 Okay, so now we are going to start the next part in this chapter, that is, that's continuity. Okay, this is the last part of thermodynamics, and then one more section we have in this that is thermochemistry. This is not that big, we can finish this in one class, max to max one class, not more than that, two to three years, anytime you can. So, spontaneity means what? Okay, heading all of you write down spontaneous and non spontaneous process. Yeah, heading all of you write down spontaneous and non spontaneous process. What is spontaneous process, spontaneous process is the process which happens on its own, or after a proper initiation. Correct, it doesn't require a continuous support. Okay, I'll give you the definition here. First try and understand spontaneous process are all those processes which happens either on its own, or after a proper initiation like for example you see you have been in your hand you just leave the pen right it will go down. It is a spontaneous process. The flow of water down the hill is spontaneous process diffusion of gas from high pressure to low pressure spontaneous process flow of current high voltage to high voltage to high no potential to low potential spontaneous process. Correct. All these processes are what spontaneous because it doesn't require any support external support a continuous external support. For example you see a burning of fuel, burning of fuel is also an spontaneous process because because it just require an initiation just spark it, it will burn right so continuous support is not required. It's spontaneous process. It is burning, right, and when you cover the candle with a, any, you know, box or any glass right, what happens after sometime, the candle extinguished right, it won't burn for so long. Why, why, why, why does this happen, because the flow of oxygen is hindered in that case. Right, whatever the oxygen is present in that particular volume. That oxygen gets consumed and once it's done, the candle extinguished. So what is the condition for the continuous burning of candle. The condition is the continuous flow of oxygen or air you can say. So that is what that is a non is spontaneous process. Okay, it requires a continuous support, non is spontaneous process. All the process you reverse like diffusion of gas from low pressure to high pressure. Continuous support is required, you have to push the gas, right continuous support is required. That is non is spontaneous process. Okay, so like this we define spontaneous and non spontaneous process understood all of you. Right definition you write down spontaneous process. One second. Okay, so spontaneous process definition you write down a process which has a natural tendency to occur. A natural tendency to occur by its own by its own or after or after a proper initiation initiation may just you need to initiate the process and then leave it proper initiation under a given set of condition that is spontaneous process. Many examples we have flow of water down the hill flow of water down the hill flow of electricity flow of electricity, high potential to low potential spontaneous process burning of fuel of fuel. Spontaneous process. Okay, this requires initiation spontaneous process, resting of iron, resting of iron is spontaneous process. Okay, neutralization reaction is spontaneous process. All these are the example of spontaneous process. You know you write down all natural processes, all natural processes, all natural processes are spontaneous and irreversible in nature, all natural processes are spontaneous and irreversible in nature. Correct. Next one is non spontaneous process, non spontaneous process, write down a process which can, which can neither occur, neither occur by itself, a process which can neither occur by itself, nor by, nor by initiation, initiation is called non spontaneous process, non spontaneous process. Next slide and write down. It doesn't mean, it doesn't mean that non spontaneous process, non spontaneous process does not happen at all, does not happen at all. It is possible with continuous external support, continuous external support. Copy this down. Correct. What is the example of this, the example of this, I'll give the best example you can understand. Few days back we were doing hydrogen, right, the preparation of hydrogen, right, we have done one process that is electrolysis of water remember. Yes, electrolysis of water in which hydrogen discharge at cathode and oxygen discharge at anode, right, so electrolysis of water, what it, what it requires, it requires continuous support of electricity from outside. We need to connect the electrode with the external source of voltage, you know. Yes. So, since it requires continuous support, the process is non spontaneous. Okay, you remove that the voltage source, the process won't happen, right, it will stop. That's why electrolysis is what the non spontaneous process, right, flow of heat from hot to cold body, flow of heat from cold to hot, cold to hot body, non spontaneous process. Like, like this we have many examples of non spontaneous process. Which process are you at what stage, which process you're saying electrolysis. You see, we don't say when after what time the process will get complete simple thing. In the solution. If there is no more H plus or O minus