Free Chemistry NEET Video Lectures for Chemical Thermodynamics

00:00:01 [Music] [Music] Hello, my name is Debbakar Dhara. I belong to chemistry department of IIT Karapur and I'll be teaching you the unit on thermodynamics. So if in this uh lecture in the probably in the first two lecture first two hours of this unit we'll talk about uh essential concept and definitions and then heat we'll talk about introduce the concept of heat work energy and internal energy and then we'll talk about first law of thermodynamics and then we'll go in the calculation of

00:00:57 work heat in different processes for an ideal gas and we'll talk about enthalpy and heat capacity and we'll then talk about determination of experimental determination of delu and delh we'll come to those terms in a minute so this is just a basic outline of the first two lectures and uh obviously we'll move to other lectures I'll show you the content of the other lectures when it time comes now let's begin with some pictures you know you So this is now it's a winter time here and you can see

00:01:35 people are trying to get some heat just burning some leaves here. So it what is happening here is the chemical energy stores in these leaves they're basically reacting with oxygen and getting burned and in that process it is producing some heat and of course some light also will be there. So in this case the chemical energy is convert getting converted into heat energy and light energy. In this picture you see cars are running and most of the cars still run on fuel petroleum or diesel. And what happen in this case

00:02:17 the petroleum petrol or diesel get burned in the engine and as a result you this mechanical energy the car starts running. So this is the example where chemical energy is getting converted into mechanical energy. Now of course you can argue that there are cars nowadays which runs on battery. So this is just a picture of one battery and so what happens battery basically because of the chemical reactions in the uh battery electrons are pumped uh in the outside circuit. So as a result you get electricity.

00:03:03 So in this case you get uh you basically getting uh electrical energy from chemical energy. If it is a chargeable rechargeable battery then you can charge it again by applying electrical energy from outside and uh getting back the chemical reaction. So basically while charging you are converting electrical energy to chemical energy and while discharging you are uh basically converting the chemical energy into electrical energy. So these basically are examples to show you or convince you that probably you know already all these

00:03:42 things that energy are interconvertible. So you can actually convert one form of energy into the other form. And thermodynamics is uh the the part of science which deals with uh the intercon conversation of energy and in broad sense uh it deals with uh macroscopic systems and uh in this unit we'll talk about only the systems which are at equilibrium and the properties uh of systems which are at equilibrium which means We'll deal with only equilibrium properties of this of the systems in this in this unit.

00:04:28 Now, thermodynamics was uh first formulated by physicist and engineers uh when dealing with the efficiency of uh steam engines. But uh it is it is of extreme importance or immense help for both chemist and biologist. You know while one hand it basically explain or deals with the uh energy output from a chemical reaction. On the other hand, it basically forms the basically helps in explaining or answering the questions which are center or at the heart of biological sciences like for example how the energy gets uh

00:05:16 transmitted through the biological cells or how the large macroolelecules get assembled within a small volume of cells. So these are the the main questions the biological science poses also had uh to be answered by from the knowledge of thermodynamics. So basically you know that thermodynamics is a very important subject and we should learn thermodynamics. So if we just write once more that uh and in this unit as I said we'll be dealing with the equilibrium systems and we'll we will not consider Ed systems having

00:06:14 very few molecules. We talking about systems which have great number of or many molecules. So this thermodynamics is does not apply for systems which has very few molecules. But it applicable only on systems which had large number of molecules. So if I just write we'll talk about systems containing large number of molecules. So that we are talking about a macroscopic macroscopic system not uh a microscopic uh systems. We are talking about microscopic systems. Now we have used these terms uh quite a few time or

00:06:57 talked about this terms systems. Now obviously what is systems or what is a system? system is uh uh the part of the universe which is of our interest. Our interest means which is of our interest in that moment. For for example, if I uh take a beaker, if I just take a beaker and uh put some water in it and we consider this is the system. So, so the beaker with the water uh in it is consist of is uh made as make the system and everything outside the beaker is what we call surroundings. So basically when you are doing a say

