 Hello, welcome back to the next segment of thermodynamic discussion. So far we have discussed the important thermodynamic properties work, energy and heat and how they are connected to each other with the first law of thermodynamics. So in the next part we will discuss one important parameter enthalpy. Let us take a look into. So now say we have a particular surrounding and over there we have a system where this system is expandable say it is a gas. So this system has a initial volume of V1 then we are making some changes in this full system where the initial system was this. And now we are expanding the system and now the system has increased this volume and we say it is the final volume after we completed that process is equal to V2. And in this process to make sure that this volume change happens we have to give some heat to the system. And as we have discussed earlier we will say that as plus Q because the heat is going inside the system. Now what is the overall energy change when we go from the initial state to the final state. So it is given by energy of the final state minus energy of the initial state. This particular system we are actually trying to find out. Now over here let us say I am writing this as E2 minus E1 where 1 is the initial state whereas 2 is the final state. Now over here with respect to first law of thermodynamics we can write it is Q minus W we are writing is minus W because the volume has increased and that increase has happened because the system itself is doing the work. And when system does the work we put it a minus sign. So it is Q minus W. Now if I rewrite that as Q equal to delta E plus W. So it is just bringing the W from the right hand side to the left hand side and I am writing that again. Now over here what is delta E? It is nothing but E2 minus E1 as it is given over here. Now what is the overall work done? So over here the work done is actually the increase of volume and now say I am assuming there is very critical assumption is that this process actually taken place at constant pressure. So if that process actually happened under constant pressure then the work done can be given in the form of P into V2 minus V1. So the volume change that we did during that process into the pressure. So with respect to that I can again rewrite Q equal to E2 minus E1 plus P into V2 minus V1. So this is I am expanding the W. Now over here the Q becomes E2 plus P into V2 minus E1 plus P into V1. So I am just putting all the things with subscript 2 at one side all the things with subscript 1 into the other side. So with respect to that I can say these parameters are giving me the idea what is happening in the final state and these parameters are giving me what is happening in the initial state. So instead of writing E2 plus P V2 or E1 plus P V1 I can write a totally new parameter which actually come with both this thing combined together and I write it as H and writing H2 to correspond with that 2 state or final state and this is H1 or the initial state and this is I can write it is a change in this particular parameter H and this H factor is known as the enthalpy. Again coming back to this particular system we will have a system where it is surrounded by surrounding I am giving it to some heat and at constant pressure it is increasing the volume from V1 to V2. During that pressure at constant pressure when it is increasing the volume the overall work done is P into V2 minus V1 and over there during this process what is happening following the first law of thermodynamics the heat that is actually given to the system can be explained as a difference of a new factor a new property known as H and this H is known as enthalpy which is mathematically written in the form of E plus P V. So, when we discussed about enthalpy and I am saying H is equal to E plus P V what does this particularly mean and as we just say that Q is nothing but delta H we can say this enthalpy factor has something to do with the factor heat and that is why we can say H or enthalpy is nothing but the heat content of that particular system because it is given by the heat parameter and it is a combination of energy and work done of the volumetric work where it is getting expanded all together connected to this enthalpy. So, now if we want to discuss further more we can take a look that the delta H is equal to nothing but delta E plus P delta V which is nothing but E2 minus E1 and V2 minus V1 that you have done earlier and over there again the assumption still corrects is that we have done that under a constant pressure. Now, once this particular parameter we got delta E plus P delta V we can say the change in enthalpy delta H is nothing but change in energy plus the expansion work done over here. Now, consider three different states solids, liquids and gas what happens over there. In the case of solids and liquids we know the change in volume is pretty low. So, over there delta V almost close to 0. So, in that scenario delta H becomes delta E because this particular factor over there P delta V it actually goes to close to 0. So, over there we can say in this case the enthalpy is nothing but change in the energy of the system. So, that is giving us an idea what is happening inside the system during the process. So, enthalpy is a very intrinsic process, intrinsic parameter for a particular system. Now, what happens for gases we know delta V is greater than 0. So, over there what will happen delta H will be equal to delta E plus a nonzero P delta V value. So, we can say at this case it will be greater than delta E. So, in case of the gases some work can be done by the system that means the gaseous system and in that case the enthalpy change will be higher than the energy change whereas in the solids it will be almost equal to the energy change. So, that is a big difference over there when we are doing this system under constant pressure and we are doing that for different systems like solids or liquids or gases. So, with respect to that we can connect enthalpy to the