extracted out of compounds or molecules and Converted into some different forms in the earlier form light as well as heat was found in the second form It is simply heat energy that was produced from chemical energy now So we understand that energy is intricately related with the way that the substance exists or there is some energy that is there inside a Substance in its molecules or In its atoms now if that is the scenario, how do we really study this chemistry and and its energy changes? And for this particular purpose we need what we call as a thermodynamic system Like we have free body diagrams in mechanics like we have circuits in electricity Or or you know in sound energy also we have systems where there is an observer and there is a sound emitter The very same way in thermodynamics, but we have something called as a thermodynamic system Where a system is that quantity of matter or region, which is what we want to study So anything that we want to study observe decide upon that is put inside a certain Constriction of space or constriction or what we can say an envelope of space and that is what we call as systems now Anything that is outside of the system is surrounding So from the surrounding either the observer could be anywhere it he could be inside the system he could be outside the system that is in the surrounding and The surrounding and system can play with each other giving energy or taking energy, but all of that put together Surrounding and system put together we get the universe and how do we really strictly if you have to separate the system out of the Surroundings we'll have to separate it out of an imaginary boundary now Please note this boundary actually looks two-dimensional, but it's not a two-dimensional boundary It's actually a you know something like a three-dimensional boundary. It's a it's a complete packet You know like a vessel or like a spear So it's a three-dimensional boundary and this three-dimensional boundary can be defined as either a real or imaginary layer That separates the system from its surroundings It is through this boundary that the system and surroundings exchange energy mass or any of the other quantities that they want to do So so so these are boundary system and surroundings and I also told you an additional definition of universe System plus surrounding basically means universe now if you really look at systems these systems basically are of three types They are called as closed systems open systems and isolated systems and we'll look at one of each of these one by one Yeah, so I think don't look at joint higher on higher on up. Yeah, so the first system is closed systems So now in closed systems, you'll see that the masses are fixed Close systems you are allowed to only exchange the energy, but not the mass So you see the mass exchange is not allowed, but the exchange of energy is possible This is possible only if the boundary is capable of exchanging energy and so therefore it should be a conductor Okay, a conductor when we say conductor we are talking about thermal conduction not electrical So this is very important So when the boundary is a thermally conducting boundary or in some cases we have also seen that the boundaries invisible There is an imaginary boundary in which case of course, you know, there will be conduction possible in such scenarios We end up getting close systems. So close systems You'll find that the mass is not exchange and therefore the mass is always constant in close systems The masses always constant now And energy keeps on varying. So this is close systems and it's also called as fixed masses system You know, that is the first type of system Now please note this close system does not necessarily mean that you know, it cannot conduct Energy completely not in pure form of energy But there is a moving boundary that can be possible and we know that whenever a boundary moves work can happen So this is a very interesting case that I wanted to show in here is that as boundaries move Even a close system is able to give work out or take work in the only problem is you cannot directly give out or take in heat Heat is not allowed or any form of other energy is not allowed But work can definitely be done. So this is a close system with a moving boundary and Which which actually helps us, you know exchange work as such and work is a form of energy Therefore energy exchange only through work is possible, but not otherwise Okay, so that's that's a close system and then we have open systems in open systems you'll see that both energy and mass they can be exchanged so you actually have an open end and You have a and which can take in or take out mass and of course energy can be Exchange and therefore it is a conducting medium as we had said in the previous slide as well Now you'll see the volume of this can actually be maintained pretty much because the volume is constant You know, you can also find this as that Control volume, which means that like a constant volume that you can come in now There is another point to to the system as the mass comes in and gets out How is the volume constant? We are saying volume of the Vessel or volume of the system is constant not volume of the gas. Okay, so Substances can come in substances can go out and a mass can be exchanged energy can be exchanged This is what we call as open systems Good now after this let's look at isolated systems now isolated systems are those where neither energy nor A mass can be exchanged now. There is a very interesting Diagram that I wanted to put in here most of the times we think that system and surrounding are only one But you can have different surroundings and one system. Let me give you an example Let's say you have a cup of tea this cup of tea is put inside a thermal insulated thermos you know and like a bag like a non-conducting bag an isolated bag and this cup of tea is is You can say close at the same time. Maybe you have some another food material like sandwiches and Maybe some vegetables now You'll see that the if we are wanting to study only the cup the energy changes in the cup of the tea It still exchanges energy between the sandwiches or the vegetables The only thing is that it will not go out of the packet out of the thermal Installation that we have put in so a system can exchange energy between the surroundings But whatever boundary we have drawn and if we say that this boundary is an isolated system boundary Then it cannot go outside this boundary internally. There can be any exchanges that are possible They can do that. So an isolated system boundary is that boundary where energy or mass both cannot be exchanged So generally we call this as the main system or several systems and then there are surroundings Which are considered as an isolated system So this is basically a main system and then the surroundings etc are called as you know Associated systems and the boundary is differentiating it from All other types of systems now So it means that you know energy can be exchange What are these types of really, you know, where do this energy really stay in a system? You know, what are the types of energies that the system can hold so for this We have to understand a few more concepts and the first concept is potential energy Now, you know that you know anyone who has climbed up a mountain or you know There's just a pic that I mentioned where these guys have really climbed atop a bridge and there is another person who is wanting to climb Diving pad on a swimming pool, you know, so when he climbs atop this swimming pool, you know, the diving pad He actually earns some potential energy. So potential energy is energy of a motionless body Please note, it is not energy of something that is moving, but it is the energy of something that is not in motion But only by virtue of its position. So depending on where in gravity it is whether it is at the top of the You know, driving pad or it is on a bridge or something of that sort You'll find that the potential energy can be assessed. So this is first form of energy But this is for very large objects How really potential energy is exhibited at molecular level now at an atomic scale or molecular level We'll find that the sodium and potassium So here so there is an nscl structure that I've shown here. This is a structure of salt So you'll find that any plus and cl minus are the cation and anions in a salt structure and both of them Attract each other and therefore there is some potential energy between them these two atoms can bond with each other and as they attract with each other if the atom is going far away from Let's say any plus is moving far away from chlorine Then you will find that the attraction will reduce if it is closer than there is a very strong attraction So because of this attraction very similar to the gravitational attraction the particles have higher or lower potential energy So these potential energies can affect because of this. I've just shown a quick small two two particles here and you will find that the nA and the chlorine can attract each other and This is how they their potential energy is very so an atomic scale also this potential energy is very much existing The same when we these potential energy can be converted into kinetic energy At a very macromolecular level or for example at a human level when we actually jump out of the diving pad We find that you know, we can actually gain the potential energy The kinetic energy which is converted from the potential energy and you can dive inside the You know in the in the swimming pool or the river wherever we are diving So potential energy gets converted to kinetic energy in the very same fashion between the atoms here sodium and chlorine the Potential energies between themselves gets converted into kinetic energy as the atoms start going further apart So kinetic energy is energy because of its motion and not of because of its position. So that's That's another form of energy that's existing inside the molecules Now this kinetic energy can be in multiple ways one is that it can be you know because of rotation You know, just so this