00:08:05 we're doing a reaction in a round bottom flask we the flask containing the round bottom flask uh the reactants and the products are basically our system and everything else which is not uh belong to the system we call surroundings and uh systems could be of uh different types. The main three types of system we'll talk about is uh open system. A open system where a system can interact or exchange matter and energy with surroundings. So basically open system can exchange I we should write exchange here

00:09:12 can exchange matter and energy with uh surrounding. So the example we talked about the beaker or say a conical flask. If we can draw a conical flask here and we're taking some reactants here and this this is not tight properly. it it is open then it actually can exchange the matter the molecules which in the system which are in the system and of course it can exchange energies uh with surroundings. So this is a example of uh open system. If you consider our own body myself as a system then obviously we exchange matter

00:10:12 and and energy with surroundings. So if you consider a human body is a open system example of a open system obviously second is a closed system. In a closed system, the system can exchange can exchange energy with surroundings. But it cannot any matter with surroundings. So basically if I had this uh conical flask and we just laid it properly airtight so that no molecules can go in and out from this system. Then this system cannot exchange with any matter with the surroundings but because it can exchange

00:11:37 energy with system and surroundings. So that would be the example of a closed system. The third system we will talk about is an isolated system. Uh obviously this is the third third category where no exchange of energy and matter. is allowed between the system and the surroundings. Obviously a closed system which or or other way it's an isolated system which is obviously has to be a closed system because it's does not allow uh the matter to go in and out but obviously reverse is not true all closed

00:13:11 systems are not isolated systems. So basically all isoed systems are closed system but not all is closed system are isolated system and we'll mostly in in this unit we'll mostly deal with the closed system one once once more uh closed systems uh are systems where uh The system is not allowed to exchange uh matter with surroundings but it can exchange uh energy with the surroundings. And a system is uh separated from the surroundings by what we call walls or boundaries. So basically we can write a system

00:14:33 is separated from surrounding. rings by various kind of walls. Walls or you can call boundaries as well. So what are the different types of boundaries or walls? First is first type is uh rigid or non- rigid. Non is nothing but movable boundaries. If I have say a cylinder with a piston, a gas with a piston. Now this can be this boundary if it is not fixed. If piston is not fixed anywhere here, the volume of this gas can change by movement of this piston. So in this case this piston surface is the boundary between the

00:16:09 system and the surroundings which is movable. So that case we'll call this as a non-rigid boundary or movable boundary. If this is fixed, if I fix it somewhere like I can put a screw here in this case and we can make this piston fixed here so that this is now this has the piston has now become non-movable and obviously these boundaries are or walls are fixed so they are not movable either. So in this case the volume of this system cannot be changed. it has a fixed volume. So in this case the boundary we call this as a rigid

00:16:58 boundary. So if the boundary is movable by which it can change the volume of the system then we call this is a non-rigid boundary or wall. And if the boundary between the system and surroundings is not movable, which means that the volume cannot be changed. The system's volume cannot be changed by changing the position of the wall. In that case, the boundary of the wall is called rigid boundary. The second type is called permeable or impermeable. Obviously if this boundary has say this piston is porous

00:18:01 which means the gas inside is allowed to move out to the system or the gas or something from outside can come inside the system through these boundaries. Then we call this boundary as permeable boundary. And if the boundary does not allow any exchange of matter between system and the surroundings then we call impermeable boundary. So obviously this is case if it is a system is surrounded by a impermeable boundary then obviously it's a closed system because this does not allow a matter to move uh uh move in or move out

00:18:55 from system. Similarly, if it is uh if the system is surrounded by a permeable uh wall, then uh obviously the system is a open system because the matter can exchange between system and surroundings. The third type we call we're talking about these are walls is adiabetic or nonadiabetic. Nonadabetic is sometimes called dia thermal or dithermic walls as well. Now in this case if the boundary between system and surroundings allows exchange of heat between system and surroundings. Then we call that is a non-aromatic