OH minus and present, then there is no movement of ions towards the electrode, and hence no oxidation or reduction. So, the moment we have that H plus and OH minus and the process will be there. And after that the process won't happen. We have some more things into that slowly what happens. These are the concepts you will study in electrochemistry was great. Slowly what happens on the electrodes there is accumulation of some compounds, which are basically impurities. So slowly it, you know, it affects the working of electrode. And after some time, it won't work radioactive decay is spontaneous. And they say not all but like the one which is required only initiation, some kind of support those decays are spontaneous process. It depends upon the process what you are talking about. Okay, definition we have, if this particular things fits into that particular definition that it is a spontaneous process. I think one it's once it starts, it goes on right. So it is a spontaneous process. Now you see, this is just the definition, obviously the definition they won't ask you why we have done this and what is the use of this that we need to understand. See, when we talk about actually one note, one point you write down first, yeah, write down. It has been observed that it has been observed that that the most spontaneous process, most of the spontaneous process. Most of the most of the spontaneous process or chemical reactions, chemical reactions I'll write down chemical reactions are exothermic, not all most of them. Now what things what what we can conclude from this particular information. I'll give you one example. When reacts with O2 gas. It converts into what H2O liquid, and it is an exothermic process spontaneous. Carbon solid reacts with O2, reacts with O2 gas, it is CO2 gas exothermic process. So all these processes are spontaneous in nature. So can we say the condition for spontaneity is delta H less than zero. So since most of the reactions are exothermic in nature, so we can say this is the condition we have that for all those reactions where delta H is less than zero, the reaction is said to be spontaneous. Okay, hold on here. Right. Now I'll give you some more example. See this H2O liquid H2O gas. I'm talking about the process of evaporation. Evaporation process is spontaneous or not. We know water liquid converts into gas easily no. You have one glass of water you leave it at the room temperature for 10 days. You'll see after 10 days the level of water is decreasing in the glass. That is the evaporation process. You all know evaporation and boiling what is the difference. Evaporation is an spontaneous process. Right. It happens at the normal temperature that we have, but boiling happens at a specific temperature like for water is 100 degrees Celsius. Boiling requires continuous supply of heat. Right. The boiling is not spontaneous, but evaporation is spontaneous. Right. So if you just try to understand the thing here, how this liquid converts into gas. When it absorbs energy from surroundings. Yes or no. So this is a kind of endothermic process kind of endothermic process. Yes or no. Respond guys. If you talk about H2O solid, you take ice from the fridge and leave it at the room temperature. What happens then it converts into liquid. Spontaneous or not. Are we doing something? Are we, you know, supporting this ice in any ways so that it converts into liquid. No. Correct. So this is also a spontaneous process. One more information I'm giving you carbon, sorry, calcium carbonate solid. You heat this, you supply energy into this. It converts into CAO solid and carbon dioxide gas releases. That is always there as it does environmental effect. We are ignoring because it will be there on all the processes. Okay. In that way, we can think. Yeah. What is non-spontaneous? Okay. No, it is not also a non-spontaneous. It is not an non-spontaneous process. This is also a spontaneous process. Okay. It is also a spontaneous process. We are just heating it. That is what I'm trying to make you understand. See, we can easily understand that when energy decreases, the system is going towards stability and hence it is spontaneous. Just a second model, just a second. We haven't done it. We haven't finished it. Wait. What I'm telling you that for all those processes in which delta H is less than zero or exothermic processes, we can easily say the process is spontaneous because we are going towards a lower energy state because some energy is going out. We are going to a lower energy state. We are going more stable state. It is, and hence it is spontaneous. We can say that happily it will go into the more stable state. Right. So that's a natural tendency we have. Correct. Okay. But what about these processes? How do we differ? I'm just giving you this information. I'm not asking you. I'm not asking you whether it is non-spontaneous or spontaneous. I'm telling you this is an spontaneous process. So first let us take this as it is. It is spontaneous. And that is what I'm trying to explain that what is that factor which makes these processes spontaneous. For exothermic too, we can understand it is going towards lower energy state, more stable state. There's a natural tendency. So it's spontaneous it is. But what about these processes here? The energy is