energy change and the possible work can be done by that system itself. Now, we go in another system when if we do the same system at a constant volume. So, over there I am not changing the volume at all. So, that means delta V is equal to 0 and in that case delta H is again going to become delta E. So, in the case of a constant volume system the enthalpy change will be equal to the energy change of the system no matter what whether this is a gas liquid or solid state. So, that is a background of the enthalpy change and the enthalpy change is directly connected to the overall energy change and we can connect it to the heat. So, that is why H or enthalpy is nothing but connected to the heat content of the system and this H or enthalpy can be used to figure it out what will be how much energy the system can take or give in the context of heat or heat capacity. So, this particular heat capacity is directly connected to the enthalpy factor and this is beyond the scope of this particular module, but later you can find it out all this heat capacity is we found Cv at constant volume or Cp at constant pressure they can be connected to this enthalpy factor and their actual derivation of their expression is directly connected to the enthalpy. So, that is why another reason to think about that the enthalpy has nothing but giving you an idea how much heat a system can actually hold during a process how much heat it can get exchange with the surrounding. So, that is why enthalpy becomes a very crucial factor. Now, we will go a little bit on some applications of that enthalpy factor. So, now when you talk about enthalpy we talk about the enthalpy of a particular process. So, let us say we are looking into a enthalpy of a reaction. So, how do I define an enthalpy of a reaction? So, enthalpy of a reaction is nothing but equal to enthalpy change during that reaction and we can figure it out by adding all the enthalpy value of all the products we are forming summing that up minus all the enthalpy of the reactants and whatever the value we will get that is going to be the enthalpy change for this particular reaction. Now, this is very important because that is going to connect with the exothermic and endothermic reaction that we have discussed earlier. We have discussed earlier that endothermic reaction is what where the energy is getting adsorbed into the system following a process where exothermic process is where the energy is getting released following that process. So, over here we can easily found if our delta H is greater than 0 that means the enthalpy of the product is higher than the reactant how we can do that. So, that means if I draw in a scale where I am drawing enthalpy and in this x axis I am drawing where the reactant or the products are if this is my reactant and I am saying delta H is greater than 0 that means delta H products is greater than delta H of the reactants and this can happen when the product enthalpy is higher and then I can subtract the product enthalpy to the reactant enthalpy and I can say delta H is greater than 0. For these things to happen my enthalpy of the product should be higher. So, I have to give this extra enthalpy to the system and where it is going to come from that is going to come from the surrounding to the system. So, that means system is actually nothing but absorbing energy. So, is delta H equal to 0 system is it presents a endothermic system or endothermic process. So, when a delta H is greater than 0 that means product enthalpy is higher than the reactant that extra enthalpy going to the product has to be supplied from somewhere and it is actually coming from the surrounding and this actually signifies there is a energy absorption during that process or nothing but endothermic process. So, this is for endothermic process and very similarly if we have a delta H less than 0 that means summation of my enthalpy of my products is actually lower than the summation of the enthalpy value of the reactants. That means over here if I want to draw there is my reactant my product is actually now low and that is why my delta H will be less than 0 and over there during this process what is happening my product is actually going down in enthalpy. So, it is releasing some of the enthalpy what was there in the reactant. So, from the reactant to product some of the enthalpy is lost and this enthalpy goes from the system to the surrounding that is the excess energy released. So, that is why this is nothing but a exothermic process. So, previously we have discussed what is an endothermic process. Now, we have a parameter that we can easily measure or easily quantify that can give me an idea whether I am having an endothermic process or a exothermic process and it is happening with respect to enthalpy. The enthalpy change we are going to measure which is nothing but enthalpy of the products minus enthalpy of the reactants and if this value is actually greater than 0 then this endothermic process if it is less than 0 it is exothermic process. So, do I need to remember it or we just need to remember the logic the logic is product minus reactant if the product is higher that means it is extra enthalpy. So, it is going to absorb some energy from the surrounding endothermic process and if my product is lower than the energy of the reactant. So, it is actually getting stabilized and releasing some energy that energy is going to be released. So, it is an exothermic process. So, that is why enthalpy we can use in the form and over there we can find out whether it is an endothermic or exothermic process. So, some example of endothermic and exothermic processes are as following. So, as we just said delta H greater than 0 it is an endothermic process also delta H less than 0 it is a exothermic process. So, over here for an example if we look into the melting of an ice this has a delta H value greater than 0. So, it is an endothermic process. So, some energy has to be absorbed in that system. Similarly, when we are looking into a combustion process in