is a water molecule. For example, this red one is an oxygen and the white ones are Your hydrogens Yeah, so so high as it is joined in I guess. Yeah, so because of rotation and because of vibration There is kinetic energy and also because of translation translation means only going Straightforward vibration means the hydrogen and oxygen are vibrating between themselves Please note the vibration is between hydrogen and oxygen So this is the way that the molecules vibrate between themselves, you know, this is how it will vibrate This one will keep on vibrating this way. So these vibration energies And then you also have the rotational energy where you can you know, the molecule completely spins So there is a kinetic energy because of that and then the molecule is going straight This is there is a kinetic energy because of translation so these three plus potential plus your kinetic energy is five and then the Interactions between the electron electron and the nucleus nucleus. So those interactions all put together So, yeah, I'm sorry. Yeah, so all these energies put together you end up getting a summation of total energy is that basically a Particle has and that is what we call as internal energy. So we just understood what is a system and What is a surrounding we understood what is a boundary for people who have just joined in high Neha Aditi so we have we have seen Systems types of systems surrounding boundaries and then we are talking about different types of energy that These molecules basically possess and in that we saw there are seven types of energies Translational rotational Vibrational potential energy electron electron energy electron neutron energy or nucleus energy and nucleus nucleus interactions as you can see in these two figures And so all of these put together the energy together is called as internal energy Because all of them individually cannot be a certain but all they they together can definitely be a certain or their value can be Established so because of this establishment of the value we call them together as internal energy now now so so the internal energy is represented generally by you or you and Interities this internal energy of the system Basically depends on Not only the number of particles of course because the number of particles will increase the mass and we know that kinetic energy is half MV square so definitely number of particles actually Change the internal energy Ketana has also joined in high Ketana now and it also depends on the type of particles What do you mean by type is basically are the particles that make are the particles try to make or polyatomic? Why because the vibrational energy is different for different particles So this internal energy also depends on type of particles at the same time it depends on temperature So let's say I've just shown you an example here So there is a lot of energy inside this small bomb, you know the one that you the cracker that you use During our festivals and let's say if you are wondering how much of energy does this cracker really have And this cracker actually depends their energy also depends on temperature. Let's try and increase this temperature So basically when we light the cracker, we are trying to increase this temperature So in the solid that the all the particles were setting pretty You know arranged pretty regular in a regular fashion suddenly when it goes to the gaseous form all the particles are let loose so The potential energy and the kinetic energy are changed when you go from solid to gaseous and that's exactly what happens when If the cracker bursts, so all the internal energy that was there inside the cracker is now into into the Gaseous form and its energy actually has increased because of the rise in temperature So the higher the temperature the higher the internal energy and so the change in temperature can directly be a certain to the change in Internal energy so at any point of time Let's say if you have to know what is the change in the internal energy some You know like we write it as delta u or delta e either of those So I mentioned both of these signs. This is proportional to the change in temperature. So please know this. This is one You know way to really measure the change in Very fast from the change in temperature. So that's that's internal energy for us now Now the other way of actually exhibiting the energy is also work So now this whatever this change in internal energy happens It gives out this energy in two forms either it will give out energy which primarily is heat See in this point. We are not considering or thinking about light energy or electrical energy all of those Although going forward. We'll also look at that But primarily whatever this change in internal energy has it happens this this shows our exhibits itself in two forms So what we have seen so far for we firstly saw what were systems and we saw that systems have energy and then we said Okay, where is this energy really lying? You know, how is it confirmed inside the substance? So we said okay There is some cause sorts of multiple seven forms of energy all of them put together give us internal energy Now we are talking about change in internal energy and we are thinking about how this internal energy really exhibits So the change in internal energy is given in two forms either through a heat or through work Now whenever heat is given out These are the sign conventions that we follow in chemistry and I have always said this You know chemistry guys are more generous. So if the system is really gaining merit the You know, we will call that as a good system. Yeah. Hi. Momica. Momica is also joined in So so we say that you know Whenever heat is given to a system in our system. We are defined system something like this So when heat enters the system, we call it as positive heat and when heat leaves the system We call as negative heat so energy transfer as a result of temperature difference I've already said that a temperature is something like Electrical potential so in for example in electricity as we have potential difference and therefore electricity flows in Thermodynamics, we have temperature difference, which is very equivalent to potential difference and therefore heat flows Now you will see that we have small terms like Qp and Qc We are going to look at that in just a few moments But for the time being just understand that you know endothermic which means it is taken inside the system is Always positive and heat given out of the system is always negative So now when we talk about the work our work is nothing but force into distance and therefore whenever we do work on the system We will say that the work is positive So that the bottom line of this sign convention is only two things if the system is benefiting and when I say benefiting when the Systems energy increases. We always call it as positive and when the systems energy decreases We always call it as negative and I had given you a thing to remember because of this that chemistry guys are always generous Which means that they only think about systems benefit not their own benefit in physics It is complete the opposite completely the opposite in physics We think about our benefit and therefore if work is done for us Which means if you are able to get the work output then it is considered as positive But here it is not that such if it is work is done on the system Then the systems work is considered as positive because the system is going to gain energy And if the work is done by the system, it means the system is losing energy the system is getting tired You know had it been a living thing So as the system loses energy we considered that energy as minus w now one quick derivation from w2 minus pdv is that Force into distance is basically our work if we simply divide by area and multiply by area Then force by area actually is nothing but pressure and distance into area is nothing but change in volume Okay, but because the change in volume in expansion is going to be positive So if something is expanding delta v is going to be positive because we have delta v is nothing but final volume minus initial volume So if final volume is going to be greater than initial volume We'll find that delta plus is going to be positive But so therefore p delta v will be positive But according to us if the system is doing work, which means if the system is expanding Then our work should be negative. So therefore we put a minus sign here Okay, so w is equal to minus p delta v actually comes from the very basic thing that the work is being done by the system One quick second. Yeah work is done by the system So please remember this minus sign is something that we put in Because we want to make the work negative and a certain to the value So there is another quick thing that I will tell you whenever expansion happens work is negative And whenever contraction happens work is positive Okay contraction means positive work and expansion means negative work. Okay. Yeah now So so these are these are a few things about work and heat So we had said delta u that is the change in internal energy Actually gets converted into these two forms. Hi, Sachi has joined in Okay, now next so let's understand what is a state and equilibrium now This is one very important thing which is actually not present in the text But to understand the concepts pretty well, this is something that we need to really understand Now a state is something that is a position. Okay in this thing. I am talking about one instant Okay, one instant. We are not talking about something that is moving For example, let's say, you know, I always give this example is that let's say you're moving from Bangalore to Mumbai Now in Bangalore when you started at the moment of starting it is an instant that is called as a state While you are in the travel it is called as process or path or being in motion But let's say you stopped at a point in Darbar while moving to Mumbai or in Bilgaon, right? So at that instant that is another state So what is state of a system state of a system is that part where you will find that the system