increasing. No. Solid to liquid, liquid to gas. Solid to gas. Energy is increasing. We're providing energy into it. But then also this process is spontaneous process. Right. These are also spontaneous process. So point I'm trying to make is only delta H is not the criteria for a process to be an spontaneous process. Would you agree with me on this? There must be something else. There must be some other factor. Yes. Would you agree with me on this? All of you respond correct. Fine. So this is the one thing we have right that delta H obviously it is a condition less than zero. But we have few examples in which delta H is not less than zero, but still the process is spontaneous. No, wait, wait, wait, just a second. Just a second. So we have one more factor over here. Let's not think about the condition for spontaneity. I will come to that point. Okay. I'm just trying to make you understand what are the possible conditions we have. So we'll discuss all and then we'll see the overall effect. Okay. So delta H obviously one of the criteria we have, but this is not the only criteria because we have some reactions in which delta H is not less than zero, but it still is spontaneous. Right. So what is the other factor? The other factor that we define here is randomness. Have you heard this term? Randomness or disorderness. Correct. Randomness is what? How do we say it? Anything like if the, yes, correct. I'm coming to that point. So randomness is what? The tendency of the particles. Yes, the degree of randomness also you can say. The tendency of the particles to move randomly, how random the particles can move. If there is random motion, there's no pattern here and there like it is moving random motion, then the process, it's said to be spontaneous. Not only. Again, one of the factor we have. Correct. So randomness is the tendency for a particle to move random in all possible direction. Right. So this is the another factor we have apart from delta H, which should be less than zero randomness is the another factor. Now, how do we measure randomness? Now to measure randomness, we have given a new thermodynamic term that is entropy. That is entropy. Entropy we use to measure randomness of any substance. If the entropy of substance A is more than to that of B, we'll say A has more random motion. Are you getting? Tell me. See how the topics, you know, you need to understand how it is connected. Otherwise, randomly, if I pick over directly, I'll take entropy. I won't tell you the history behind it. You won't be able to connect things. Okay. So entropy is again, a new thermodynamic term defined, okay, which is represented by S. And why do we define it to measure? To measure what? Randomness to measure randomness or disorderness, disorderness. Right on the definition next. Randomness and entropy definition right now. It is a thermodynamic state quantity. It is again a state quantity. Okay, like internal energy enthalpy and all, right? It is a thermodynamic state quantity, which gives the, it is a thermodynamic state quantity, which gives the degree of disorderness or randomness of the molecules of the system. What did you write? It is a thermodynamic state quantity, right, which, which gives us or which measures the degree of randomness or disorderness of the molecules of the system. Let me explain. It is an extensive property. It is an extensive property and depends upon the, sorry, extensive property and state function. It is an extensive property and state function depends upon initial and final state. So delta S if you need to find out, it is final entropy minus initial entropy. This is what the change in entropy we have. It is expressed at unit we have joule per Kelvin or calorie per Kelvin. Now, if you think of any liquid molecules, SL, excuse me, is the entropy of the liquid molecules. Okay, SL is the entropy of the liquid molecule. Entropy of liquid molecule is more than to that of solid. Can we say that because solid particles are rigid? Okay, they cannot move. Entropy is more. Similarly, gas entropy is more than to that of liquid. Or overall if I write down the entropy of gaseous molecules is maximum, then entropy of liquid, then entropy of solid. Are you getting it? More entropy means more randomness. No, it's not. Heat capacity is not this. Heat capacity we discussed no. CPCV. Unit is same. Maybe you are, yeah, dimension wise you can say, but it, the function of this is different. Right, we also define this per mole also, joule per mole Kelvin, joule per mole Kelvin. No, wait, wait. Wait, wait, just a second, wait. Entropy depends upon something else, wait. Yeah, now write down as temperature increases, as temperature increases, because mostly we deal with gaseous. So, kinetic energy of the gaseous increases, we have done this in gaseous state, temperature, kinetic energy proportional. So, kinetic energy increases means randomness increases. The molecules will have random motion, randomness increases, means entropy increases. This statement you write down. Mathematically, mathematically, it is defined as defined as the integral of, integral of all terms, all terms involved in. In the heat exchange, heat exchange divided by, by the absolute temperature in a reversible, in a reversible isothermal process, reversible isothermal process. So, the change