00:20:06 or dithermal boundary which is basically in this case the boundary consist of thermally conductive material. But if the system is surrounded by a boundary which does not allow any heat exchange between the system and surroundings. Then we call the boundary or the wall as a adiabetic wall. Adiabetic wall is very difficult to achieve in practice. Uh the most close example we have those uh flask thermoflask where we have actually a double walled uh boundary with nearly vacuum inside. So basically that prevent

00:20:55 uh almost prevent exchange of heat between system and surrounding so that you can keep your drinks or whatever material in the thermal flask at at higher temperature or lower temperature for a long time. So that's the closest example of adiabetic boundary we have. Now obviously ideatic boundary will not allow something to permit the matters to go in out. So ideetic boundary will the system surrounded by adabetic boundary or walls has to be closed system and system surrounded by impermeable walls has to be closed system also.

00:21:49 If the system is surrounded by permeable boundary then it's a open system and if a system is surrounded by all three like a system which is surrounded by rigid impermeable and adiabetic walls then what do we call it? We cannot exchange because rigid this by moving the boundary in and out by changing the volume we can exchange energy we can exchange energy either by heat changing the like exchanging heat between system and surroundings. So if you are closing the system with rigid and adiabetic that mean there is

00:22:50 no exchange of energy possible and of course if you are impermeable me boundary then there is no question of matter being passed. So in this case matter and exchange matter and energy is prohibited for exchange between system and surroundings. As a result, we call this as an an isolated system. So as an an isolated system would be surrounded by rigid uh impermeable and ariabetic boundaries. So we talked about system and we talked about walls. Now how do we describe a system? Now a system when you you describe a

00:23:42 system you have to specify the values of several properties. Like for example, if I want to describe a system, we have to specify the equilibrium pressure, equilibrium temperature, volume, composition. So these are the some of the properties the values of which the values of which need to be specified or mentioned. Then only we'll be able to describe the system and once we do that we call that a state of the system. So the state of the or thermodynamic state of the system is specified by or described by specifying the value of

00:25:03 the properties of the system and as I told at the beginning that in this case we'll be in this unit or in this thermonomic uh course we'll be dealing with equilibrium properties. So when you talk about pressure, temperature, volume, composition, we are talking about the equilibrium pressure, equilibrium value of the pressure, equilibrium value of the temperature, equilibrium volume of the uh system and so on. So if we have two system so I have a water here a bottle of water uh in this case which have say 25°

00:25:43 centigrade and I have a volume or mass is I know what is the mass and I know what is the temperature I I know what is the pressure inside. Now if I have another bottle which has have same amount of water with same amount of volume and temperature is same pressure is same then we call that these two are of same thermodynamic state. So basically thermodynamic state if I consider this as a system then the obviously the bottle outside will be the boundary and to specify this uh system or describe the system I have to tell

00:26:25 how much amount of what is the amount of water in it what is the temperature what is the pressure and obviously if you mention these three the fourth one the volume will obviously be linked with uh the fourth. So we don't always need to specify all the volumes because sometimes they are linked with each other. For example, if you you know that ideal gas they are linked with the the pressure, volume, temperature and the number of moles are linked with this. So if you know three of them n, t and v

00:27:13 then you will be know able to know the fourth one. So you don't have to mention all the time all the thermodyamic properties because sum of the ther value of the sum of the thermodyic properties can be obtained from the relations between them and these are called the relations between the thermodynamic properties are called uh equation of states. Now these states or these value these properties are called also state variables. What is state variable? What are state variables? For example, the values of pressure,