is not Undergoing any change all of it all of its properties are known and they are not changing and what properties are we talking about for example? Pressure volume, you know temperature. We are talking about internal energy. We are talking about Total energy. Okay, so all of these things are pretty constant and they are not changing. Hi, Ananya. Good to see you Yeah, so all of these these are remain here and this is this properties do not change There is called called as a state of a system now Even if one property changes then the state of system has to change has to change. Okay Now now what is a chemical thermodynamic equilibrium? It is very similar to chemical equilibrium in chemical equilibrium We say that the reactants and products are in equilibrium with each other Which means that their concentrations are not changing one forms the other Simultaneously the other is also forming the first one. So this is how it was it was happening But in thermodynamics we call as the system being in equilibrium, which means that the temperature exchange is constant You know, I if there is a heat getting from one place to the other the heat is also coming back Similarly, the pressure is constant Similarly the phases let's say solid is changing to liquid and liquid is changing back to solid again Then both of them are in equilibrium. So all of these whenever there is an equilibrium again Chemical equilibrium is important to thermodynamic equilibrium is a very larger super set You can say of all small equilibrium and when all of them are in equilibrium We say that okay, there is a thermal equilibrium that is possible now Why are these two things needed whenever we are going to look at an enthalpy change or whenever we say that there are Certain amount of processes that are happening. We talk from a system state to another system state Okay, so the states change and therefore processes happen. That is the that is the reason that states change So therefore state is necessary and equilibrium is a way so that we can measure things if something everything is happening randomly If there is no equilibrium, then it is very difficult to ascertain the properties that are there We cannot really give values to them and therefore Thermodynamic equilibrium is also an essential important thing to really take a note of Well, let's look at parts and processes now that we have understood state and taking our same example of traveling from Bangalore to Mumbai when we are actually traveling That's when the process is happening and how we are traveling are we going by a flight? Are we going by a road and stopping somewhere in between again taking another road and reaching Mumbai that is the path Okay So path is the way that you are going you're doing the travel and process is the entire travel is the process So when the process is happening the process could be isobaric process Isothermal process so what is isobaric pressure remains constant isothermal means temperature is constant isoporic is volume constant Isoentropic means entropy is constant So all of these there are you know different types of processes and all of these processes can be thought but I just thought I will give you some examples so that you can actually remember that we are also going to see in this process is in slight debt or slight more detail now in part You know, whenever we are traveling, let's say I'm traveling right so at each point of time I know what is the velocity? What is the acceleration? What is the distance traveled? So I know all of these parameters so all of these parameters when we relate them to let's say thermodynamics We call that as part so it is a series of states through which the system passes while it is going through a process So it's like while I'm traveling. How am I traveling? Which are the cities that I'm going through? Am I really stopping over at some city or not all that is a part of the path But just the travel whether you are flying or whether you are going by the road You are traveling so that traveling is the process and this process is defined in multiple ways Similarly path also is defined in multiple ways reversible path irreversible path so and so forth So to you know, really give some examples of different types of these processes So let's say you have an isothermal process now freezing of water to ice at minus 10 degrees Celsius Okay, now if it is freezing now, please note You know I've not taken it as zero degree Celsius purposely because I want to make sure that it can be Because of different changes in pressure or temperature you can have at any other point as well So the moment you have freezing of water at any temperature But once the temperature remains constant because the change in state is happening you call that as an isothermal process Now we also know that latent heat is given out the very definition of latent heat etc was well Okay, so all that really works Hi, Brian. Good to see you here, Brian But this is an autonomous syllabus. So if you want to look in you can always are welcome But just just know that this is an autonomous syllabus that we are looking at Okay, right. So now So isothermal processes is once where temperature remains constant and the change in the substance really happens Now in isobaric process the pressure is going to remain You know constant the pressure is something that will that will always be the same. So in isobaric process You know one of the examples if you have to give it is like heating of water in open air, you know under atmospheric pressure so Here because the pressure is only of the atmosphere the pressure never changes and therefore you will find that You know the you know the change in pressure does not happen now Similarly you have isochoric where volume is constant So in isochoric processes, you will realize that The volume is same and therefore one of the examples of having the same volume is Basically in a seal metal conduct container you trying to do some experimentation. Okay So heating of a gas or doing a chemical reaction. So all of that will actually help us having, you know Isochoric process similarly you can look at reversible processes in reversible processes The system is close to equilibrium at all times and the changes that happen are very Infinity similarly very small, you know, they happen in a very small state You know moment by moment. So that is the reversible process in which the equilibrium is between system and surrounding with the original state and it keeps on giving in You know helping us have achieved equilibrium at every small points within the reaction. Okay, so these are reversible processes That are possible and then there are cyclic processes which are basically going from an initial state To a final state and then again returning to an initial state So whenever you have reactions going from one to the other and coming back These are called as cyclic processes. So For example, you know, whenever heat and work are done so There is one caution that we have to observe that in cyclic processes neither the heat nor the work are zero, you know So both of them actually would Maybe non-zero and at the same point of time It will still help us Have a cyclic process and then the last process is adiabatic process Which is where no heat is added or removed from the system So the you know, there is an exchange of Only internal energies that are possible as an internal energy can go up and down, but heat is not exchanged with You know with with the surroundings and then there can be combination of such processes. This is just an example To you know to understand, you know, how processes really work, but you know It also helps us knowing what states and processes are Yes, so hype high prenup high middle good to see you Yeah, now let's go back to understanding the first law of thermodynamics now This is something that we have, you know seen Since very long right right from seven grade. We are saying first law of thermodynamics is nothing but conservation of energy law it says energy can neither be created not destroyed at the same time the total energy of the universe remains constant so Energy can be converted from one form to the other But and vice versa, but there is no loss of energy at any point of time. So that's first law of thermodynamics now Just to have a quick recap So we understood okay, there are substances and then there are systems which we use to understand You know all the energy changes that are happening these systems further end up Giving us energy now we also understood that this energy is stored inside the system as internal energy And whenever there is a change in internal energy either work is done or there is a You know there is a heat given out now when both of them are happening They happen through a state the states change and as the states change They might change through a process or through a path. So this is what we had understood so far Now we are actually we have understood the complete background of the things now We are trying to understand. How does this energy changes really happen? So these energy changes will happen by the by obeying the first law of thermodynamics where which says that energy can either be created or Not destroyed. Okay. Now To really understand this in more depth they defined another entity which is called as enthalpy Okay, so to have the first law of thermodynamics very well known, you know enthalpy was introduced So what is enthalpy enthalpy is nothing but it was defined as you know Please note that it is a definition that you we have here so enthalpy is defined as You know e plus pv where is the internal energy at any temperature and pv is the work function at that temperature Please note pv is not a Like a physical quantity, but it's just a product of pressure and volume at that temperature And this is what we called as enthalpy now Why is there a need to define one more term basically if you define this term We will realize that you know, whatever delta u Going to you know either heat or work These can be very well tackled by this definition. We'll shortly see how it is how