in entropy ds is equal, this is the fact we have, okay, this information is fact. Let's take it as this, dq reversible, q reversible, reversible process heat exchange. So, the change in entropy is equals to, we have q reversible by temperature. This is the mathematical definition we have of entropy change. One note you write down, any process has natural tendency to move towards more randomness. Any process has natural tendency to move towards more randomness, okay. So, what happens, yato will have this one, delta h less than zero, going towards stability, it will happen happily, right, spontaneous, or in the process delta s must be greater than zero, entropy must increase. So, if this condition is satisfied, or this condition is satisfied for the spontaneous process. So, it has been observed that the process goes in that direction only where the entropy is increasing, randomness is increasing, okay. Now, look at this example, this example that we had discussed here you see, you see this, liquid to gas, we know gases particles has more randomness, right, that's why the process is spontaneous over here. Gas to liquid, liquid has more random, you know, more entropy than this is spontaneous in nature. Here you see it is solid. This is also solid, but this is gas. Since we have the gases particles this side on the product side, so the reaction has the natural tendency to move towards the product side. So, what happens, correct, in the process is spontaneous in nature. Did you understand this, right, so either you'll think of enthalpy change delta h should be less than zero, or you will think of entropy change delta s should be greater than zero. So, we have basically till now, till now we have two criteria for spontaneity. What are those criteria, we have two criteria for spontaneity, one is enthalpy change, other one is entropy change. Later on, we'll also see that instead of considering two factors, we'll combine these two factors and we'll give a new thermodynamic term in order to understand the spontaneous behavior of a process, because both are temperature dependent process. So, both process with temperature will have, will combine and we'll get a new thermodynamic term that we'll see later on and the reason also we'll see why we do that. Okay, let's discuss few things about entropy, because entropy again is a major portion over here, you'll get questions on this entropy based questions so we'll discuss few things about entropy and then we'll move on to the other things. Okay, see, on the basis of few conditions, we will be discussing this, see what condition I'm taking the first. First is, for an spontaneous, for an spontaneous isolated system, for an spontaneous isolated system. So, since the system is isolated, right, there's no interaction with spontaneous process, I'm sorry, for an spontaneous process in isolated system. It's spontaneous process in isolated system. So we have isolated system. And what is isolated system we know isolated system does not interact with surroundings at all. Right. So obviously between system and surrounding there is no interaction. Right. But if the isolated system consists of gases. This is the isolated system we have. If it consists of gases. And if there is intermixing of gases here, then the entropy change will be positive. Why this happens, if there is intermixing of gases. It cannot interact with, it cannot interact with the surroundings is still the entropy may increase if the gases are interacting with each other. The gases are mixing. Okay, so that is a commission if the gases are not mixing, then delta S will be zero. Okay, now you see, if the system is not isolated is not isolated. Then we have two possibilities. What either the system will gain heat from surroundings, or it will lose heat to the surrounding, right, it will give heat to the surrounding. Correct. Two possibilities we have. If the system gains heat from surrounding, its entropy increases, and the surroundings entropy decreases. Right. So like this we see, we have a system it gains heat from surrounding. So what we can say delta S of system. This will be positive because it is gaining heat from surrounding. Okay. And by the value delta S of the system increases with the same value delta S of surrounding will decrease. And the total change in entropy if you see delta S total delta S total is equals to delta S of the system plus delta S of the surroundings. Okay, so system entropy will decrease will increase surrounding entropy will decrease. So there will be a competition between the two. The two processes are opposite right system is taking heat surrounding is losing heat, right, all these things we have. So right down to this the next point, during the spontaneous process, during the spontaneous process, the entropy of the system, during a spontaneous process, the entropy of the system, increasing, the entropy of the system, sorry, entropy of the system increases till the system attends equilibrium state, till the system attends equilibrium state. Hence at equilibrium, the entropy is maximum. So entropy keeps on increasing till the equilibrium is set. And at the equilibrium state, it becomes constant, it won't change further. Okay, hence right down the next time. So at equilibrium, the entropy of the system is maximum. The