00:27:55 volume, temperature you don't have to if you mention what is the value that is enough. So we don't have to tell the history you know how the pressure is achieved or how the volume is achieved how the temperature in this case is reached. It does not matter the history of the system. It will only the present value with dictate uh the system or describe the system. So these are called state variables. So if I take again the bottle of water and talk about the pressure inside is one atmospheric

00:28:32 pressure, temperature is 25° centiggrade and volume and so and so. So I don't need to mention that whether the water was obtained from melting ice or water was obtained by condensation condensing the vapor. It does not matter as long as I mention the the present value of the temperature, volume, pressure, then this system is fully described. Which means that these are called state variables. So the v the the value of the state variables do not depends on the history of the system. It depends on the present

00:29:09 value only. So these are called state variables. We have some other v some other classification of variables. One is extensive extensive variables or extensive parameters another intensive parameters or variables. Now extensive var variables depends on the size of the system which means if you double the size of the system the the uh the value of the variable will double. For example, the volume. If I increase the size, increase the mass. If I have this water bottle again, if I increase the or double the amount of

00:30:19 water keeping the pressure temperature same, then the volume of the water will be doubled. So volume in this case is extensive quantity or extensive parameter. How do you call all are equal and basically the total volume of this bottle can be obtained by summing up the volumes of each part of this bottle. So the value of any extensive variables can be obtained by summing up the value of that particular variables in all part of the system. On the other hand, intensive variables they do not intensive variables do not

00:31:10 depends on the depend on the size of the system. Generally the value of intensive variables are obtained at any point of the system. If I want to get a temperature of this bottle of water then I can measure the temperature on the top or I can measure the temperature on the bottom it should get the same value of the temperature. So it does not matter I have a half bottle of water I have a full bottle of water temperature would be same. So in this case the the value of temperature in this case it does not depend on the size

00:31:54 of the uh system. Now some systems may contain different phases. What is it called phases? See if I talk about one term density. What is density? is extensive quantity or ext intensive quantity. Obviously density of uh a system will depend on what will depend on the size or it will depend on it will not depend on the size of the if I talk about what is the density of water in this bottle. It obviously does not depend on the size. So density is in intense in insane in intensive variable property. Now if I add some sugar in it,

00:32:46 keep on adding, what will at some point of time the water will get saturated and there will be sugar lying on the bottom of this bottle. So obviously this the system will have both water and the sugar which is uh in the bottom lying on the bottom. So now if I want to find out density in different parts of this system then obviously there will be different value for like in the solution there will be one value and the sugar which was lying which is lying on the bottom of this bottle which will have different value.

00:33:28 So in some cases if the system does not have same value for all the intensive variables across the system then we call that as a heterogeneous system. Obviously pure water will be homogeneous because the value of density of all the intensive variables value of all the intensive variables same across the system. For the example I have I just given you where some sugar molecules are sugar are lying bottom of the bottle after the saturation is reached then you'll have different value of density

00:34:12 in the solution and the sugar sugar. So though this is example of heterogeneous uh heterogeneous system and the solution part and the sugar part will will mention at different phase. So there's one phase which is the solution of sugar in water and other would be the solid sugar phase. So that's a heterogeneous system contains more than one phase and if the uh system is contains only one phase basically uh the the value of the all the intensive properties are same across the system then we call as a

00:34:53 homogeneous system. So I just uh gave you example of two types. One is a heterogeneous system and homogeneous system. So we know what is uh the thermodynamic state of a system. Now if we change if we say if we So we have a state one values are say P1, T1, V1 with some n is the value of number of moles amount of substance and we change to we change the value of say pressure and temperature P2 and T2 and say volume also and without changing the number of moles. So if you change it will be a new state

00:36:07 of that system and how the change is brought about that is called process. you know a process by which the state of a or thermodynamic state of a system is changed and there are many different types of process possible and I'll just try to name few of them for example we can talk about isothermal process isothermal process where temperature is fixed throughout the process all the time. Not that the initial temperature and the final temperature is fixed. Isothermal temperature is isothermal process is the process where