it really works So once enthalpy is defined we can actually talk about change in enthalpy, which is delta h And what is delta is delta h is nothing but delta e plus delta pv Most of it names because pressure is constant pressure can be taken out and we can write this as delta e plus p delta v now Delta h is the in you know enthalpy which is also called as the heat energy So this can actually be written as qp qp. Why because it is the heat given at constant pressure So whatever is a change in enthalpy is nothing but heat given at constant pressure And therefore delta h is qp, which is another relationship that you can devise now So from one to the other, you know as I mentioned that delta u which is a change in internal energy can give heat or work If this this both of them are happening at a constant pressure You can simply say that whatever is qp will be the change in enthalpy of that particular substance So therefore we write it as qp is equal to delta h. Okay now Just to understand some more of these Whenever you have a constant pressure again recalling the definition that delta e which is delta u is q plus w as I was mentioning A few minutes ago You'll find that we can also write delta u as qp minus p delta v because now the pressure is constant So work can be written as minus p delta v and internal energy can be related with qp and work Okay now, so this is this is how we can we actually derive the first law In in form in the form of delta u q and w. Okay, so this is this is how now We have been able to relate q w and delta u or in year in our case. We are writing it as delta e Okay, so this is the you know heat at constant Pressure. Okay. Now now let's look at How what what really is happening? So as we have mentioned, you know, enthalpy is used to quantify a heat flow into or out of a system in the process You know, whichever is happening at a constant pressure So you'll see let's say we take a slab of water and this Water is actually formed out of hydrogen and oxygen now as it forms water heat is actually given out So this exothermic process, you know heat is given off by the system and therefore the delta h is going to be negative But at the same point of time, let's say from solid if we are going to gaseous form It is actually absorbed by the system now. We can always mention here This is the enthalpy of you know h of h2o, which is in the liquid state And this is actually enthalpy of gaseous h2 plus o2 Similarly here we can say this is the enthalpy of h2o, which is in the solid phase and this is enthalpy of sorry hg o this is hg o in the solid phase and This is enthalpy of Hg which is mercury plus enthalpy of oxygen in the gaseous state So in both of these we understand that energy sometimes can be given in or can be taken out Enthalpy of the system can be defined here and system can be defined here as well Please note that there is no difference in masses here Also, we had 4 hydrogen and 2 oxygen here. Also. We have 4 hydrogen 2 oxygen here We have 2 mercury and 1 oxygen we have 2 mercury and 2 mercury 2 oxygen here We have 2 mercury 2 oxygen so this so you can you can imagine that both of them are actually having the same mass So it could either be an isolated or a closed system But the change in enthalpies here simply by you know Subtracting the enthalpy of the lower one So let's say we we call this enthalpy of the products minus enthalpy of reactants We are able to end up getting delta H and this delta H can then further be related to q and w as we had seen in the previous slide Okay, so now in in this scenario in exothermic processes You will find that because the heat was given out the reactants have more heat than the products because products You know were found by giving out heat and here you will find in the other scenario You know the enthalpy of the products is more because the products actually took in heat from the reactants To form delta H and delta H is greater than 0 right? So now let's look at a chemical reaction, you know So this is called as the enthalpy change of a reaction where we write delta H products minor Delta H is equal to H products minus H reactants So you as I was mentioning in the previous slide from H initial to H final as the enthalpy is decreasing Delta H is less than 0 and the heat is given out Whereas when heat is getting in you'll find that the enthalpy is actually increasing and We are going from a lower enthalpy initial to a higher enthalpy final So delta H is less than 0 then Qp is less than 0 similarly And therefore reaction exothermic and endothermic So this is a quick way to really look at enthalpy changes as well as being and or exothermic Now now how is this related all of this enthalpy q w all of this related to our chemical equations So the first thing that we need to understand is you know the thermo what is thermochemistry? So thermochemistry is basically the study of transfer of heat in chemical reactions So whenever there is a reaction happening and if there is heat being transferred Then whatever studies we do with regards to the heat as well as the chemical reaction That is what we call as thermochemistry Exothermic for example in exothermic reactions the transfer of heat is from system to surrounding at the same time There is a chemical reaction happening. So therefore it is a thermochemical reaction Similarly in and so H2 twice of H2 plus O2 giving twice of H2O plus energy This is actually a thermochemical Reaction please note here in exothermic reactions energy is actually like a product So energy is given out like a product similarly in endothermic reactions The heat is from the surroundings to the system and you will find that energy is like a reactant So energy is taken by the reactants and therefore the net energy of the products actually increase Here's a quick diagram, you know I'm so I'm trying to show you in multiple ways how enthalpy change really happens from a product to reactant or vice-versa now since you know enthalpy cannot really be You know, it's basically the heat content of a substance at a constant pressure You cannot measure the actual enthalpy directly. You can only measure the change in enthalpy This is also one of the problems that we had basically with our internal energy as you know off when internal energy you find that You know the enthalpy Internal energy cannot be directly measured very similarly enthalpy also cannot be directly measured and therefore we mostly talk in terms of Enthalpy change so you can see that, you know in in this in the first Reaction you go from reactant to products So the net energy goes from this point, you know Where our cursor is to the point that is much below therefore this delta H, which is in red is actually negative or is is You know Because of an exothermic reaction Whereas delta H here is positive because of an endothermic reaction So again, you know enthalpy of reactants is more than products here enthalpy of reactants less than products So delta H here is negative delta H is positive heat is given out and heat is absorbed. We've just seen this Now now that we understood okay in exothermic endothermic We have seen in so detail that okay energy and enthalpy changes and they are either exchanged with One other one of the most important things is to really define enthalpy You know in its most standard conditions So therefore we use a standard condition Why because enthalpy varies with conditions with pressure it varies with temperature it varies all of that So therefore we fix enthalpy with certain conditions and say this is our standard enthalpy and from here I'm going to measure everything so for example with pressure you measure it as 100 kilo Pascal's and that is basically one atmosphere and The temperature is taken as 25 degrees Celsius Please note one common mistake that students do is take temperature as zero degree Celsius No, it's basically like the room temperature and therefore at 25 degrees Celsius You'll find you'll see that You know at normal all these Substances which actually exist as they are in the normal lab conditions whether they are gas gaseous solid or liquid at that point We call it as a standard enthalpy and from there if we are measuring any enthalpy change We will call that as the standard enthalpy change So firstly we define a standard enthalpy which is that one atmosphere and 25 degrees Celsius or even at these pressure temperatures We will measure all the other changes that are happening and those changes are called as you know standard enthalpy changes So so it's sometimes also written as with a not sign You know like a like a simple circle or a circle with a bar in it called as a delta H not So that's that's the symbol for standard condition So now let's look at some some enthalpy changes that we have okay So one of the most important enthalpy changes is enthalpy of formation now enthalpy of formation basically means is that How is the substance is forming from its root causes? Okay, this is most of the times but not always you know this most of the times exothermic You know in a very few circumstances. It is endothermic. For example, you will find a melting of ice You know so water formed from ice You know is an endothermic process. Okay, but it's not typically a delta H formation But I'm telling you a process that is endothermic yet very natural and spontaneous So so most of the time the enthalpy of formation is actually You know Exothermic process, please note. We need to form one mole of the substance Okay, it's not one mole of reactant. It's not one mole of you know intermediate It is one mole of this product that is found so in all the cases We'll find that it is one mole of product that is getting formed We are taking this as a fraction that is okay, but I need this as one mole okay, so whenever we make one mole of a substance form we call that as enthalpy of formation and So you see I've written a note here that it is always one mole on the RHS of the equation RHS is the right-hand side of the equation So here we have to have one mole whatever fraction you want to put on the left-hand side you can do that Now one more