entropy of the system is maximum. Hence, there is no further. Maximum and there is no further increase in entropy. So since at equilibrium, the entropy becomes constant. So delta S at equilibrium is equals to zero. For non-spontaneous process, for non-spontaneous process, delta S is less than zero. It decreases in non-spontaneous process. So there are a few things you need to understand for entropy. Now we are considering a situation here, like in which we have an isothermal reversible process. Isothermal reversible process. Okay, isothermal. So we have to constant temperature. What we are assuming, suppose the system absorbs Q amount of heat, you can assume anything, I'm assuming this, Q amount of heat, anything in the sense I'll tell you what, Q amount of heat from surroundings, surroundings at temperature T. Okay, you can also assume the system releases Q amount of heat, right, at temperature T. I am assuming absorbs both we can assume. Right, so system is absorbing heat. So what is the change in entropy of the system. It is plus Q by T. Yes or no definition we haven't entropy definition is this only plus Q by T. What about surrounding? Could you tell me surrounding entropy decreases or increases surrounding entropy decreases by same value right Q by T minus Q by T, because surrounding is losing heat and system is getting it. So what is the total change in entropy? Delta S total. This would be equals to delta S of the system plus delta S of the surroundings, which is nothing but plus Q by T plus minus Q by T, which is zero. So delta S total for a reversible isothermal process is zero. The change, total change in entropy for a reversible isothermal process is equals to zero. So we have done half of the discussion of entropy. We have few more things left in this chapter. Okay, that will do in the next class, because we don't have time we cannot start a new thing. If it is, if it is reversible then DS total, if it is reversible then I have given you just a general thing over here that total entropy change would be equals to system plus surrounding. Now we are applying condition into this, right? System is not isolated. So system surrounding interacts, then this kind of possibility we have total entropy change is this. Now depending upon the condition, what are the things we'll get that we are seeing. See in irreversible process, there will be change in entropy, right? And we cannot count them. See, reversible is kind of ideal, no. So system will lose heat and surrounding will gain. We are not talking about the heat content over there. It is also possible the system lose, absorbs, you know, heat and some heat is contained in the system. Right, that is what we are not talking about. In irreversible process it is, it is different. Okay, why it is different? Because it depends upon, first of all, the system is at what temperature and surrounding is at what temperature. Right, isothermal process we have constant temperature everywhere. Right, then also we have exchange of randomness because of the motion of the particles. In irreversible process generally we have two different temperature and in that what happens we'll have a small derivation for that, that we'll discuss next class. Okay, we'll have a small derivation and we'll see the delta S of the process in irreversible, a process if it is there, then entropy of the system always increases. Right, entropy always increases. That's why I have given you one term there that all naturally occurring process are what? All natural occurring process are irreversible and spontaneous. Right, so naturally whatever process is happening is irreversible and hence the entropy of the universe is continuously increasing because irreversible. So for irreversible, change in entropy is always positive, how it is positive, we'll have a small derivation of that, we'll discuss that next class. Okay, understood. Tell me guys, first, 16th one. One second. Yes, tell me. See, we are not considering that because we cannot calculate that two cylinder A and B, when the expansion is there, we are not assuming that the energy is getting lost in this process, that's how we are conserving the energy. In actual scenario, if you see, then yes, it is possible in the form of heat due to friction, some amount of energy will last. Right, but here we are not considering that we are considering ideal condition ideal case where there is no loss of energy. Yeah, work done for expanding whatever work is done by the system, but there is no loss of energy in that process. There is no loss of energy if you consider, then we cannot apply all these expression that we have delta h delta u because everything is based upon the no, the conservation of energy. That's why you see whenever we have piston cylinder system, we always have friction less piston. In physics you may get some friction but we have some terms over there with by which you can calculate, but mostly here we'll get friction less system. Or there is no loss of energy due to heat or friction. That is what we consider. Correct. So heat loss you don't consider. Okay, you won't get that question. It's not like we do not calculate, but it's not there in our syllabus now. Fine guys, see you in the next class. Okay, I'll share the assignment on this also. Yeah, bye. Thank you. Take care.