00:37:12 temperature is fixed during entire duration of the process. Okay. Not that only Initial temperature is same as the final temperature. Similarly, isobaric process in this case pressure is fixed throughout the process. Again it's not only the initial pressure and final pressure is fixed. The pressure is fixed during entire process isocodic process where volume is fixed throughout the process. Now if there is if the process there's a process under uh happening the state one is getting changed to state two without any heat

00:38:30 exchange that means the system is surrounded by ideiabetic wall. So the and there's a process going on inside the system which means the system will and there will no heat exchange between system and surroundings. In that case we call that process as adiabetic process. Adiabetic process are the processes where processes taking place in the systems surrounded by adiabetic wall. Obviously there are other names like cyclic process where the initial state and the final state of the system is same.

00:39:18 So there are the many other possible names which uh will come across uh when required. So we talked about different processes what you know what is a processes and then we talked we talked about isothermal process isobotic process is process and adiabetic process. Now let's come back and talked about the system we discussed earlier. Let's talk about again that uh cylinder with a containing a gas and a piston. This this we consider a frictionless piston so that when it moves there is no friction associated uh in the walls. So

00:40:12 there is no energy exchanged during the friction of uh the piston and the system. Now how many different ways a system can exchange energy with surroundings? Say this is a dithermal wall. So we can exchange heat and this is a movable piston. Now if I place this system in a bath which is slightly higher temperature then heat will come in mean there will be heat exchange between surroundings to system. So heat will come in as a result the volume will get expanded. So there is two type of exchanges

00:41:09 possible. One is heat exchange between system and surroundings and second one we are talking about the volume change and we call mechanical exchange. Mechanical exchange is nothing but the exchange of energy between system and surroundings because of the movement of the boundary between system and surroundings. Now if I consider that this is fixed now this boundary is fixed not movable or rigid boundary and then again we heat it up in that case there will be no volume ex expansion or no volume increase.

00:42:15 So in that case only heat exchange is happening between system and surroundings. In the third case, if I have say this wall is a adabetic wall which prevents any exchange between system any exchange of heat between system and surroundings and this is movable. So if I change the pressure just apply more pressure than inside then this piston will move in volume will decrease. So there will be exchange of mechanical exchange we call there's a work the surroundings is working on the system and if

00:43:02 the pressure outside is lower than inside then the system will move up I sorry the piston will move up volume will increase and we call that the system is working on the system and there's a mechanic mechanical energy exchange mechanical exchange between the system and system and surroundings. this the work due to the the volume change also sometimes called PV work which is nothing but the mechanical energy exchange uh uh as explained here we call work exchange as well. So work exchange happened the energy

00:43:56 exchange between system and surroundings. Energy exchange between system and surroundings happen as work when the non rigid Wall move due to difference in pressure inside and outside the system. Similarly, energy exchange between system and surroundings happen as heat. I I'm not writing that again. Okay, I can write uh that energy exchange between system and surroundings happens as heat. Earlier it was work. Now it is heat. When There is a temperature difference between system and surroundings.

00:46:00 So basically we now know that a system and surroundings can exchange energy by two process two modes. One is work and another is heat. System and surroundings can exchange energy between them either by work. What comes when the movable boundary moves and the system and surrounding can exchange energy as a heat when there's a temperature difference between system and surroundings. Now we talk about the energy of the system. You know we are talking about the energy of systems. Now what are the different types of

00:46:47 energy a system have? Obviously if I have a microscopic object which is moving maybe I uh I'll give you one more example of uh heat exchange and uh uh work exchange before I come to the type of energy. Let me just give you one uh again I have a piston here and in this case I have ura and I'm doing a reaction here within this uh cylinder. So beginning I have ura this is ura and oxygen. And I keep this is in a say in in a water bath outside. And once the reaction is done, if this is having a