very important thing here is enthalpy of formation is that if you use any elements Their enthalpy of formation is always zero for example here o2 C h2 o2 carbon all of these are you know Their enthalpy of formations are always zero elemental forms because elemental forms means that they are in the same form naturally occurring right so Therefore they are they are you know Their enthalpies are zero now carbon is not taken in any other form except for graphite For example carbon can also occur in coal or can occur in diamond forms, but those are not taken So enthalpy of formation is the way that one mole of a substance is formed from its elements And these elements should also be in their standard states and that's when we call it as enthalpy of formation Now Similarly we have enthalpy of combustion. So enthalpy of combustion is when one mole of a substance is consumed Please note there on the other side. We had one mole on the RHS here We'll have one mole on the LHS so one mole of what you are burning Okay, so whatever you're burning that should be one more for example here graphite is being bought Hydrogen is being burnt your C2 H5 is being burnt all of them You'll see we have taken only one one one mole Oxygen can be anything the product that can be formed can be anything but something that you're burning always has to be one mole Okay, so that that's Basically one mole of You know enthalpy of change for combustion Yeah, hi Moncy, okay, no problem But you know so you know guys who have joined in late if you if you missed out something you can always go back Look at a variable feed connect to the topic that you have really not understood and you know Revised things you can always message me if you have any doubts guys you can message me on the You know chat window here or on WhatsApp in either of those I will be able to answer you Okay, so so that's a that's a quick look at Okay the enthalpy of combustion now Please note enthalpy of combustion is when you know it is going to a certain complete combustion Okay, yeah, you'll find that there is one more enthalpy of combustion where you have carbon plus You know oxygen actually giving CO okay half or two giving CO now this is not complete combustion Okay, so this can be the enthalpy of formation of CO, but this cannot be the enthalpy of combustion of carbon Okay, very important point because here you'll find that carbon has not gone complete combustion It has not gone to its maximum state of combustion or naturally occurring You know compound of oxygen unless it's CO2 is formed combustion is not typical, okay So so that's a that's a very important point of this. Okay. Now, let's look at Okay, so this is when we have We have seen enthalpy of combustion now the next is enthalpy of neutralization So these are different enthalpies that are possible So we have seen formation combustion and then this is the enthalpy of neutralization now in enthalpy of neutralization Basically one H plus and one OH minus forms one H2O Please note the basic definition is when one mole of water is formed So water has to be one more whether you are taking it from, you know How you really are making it from whether it's formed from H plus or OH minus or you take at CL and NAOH or you take half of H2SO4 and You know, maybe half of COH twice all that is irrelevant At the end of the reaction you have to have H2O plus whether if there is a salt form that is okay Salt form is okay, but one mole of H2O is very important if one mole of H2O is formed That is what we call as enthalpy of neutralization Now, please note that enthalpy of neutralization of any strong acid is strong base Which means at any point of time only H plus and OH minus is reacting It is always minus 57 in fact minus 57.1 kilo joules per mole. Please note It is kilo joules per mole. I see a lot of errors where people write it as joules per mole You know, they sometimes forget the sign minus sign is for what I'm see minus sign because this is an exothermic process Please note. Enthalpy of neutralization is always always an exothermic process So therefore whenever Enthalpy of neutralization is dealt with you should always consider a minus sign Now one more very important aspect of this is that we have mentioned strong acids and strong alkalis because We imagine that strong acids all entirely give out H plus and strong alkalis entirely give out OH minus What happens when there is a weak acid for example, you have CH3COH Then you will find that one mole of CH3COH is never going to give entirely H plus Maybe you will only end up getting 0.02 moles of H plus So therefore there is some energy lost in dissociating also and hence this minus 57 comes down to probably let's say minus 51 Sometimes it is minus 49 also. Why is this 6 joules of energy which was which was supposed to come to us is Now not coming you're not reaching us because the molecule itself is utilizing this energy to dissociate itself The CH3COH is dissociating into H plus and that's how the action You know the neutralization is happening. So that's enthalpy of neutralization again Just to quickly have a look enthalpy of neutralization is when one mole of water is formed So I have taken one mole of water again. Okay, that's a quick look at the enthalpy of neutralization Now there's another very interesting and very important enthalpy which is called as the bond dissociation enthalpy. So you'll see Whenever energy is required to break one mole of gaseous gaseous bonds to form gaseous atoms One mole of bonds, so please note. This is not one mole of atom one mole of molecule one mole of Any substance? No one mole of bonds now see in cl2 one mole of cl2 will have one mole of bonds But two moles of atoms is formed, which is okay In OH minus there is one mole of OH will give one mole of oxygen and one mole of hydrogen Now, let me give you another example Let's say I have CH4 then one mole of CH4 will give me four moles of ch bonds Why because each cl is connected to four hydrogens and therefore there are four moles of ch bonds So I will have to ideally take one fourth of ch4 dissociation into ch to actually get the bond dissociation energy Okay, now bond dissociation energy is most of our times in fact All the times you'll find that it is actually an endothermic process because you will need energy to break the bond So you will have to give energy to break a bond. So bond dissociation enthalpies are always positive Okay, they are always positive energies or positive delta H okay, uh Now making bonds is exothermic because it is uh, you know, uh, opposite of breaking a bond So when you are breaking a bond, it is always endothermic for diatomic gases, you know, bond enthalpy is 2 into Enthalpy of atomization. Okay. Now, this is a very important thing. Now, please note What is enthalpy of atomization? Enthalpy of atomization is to form one mole of atom chlorine, okay So if I were to form one more of chlorine, I'll have to take half of cl2 Half of cl2, isn't it? So if I'm taking half of cl2, you'll realize that we are basically talking about half of bond enthalpy cl cl So therefore I've mentioned here that for a diatomic gas bond enthalpy is equal to twice into enthalpy of atomization Okay, which means that, uh, you know, whatever, uh, Atoms are getting formed. What is enthalpy of atomization? Enthalpy of atomization means energy required to form one mole of atom So there are multiple of these enthalpies that you can define But in all of these cases, most of the times it is only one mole that you are looking at Uh, what exactly is one mole of bond? Okay, so pranav is saying what exactly is one mole of bond see pranav one mole of bond means, uh, you need to have Bonds which are equal to one mole in quantity Okay, what does that mean? So for example, let's say I have ch4. Okay Now if I take one mole of ch4, then I'm going to take basically 6.022 into 10 to the power 23 molecules of ch4 Now in each of these molecules, I have one two three and four bonds of ch Each so if I have so many ch4 molecules, I basically have so many divided by four ch bonds You know, uh, sorry into four ch bonds The other way around I basically have so many molecules into four because each molecule has four into four ch bonds But for us, we only have to define or deal with one mole of bond which means only this many number of bonds So whatever is the formation energy of ch4 That divided by four will be will be giving me the bond enthalpy or bond dissociation enthalpy of ch Are you getting this? Yeah. So one mole of bonds means basically one mole in number one more like like I have one mole of carbon One mole of oxygen one mole of You know hydrogen atom. So this is one mole of oxygen atom one mole of hydrogen atom Similarly in this thing, I have cl cl so I will have one mole of bonds inside them Okay. Yeah, so that's that's one mole of bond enthalpy now Of course now smaller the bond enthalpy weaker will be the bond and easier to break if the bond enthalpies are very large implies They are very strong bonds and they will be much harder to break. Okay Now, okay, so that's a that's that's bond dissociation enthalpy. Okay, sorry Now, let's go to the next we'll see we look at a few bond enthalpies now Please note single bond enthalpy and double bond enthalpy doesn't mean that double bond is twice of single Okay, this is a common mistake that you know people do So double bond might be very different triple bond is a different kind of a bond. So triple bond enthalpy is not that Equal to single bond. I have shown you here. For example, 346 is a single cc bond enthalpy But if you look at the triple bond enthalpy, it is 837 it is not 3 into 346 which will be somewhere around a thousand kilo joules It is only 837 kilo joules. Okay So the actual value is something that is Experimental and for each of them you will end up getting some bond enthalpy. Okay So I've given you all the bond enthalpies for different atoms. Uh, for example, hpi just just for you to really have an idea Now one mole of gaseous bond your two form gaseous atoms, right? And they are always into a familiar theme there Okay, now most of the times it is a mean value that you will take if there are multiple bonds that are present Okay, for diatomic substances, we have also said, you know, bond atomization and smaller bond enthalpies. Okay So this we have really seen Once again, okay now Sorry Now let's go to the next Uh, the next is now that we have understood bond enthalpies and we have understood first law thermodynamics A quick recap for everyone who joined late because this also keeps us having the flow Firstly, we understood what was the system and we said Uh, in this system one second in this system We said that there is energy and this energy is getting exchanged with others. So we said, what is that energy? Where is it deciding? So we said, okay boss, there is something called as an internal energy Which comprises of seven factors. Okay, we said that's fine. So what happens when internal energy changes? We said it either gives us heat or it works as it gives us work Is it okay? But how do we measure that? So we brought in our term called as enthalpy And then the enthalpy was there. We said delta h is something that is Changed in enthalpy. We said, why are we talking about delta h? We said because We can only talk from state to state. We don't need to talk about all the processes that are happening in between So if you're talking from state to state, we can directly talk about internal energy Uh, and uh, you know get the work done with so we said, okay, that looks great So therefore delta h is nothing but qp is what we derived Okay, it is it is the change in heat at a constant pressure So after we have understood all of this then we said there are multiple types of delta h So we saw delta h combustion. We saw delta h formation. We saw delta h which is bond dissociation Then we saw delta h atomization Delta h neutralization We also saw delta h, you know, there's another delta h which is called as fusion or delta h vaporization So all this is basically for different Enthalpies that can change from one spot to the other. So all that is okay But now sometimes all of these enthalpies we cannot really measure Okay, so then we are not able to measure enthalpies. How do we really understand what is happening? So what we do is we use indirect methods to measure these enthalpies for example Uh, there is you know has gave a very interesting law, which is a very important law And he said whatever enthalpy change you're doing, it is independent of the path. Okay Don't worry about the path. You simply go and do the enthalpy change at the end of the day The enthalpy changes will be the same So he said, okay, look one you are going through flight and let's say you're going through road At the end of the day, you will be reaching Mumbai from Bangalore on this. Okay So if you are reaching Mumbai from Bangalore just by looking at these two I will be able to tell you at least some information For example, I will be able to tell you what is the net distance you traveled I will be able to tell you what was the net time you took. Okay I will be able to tell you where is your geographical location. Okay, so maybe not all This is a very important point. Maybe not all but some entities I will definitely be able to tell you This is what has said. Okay. So he said, okay, definitely one of the things that I can surely say is about enthalpy So he said, okay interesting looks interesting. Let's understand how So he said when enthalpy changes, you know going from a to b, uh, you know The values of enthalpy changes from let's say from one road it is from a to x x to y and y to b And let's say at each of these it is delta h to delta h to delta h3 Then the summation of this second path and summation of the first path will be the same Tomorrow if you go through some other processes, let's say one two three four five and then six All of them put together also will end up giving you the same enthalpy through as as through this process h Now, please note when we are directly talking about a and b we are basically talking about then enthalpy of reaction Because we just a few minutes above we discussed enthalpy of reaction is nothing but enthalpy of products minus enthalpy of reaction So when we are simply talking about enthalpy of reactions, you'll find that delta h1 delta h2 delta h3 could be any other enthalpy But their product will come out to be the same Okay, now here is a few, uh, you know, uh, so what happens if you go in the opposite direction? Okay, uh, now if you go in the opposite direction of the arrow, uh, instead of adding the value, you'll subtract the value Okay, so let's say for example, you know, uh from x to a, uh, you know, I went in the opposite direction from, you know, uh It's a delta h2 is this So in x and y, this is delta h2 and if I have to go from x to y I'm going to go in opposite direction here Then I'm going to go in the same direction and then I'm again going to get into the opposite direction Because I went in the opposite direction at delta h1. I have written a minus delta h1 Now delta h2 is what is through one path. This was on my path one. Okay, and this was entirely my path two So the path one delta h2 is equal to minus of delta h1 because I went opposite I'm going in the same direction. So I'm keeping delta h3 hr as the plus and here also I'm going in the opposite direction. So therefore I will make it as minus So minus plus minus that is how you can actually end up getting the same, uh, you know, uh, delta h in both the direction So if you change the direction of the reaction, you will realize that, uh, your delta h will become negative There is one more interesting thing here. What happens if you trace the same path twice? Okay, so let's say this was my path one and my part two is this Then I come back again Then I go back again and I come here and then go here. Okay, maybe some path like this So what I will do is because I did this twice because because I went twice upstairs I will write this as minus two into delta h1 Okay, please note this. I would return return it twice. Why because I went twice up Okay, and I came one down. So I am going to take this as plus delta h1 Okay, only at these three steps Then the other things will remain same plus delta h are, uh, you know, minus delta h3 will remain same But the important point is so if you trace one path twice You will have to add it as many times and remember the direction if it is still twice in the opposite direction You will write two negative and once in the straight direction, you'll write one positive Okay, so that's that's one interesting thing now Now, uh, a few more applications of hs's law. So how do we really use hs's law, right? So hs's law enables one to calculate those enthalpy changes, uh, you know, which we cannot directly actually calculate Okay, so I've given some examples here So let's say changes which cannot be measured directly like lattice enthalpy or enthalpy change of a reaction from bond enthalpy So, you know, we know bond enthalpy, but we don't know change of a reaction which is delta hr But I know what are the bonds, you know, for example, ch4, I know the bond energy But I don't know what happened when the reaction is suddenly co2 and h2 was formed So if I have to form this energy, then I know the energies of co2 I know the energies of h2 and I know energies of this guy So I will take this as let's say a I take this as b so a minus b will be my enthalpy of reaction So enthalpy of reactions from bond enthalpies can be formed Uh, similarly, you know enthalpy of reaction reactions from formation enthalpy or combustion enthalpies can be formed So using different other enthalpies those enthalpies that is missing with you can be traced So these are these are a few, uh, uh, you know ways to really write answers law now, uh Just to really do one example, you know, how we can really do it from bond enthalpies So basically, you know, let's say imagine we have, you know, during a reaction The bonds are being broken, uh from individual atoms and they are forming new bonds That is what basically happens in a chemical reaction, isn't it? So let's say these are reactants and these are products now the energy of the reactants Was more than the energy of the products, which means that the reaction was basically an exothermic Okay, the energy of reactants then it also means that I basically broke all of these bonds So therefore I needed a bond dissociation enthalpy here Okay, and then I formed gaseous atoms and from here I formed products So I had a reverse of bond dissociation, which is basically bond enthalpy Okay, so here I the energy is given out and therefore there will be a minus delta h here and here there will be a plus delta h Okay, so so from reactants to products as we go You we will realize that, you know, the bond enthalpies of reactions can be utilized So as to imagine that the bonds were being broken and as the bonds are getting formed the You know, the bond enthalpies can then be added one more time So energy is put to break, you know to break bonds to form separate gaseous atoms And then energy is again taken out as the bonds are formed. So energy is released. So by using Hess's law Basically and adding step one and step two you can form find the delta h r between reactants and products So we are trying to find the energy of the reaction going from reactants to products through bond enthalpy additions and bond enthalpy You know bond bond breakages and bond additions Okay, now so so that's that's uh, uh, you know bond enthalpies Uh, now let's let's look at an uh, you know another way, you know the way that we can actually uh understand this is See, uh, when we actually broke the bonds we had to give in energy So let's say reactants were here the this was the energy to make them into atoms So I gave energy so the energy of or the entire system has risen by so much Okay, now from here as the bonds are you know are formed new bonds are formed the energy simply drops Okay, so all the energy that has dropped from this point to the points of product You'll find that the energy is given out and as the energy is given out You'll realize that