00:48:26 movable boundary then because of you have now more number of the volume of the gases increases the volume will now increase. Maybe I'll draw here plus water here. Liquid water here. and you have this is placed in a water bath. Now in this case the system is doing some work on the surroundings because of the volume expansion plus there will be some energy exchange between the surroundings which is a water bath here. And if you can measure the temperature before and after the reaction in the surroundings which

00:49:48 might with a very sensitive thermometer we'll see there is a temperature change in the water bath here. Now if I do the same reaction with a fixed piston. So there is no volume changes allowed in this case. water then what we'll see this case that there's no exchange of energy as a work but in this case the temperature difference between the system and surroundings or uh the temperature change you will observe in the surroundings in this case water will be higher compared to the first case. So if my drawing is probably

00:50:53 not uh good here, but anyway, what I'm trying to say is that I'm doing a reaction burning of ura inside a cylinder with a movable piston and the the container is kept in a water bath. Now because there's a the volume of gas is increasing if this piston is movable then it the volume of the system will go up which means there is exchange of energy between system and surroundings as work and there will be exchange of heat between system and surroundings and there will be change in temperature in

00:51:38 the surroundings here in the water bath. If you do the same reaction in fixed volume where the piston is fixed then there will be no exchange of work next no exchange of energy as work between system and surroundings. In that case the difference between the initial and final temperature in the surroundings will be more compared to the last case. Okay. So basically we now know that there are two ways system and surroundings can exchange uh energy. One is uh heat and the other is one is heat and another is work.

00:52:27 Now what are the different energy in a in a system? If we just talk about say this pen this is not moving so the macroscopic kindinetic energy of this pen is zero and if we though it's in a height maybe in a table so there is some bit of potential energy gravitational potential energy but that's also we can neglect So you're talking about and if there is no external field from out applied from outside then there is no potential energy either. So in this case if I instead of a pen I'm taking a beaker or a conical

00:53:23 flask where I want to do a reaction generally it does not the beaker or conical flux does not have any microscopic kindinetic energy or potential energy. Then what is the energy a reaction medium a chemical reaction medium will have? The energy is from the molecules which are present which are present in the system and that energy is called internal energy. The internal energy is basically the energy due to the molecules present within the system. And what are those energies which are associated with those

00:54:15 molecules? I'll just explain in a minute. So as I said, if I just have a conical flask uh and we talk about a process from state one to state two. So and if I tell K is the microscopic kindinetic energy then del K is zero. Obviously before and after both kindinetic energy is zero. So, so K is microscopic and when you're changing the state of the system there's no change in microscopic kindinetic energy. If you don't apply or change the potential from outside then the macroscopic potential energy V

00:55:31 is also zero. So there is no change in macroscopic quantity of kinetic energy or potential energy. So what could be the change between say state one thermodyamic state one to thermodynamic state two that could be change in internal energy. If you express internal energy as U then the change would be delu. So if I want to find out total energy change of the system for going state one to state two would be given by del total energy del k is total change in macroscopic kindinetic energy macroscopic potential

00:56:35 energy plus Del U and obviously as we mentioned the type of system chemical system we will be dealing with in this unit or generally we deal with in thermodynamics these two terms are zero. So the total change is equal to the total change in internal energy. So basically now onwards we'll be focusing on when you're talking about the change in total energy of a system we'll be focusing focusing mainly on the change in internal energy not any other energy of course now the question you will ask

00:57:15 what is internal energy so internal energy Q which is due to the molecular motions plus inter molecular interactions. So u consist of no the translational energy of the molecules. So molecular translational plus rotational vibrational and electronic energies of the molecule plus relativistic rest mass energy. E M rest C² of the electrons and the nuclei plus the potential energy of interaction. ction between the molecules. So this lecture I'll stop here and uh in next lecture in lecture two I will

00:59:28 continue from our discussion about internal energy. So we'll just take this slide and then page and we'll continue in the next uh lecture uh more about internal energy.