uh, you know the energy of the entire system has gone down and therefore net the delta h is negative Which means that uh, you know, the reactants have a larger energy than the product Uh, reactants have yes a larger energy than the product. Okay, so this is another way of looking through an energy diagram Another way to really look at it Gages atoms combine and therefore they give out energy now because I'm going in the opposite direction I'm using a minus sign in step two Okay, I hope all of you guys are getting this and uh, you're connecting You know, please keep on responding if there's something that you really do not connect Uh, you know, maybe I can go over it one more time Um, I I hope that this this is something that you guys are really Understanding. Okay. We have done many Equations on this, but I just wanted to have a quick look at all of these individual problems and therefore I wanted to mention this Okay Okay, good Now let's look at the next You know Way to really look at that, you know another way to look at that So basically I had a molecule and the I'm breaking up all the bonds of this molecule So I've got only the atoms there are no bonds here The delta h2 was the energy given out and whether they say when when new compound is found out of it You'll find that delta h3 is energy that is Basically required. So as you go back, you'll realize that delta h3 is produced From, you know, delta h2 So so the delta h1 in this scenario will be nothing but delta h2 minus delta h3 So negative is used because I'm going in the opposite direction. Okay So that's the enthalpy of reaction from different bond So here's an example, let's say I had delta h2 given to us Now these are different bond enthalpies given to us then all the cc double bond energy was 611 T h bond energy was 413 Hs bond energy is 436 So you'll realize that the total energy to break the bonds was 2 and 2699 So delta h2 so much of energy was given out and while the new product was formed We'll realize that so much of energy we had we had basically 1 cc bond but 6 th bonds that were formed So you end up getting 2824 energy is released So 2699 minus 2824 in total you end up getting 125 kilojoules as the net energy that is given out So here's a quick example the way to that you can actually write the reaction In example, if you're able to write it in this form instead of just add you can write that AT And you can also say 611 kilojoule if you really write it in a systematic form one below the other I don't think so that there will be a problem in you know solving any of these questions. Are you getting anything wrong? Okay Good, so, you know before I go forward, you know, it would be nice if you guys can just put in a yes No, if you're connecting if everything is okay I understand that a few of you are still there a few might have dropped out Okay, I'll just look into that but Just just let me know just in case if you're connecting and all is okay at your end Okay, so uh moving forward, um, you know enthalpy of reactions from enthalpy is a formation Okay, now enthalpy is a formation are from electric elements going to reactants and product. Okay Okay, good. Uh, so uh, so basically when elements go from You know when when we go from elements to reactants or elements to products You realize that delta h formations is what we are really looking at so, uh, so basically what happens is Look at the arrows that I've mentioned. Okay minus and plus so when you're going from reactants to products to this way You will realize that I will have to use minus delta hf of reactants. Maybe I'm going to change the color Okay, this might not be visible Yeah, so I will have to do a minus delta hf of the reactions to elements And after that I will have to do a delta plus hf from elements to products So this will be a positive one because I'm going in the direction And this will be a negative one because I'm going in the opposite direction when I move from here to here So this is another way where you can use the enthalpy of formation basically to go from reactants to products Uh through either a delta h reaction or uh in the opposite direction. Okay So this is another way where you can relate delta h formations with uh the enthalpy of reaction And uh, you can actually uh solve a few problems on those Okay, so I've I've basically mentioned multiple examples just to quickly recap. Uh, we have seen enthalpy of reaction from, uh, you know Born discreet born enthalpy. We have seen enthalpy of reaction from combustion reaction We have seen enthalpy of reaction from formation reaction. So all of this is also pretty important So you realize that from elements you're going to uh, you know, uh reactions first and from reactions You're again going to products. So all the time since you're going in the direction of reactions and products Uh, you know, you'll simply keep on adding the energy if if you go in this fashion So your your this will be negative, but this will be positive So ways to really write down different ways to really write down the energy equations Uh and put the right signs, okay So delta r of this reaction is nothing but delta h formation of products Minus delta h formation of reaction both of them Now, please note that I've used the minus sign here because from reactants to go to elements You'll have to give in energy. So that is in a different way or in the Opposite direction that I'll that uh with that of actually formation of the uh, you know reactants Okay now, uh Here's an example a simple calculation that I've I wanted to show you So let's say you have enthalpy change, you know, they've asked you to find the standard enthalpy change for the given reaction Uh, then the standard enthalpies of formation are for water for nitrogen For oxygen and for h 103 they have given obviously for oxygen it is going to be zero because it is in the elemental form and They you know, you have to find the enthalpy change of the reaction of such So what you'll do is you will basically take up the enthalpy or formation of products Now, please note that there are four moles of h 103 that is getting formed Okay, so I will have to multiply four into delta hf of h 103 Minus because now the reactants are not even formed in fact, you are breaking down the reactants into elements So I'll have to use a negative sign because I'm going to go into the opposite direction So you'll multiply twice into that of delta hf of h2o because I have twice of h2o that is getting broken down and Four of no two that is getting broken down. Okay, so four into delta hf of no two And then one of o2 this is basically zero. So I don't need to really bother about it Now, please note when I'm substituting the values of delta hf. I'm substituting it with the sign Please note for no two. It is positive. That is what they are given it to us and for H2o it is negative. So I've substituted delta hf values as it is In h3 also, it is negative. So I've substituted as it is and once you solve this equation You'll end up getting the final answer, which is nothing but minus 252 kilojoules Now what I want to do is what I would suggest this after the class in all of these questions You know, I've done two problems already one with the bond enthalpy second with Enthalpies of formation. I really want you guys to you know, pause this video You know after the class because we might not get enough time right now But if you can pause this video after the class and really look at You know how these problems were done. Just look at the question try to You know solve it yourself and then come back to really understand You know, what what what did we do and how did we solve? Okay, so so that really would work Okay, now let's Okay, now let's let's look at the next one Now this is the combustion. Okay, so we have not done the entire equation with combustion as such So we are the another example that you know, I wanted you to look at so you will see that You know enthalpy of reaction is basically from reactants to Products now you'll see when whenever we do a combustion mostly co2 and h2 is formed for organic substances Okay, so When reactants go to their oxidation products Then products also go to their oxidation products in both of these enthalpy of combustion is happening Now in delta hr will go from reactants to products So here I'm going from this direction. So this will be positive, but this will be negative So you will realize that I can write delta h equal to delta hc of reactants, which is positive Minus delta hc of products, which is actually negative Okay, so minus sign comes on the product side and therefore you will be able to realize that delta h combustion can also be taken out So, you know, just to just to show you here You know, uh, delta h combustion can also be written as delta hc of reactants minus delta hc of products Okay, so both can be really looked at here and uh, you know, we can understand Uh, you know, uh the delta h reaction from combustion reactant Uh, say again the same way, you know delta We are going to the oxidation products oxidation products are see in all of these, you know, there is one trick Oxygen products I have always taken it as the lowest enthalpy But if you really look at elements, I have taken at the highest enthalpy So automatically all my direction gets fixed whenever I take their natural enthalpy. Okay So here the enthalpies are very high and here the enthalpies are very low Okay, so when I go from products to oxidation or reactants to oxidation products The enthalpies will be automatically seen. Right now when I'm going from products to reactants I will have to go in this direction some products to reactants. I'm here I have to go all the way through this, right? So when I go from products to oxidation, uh, you will see that the combustion Uh, it comes out to be uh, now look, uh, look at the direction the rent direction that we have taken Okay, uh, I'm going from, uh, reactants to the products and I'm going from this direction So the green one is positive and the blue one is negative blue one is the combustion of oxidation and products Uh, uh, so therefore minus of delta H3 products is is mentioned Okay, so so this is this is one now, uh Here's here's another question that, uh, you know, I just wanted to show you So let's see that the, uh, you know, standard enthalpy of formation of methane is taken Uh, and standard enthalpies of combustion of carbon and hydrogen and methane is here So you can realize that, uh, graphite is what we will use as the elemental form of carbon So whenever you're we are using graphite, uh, uh, you know, the formation is of, uh, ammonia from You know, carbon as well as hydrogen Now, uh, if we use the enthalpies of combustion graphite will go to CO2 and H2O will go to H2O So if the combustion is happening of graphite, they have given the combustion enthalpies as minus 394 and minus 286 And minus 390 of CH4 Using our previous equation, which is delta H, uh, C of reactants minus delta H, C of products You'd see that reactants, uh, enthalpies can be written, uh, two into, you know, please note Again, we have written two here because there are two moles of H2 that are getting Combusted, okay, or that are getting burnt So two into minus 286, one into minus 394 And since there is only one mole of CH4 again minus of minus, uh, one into minus 1890 So you will end up getting your final answer. So again, this is reactant minus products So please note, I'm time and again, I'm saying, you know, this is one of the simplest way that you can really represent Okay, so I'm telling you not only in one way, but three different ways One way I'm showing you by these arrows, you know, remember these arrows that reactant products and oxidation products can form Remember these arrows where you can have, you know, states of enthalpies from lower higher Increase decrease and the third way is you can simply write it through the formula if you can remember the formula Any of these three states, if you are able to remember, you will end up getting the right answer Okay, now, uh, just a quick summary. So what we have seen so far is that the change in enthalpy that occurs When reactants are converted to products is the same whether the reaction occurs in one step or multiple steps That's what is Hess's law. The second thing that we, uh, understood in Hess's law was, uh, It can be used for calculating all of those enthalpies which we cannot directly really calculate or directly connect to Okay, so that's that's another way. Now when we are doing a stepwise reaction to determine an overall reaction, uh, we can actually manipulate the, uh, you know, equations depending on our convenience. For example, we are given combustion Then we'll take only combustion if you're given, uh, formation, we'll take formation, so on and so forth So by manipulating all of these, uh, you know equations, you will end up getting the final enthalpy Okay, so, uh, that's what we do. The next is, uh, at each of these steps, whenever we are manipulating, we write the right delta h So I told you that delta h not only depends on temperature and conditions, but it also depends on the number of particles It also depends on the type of particles solidification and all of that So all that put together, you will realize that the delta h values for each step is rightly written And from their overall enthalpies are calculated. Okay of the desired reaction Now let's look at one more example where we are trying to use Hess's law and, uh, you know, solve Uh, but this is a more general example so far. We are only seen through combustion or, uh, you know, formation energies This is a general example So here we are taking h3b o3 which is nothing but boric acid. It's actually getting, uh, dissociated into hb o2 plus h2o Uh, and h2b4 o7 is also Dissociate with water is actually giving four hb o2 There are three reactions that have been given to us and their delta h reactions have been given. Okay So reactions and their, uh, enthalpy change of reactions are given to us Uh, and we need to find the delta h for a new reaction Which is not a part of any of the earlier reactions. Okay Now please note, uh, this reaction is not present in the above part. So what do we do? So now we write all these reactions that are there with us We try and get these reaction through a simultaneous reaction process So this is what I can do. So I need twice of hb h3b o3 So I will multiply this by two the first reaction by two The moment I multiply by two I get two to two everywhere. Okay. Now the next is I need b2 o3 Okay, now b2 o3 is here. Okay, but it is twice of b2 o3. So therefore I will divide by half Okay, I will divide by half or multiply by half So this I I found out that delta h for this reaction, which is nothing but twice into this thing Which is minus delta h core now here because I need, uh, you know only b2 o3 I'm I'm dividing by half. Okay, so so in this process you will see that, uh, uh, I need, uh The hb o2 so I'm dividing it by two, uh, and uh, here also I'm multiplying by dividing by two So I get b2 o3 and half h2o now all of this put together I found all of these now once I add them look how it cancels out So twice hb o2 cancels with twice hb o2 h2b4 o7 cancels with h2b4 o7 and once you add all of them you end up getting your desired equation Which is nothing but twice h3b o3 plus b2 o3 now Please note in the second reaction because we wanted, uh, you know twice of hb o2 cancelled out Therefore we multiplied by half. Okay, because we've had a reaction with four, you know, uh, uh, The h hb o2 reaction is here. Yeah, we had four hb o2 reaction Also, this reaction was actually reversed I only need twice of hb o2 because here I had multiplied in the first reaction I multiplied by two So I'm reversing the reaction therefore it is not no more minus 11.3 But it is plus 11.3 at the same time I'm also dividing it by two because I need twice hb o2 to be cancelled out this twice hb o2 has to be cancelled out So reverse the reaction and then divide by two because I reversed 11.3 becomes positive 11.3 Please note it has become positive 11.3 And it has also got divided by two because I divided the reaction by two the entire energy was divided by two Similarly here also we divided it by two because I did not need to reverse or anything And then simply add all of those, you know, all of these will cancel out and you end up getting your final reaction So you simply add the energies now. So minus 0.04 plus 5.65 plus 8.75 You will end up getting a final answer of 14.36. So this is one way to really, uh, you know solve, uh, this question Okay now so this brings us to the you know end of this chapter. I'm just Uh You know, we've already done equilibrium chemical equilibrium. We have already done in the very first Uh, you know, uh chapter if you want to, you know, just ping me on whatsapp and I'll send you the video YouTube link for that. So, uh, you know, uh Just just just buzz me after after this session and I can send you the video link by the way guys You know the equilibrium class the volume was pretty bad So I had actually modified the volume and made a new video out of it Which I had also posted on the group if you feel that you still want, uh, You know that to be, uh, you know, you still want that video one more time You know, just ping me on whatsapp and I'll share the video link with you. Okay Good. So, uh, I just want to pause here for a minute and you know, if there are any questions if you feel that you I you want you want to know anything more about uh, equilibrium because that's the end of the chapter We have seen all different types of uh, enthalpies, uh reactions We have seen, uh, how thermo thermochemically, uh, equations really differ or the matter So if there's anything that you'd like me to really speak about, please feel free to ask now You know, and and then I'll quickly revise the entire chapter and then we can wind up But I'm happy to hear out your questions before we really do that So I'm just pausing for a few seconds and I'll wait for you to really respond on, uh, you know, any questions that you may have Okay, so I think most of you, uh, it seems So let's let's quickly look at, uh, uh, you know, what what we've understood today So we have understood what are what were, uh, you know exothermic and endothermic reactions Uh, we have also understood the reasons for standard enthalpy changes Please recall all the definitions You know, you cannot actually say, you know, yes or no on this You know, if you have really understood this chapter well and this presentation well You should be able to do all of these things that I've written here and you can mention whether it's an yes or no Based on these two, okay? Now, uh, so we have understood what are the reasons for different changes in enthalpies and what does enthalpy really depend upon How is it connected to the first law of thermodynamics as well as the change in heat and the change in work? Uh, we have also understood how to write equations of enthalpy of combustion enthalpy of formation Uh, you know, how to recall and apply Hess's law how to also recall the definition of bond dissociation enthalpy Yeah, I mean how to how to really write bond dissociation enthalpy we've also seen standard enthalpy changes using bond enthalpy values and You know standard enthalpy changes of formation and combustion Okay, so so that's a quick, uh, you know aspect of all the enthalpy changes and energy changes in Uh, you know in in chemical reaction Uh, good. So, uh, nice, you know, I can still see that a few of you are there Some of you have already left. Uh, if there's anything, you know, I'm available. You can always bust me Uh, else, you know, we can uh, call it a you know, we can we can close on the session for now Uh, please let me know. Thank you so much if I'm I'm here for the next few Few minutes if you feel that there's something that you would like to really speak about we can do that Or else we'll close it close close up on this Okay, so thank you so much guys. Uh, I wish you all the best, uh, you know, write the answers well And I'll see you soon on the on the next session Thank you. Bye. Thanks. Thanks. Uh, Aditi. Thanks for now. Thank you so much guys