that case the delta H is actually less than 0. So, it is generally an exothermic process. So, in that way for different systems for different processes physical or chemical processes we can find out what is the overall energy change and that we can directly connected with respect to the enthalpy. And that is why we can find for different processes combustion we can find neutralization when an acid base reaction is happening over there there is a change of the enthalpy with respect to the initial acid and base and when the formation of the water and the salt happens. So, over there some of the enthalpy change can give you an idea whether it is a very favourable process or not. Similarly, formation of different compounds from its original constituent elements that you can also find out with respect to enthalpy and also fusion or melting processes for all these systems we can find a particular enthalpy change and that enthalpy change give us an idea whether or not the products are forming with respect to enthalpy it is higher or lower than the enthalpy of the reactant systems. Now, this enthalpy change is very critical because it can give us a very good idea with respect to a very important parameter as we know is known as bond energy or in thermodynamic term we can say it is a bond enthalpy. So, what is a bond energy or bond enthalpy? So, when we are actually creating a bond. So, when a bond is created and the amount of energy liberates during the bond formation and during that we are considering that the bond is forming from the two elemental atoms. So, from its elemental form when it is forming during that the energy liberated is nothing but the bond energy or bond enthalpy. So, it is nothing but we are measuring the bond enthalpy or the enthalpy of the system of the product and the reactant reactants are the two elemental atoms and product are the product we are forming. So, for an example we are combining carbon and oxygen and producing carbon dioxide. So, we are going to measure the enthalpy of the carbon dioxide as a product and subtract the enthalpy of the initial reactant that means the carbon and oxygen together and finding out what is the overall enthalpy change that is going to give me the bond energy or bond enthalpy of the each of the carbon oxygen bonds. Over there another example of this bond energy can be given in a way that considering the other direction the bond is already there. Now I am breaking the bond I am breaking a bond and the amount of energy is required to break that bond it is nothing but the bond energy or bond enthalpy. So, we are looking this system in two different ways either I have the elements and making the bonds. So, how much energy is liberated that is the bond enthalpy and sometime we have the bond and we are breaking that down and going to the elements and the energy released it is known as the bond enthalpy or bond energy. So, we can explain that in either of the direction. So, that is known as the bond energy or bond enthalpy. So, all the bond energy we are actually use all the times in chemistry physics and all the different requirements when we are actually using a chemical process all this bond energy is nothing but a manifestation how the bond enthalpy is actually been measured. So, bond energy is nothing but a direct representation of bond enthalpy change during the process of the bond formation. Now we go back to a very important term and how it is very much connected. So, it is found by a scientist named Hess and he name this law as Hess's law and I am going to write this and let you guys understand how we can actually follow this bond enthalpy to find out bond energies or the chemical energies or the enthalpy change for a particular process. So, it is given as the standard enthalpy of an overall reaction is the sum of the standard enthalpies of the individual reactions into which the overall reaction can be divided. So, that is a very important law and that is very important in the terms of measuring the enthalpy because sometime we find that we are doing a particular reaction and we are starting from A and B and going to C and I am going to find out what is overall enthalpy change for this process. But sometime it is very tricky and what we can connect it and expand it further the formation of A formation of B and their enthalpies and all those enthalpies can be connected together to give you an example. Let us take an example of the enthalpy change of C 6 H 14 which is a hexane and this hexane is in the gaseous form and in this gaseous form if I want to go to its original elements carbon and hydrogen the enthalpy change is 197 kilo joule per mole that is the unit kilo joule is the energy unit and per mole is giving like per mole of compounds. So, we know exactly how much compound we are talking about. So, it is the hexane we are actually breaking down to its original form of carbon hydrogen and 197 kilo joule is the difference between them. Now, we know if I go from the liquid state of the hexane to the gaseous state their energy difference is 29 kilo joule per mole. So, from the gaseous hexane to the liquid hexane it is 29 kilo joule per mole and liquid hexane has lower enthalpy than the gaseous one. Now, if I want to find what is the enthalpy for changing the liquid directly to the original carbon and hydrogen how we can do. So, you can just take a look into this figure and find it out the enthalpy change for the C 6 H 14 gaseous to C 6 H 14 or the hexane liquid that means for this particular process plus the enthalpy of the gaseous hexane to its original form. If we add them up together it is nothing but the enthalpy change for the liquid hexane to its elemental form change right and over there you can see this is nothing but we can actually put these things up together and find out what will be the overall enthalpy change. So, you have to just add it up and you can find out what will be the enthalpy change for the liquid to the original elemental form. So, that is why gaseous law give us a very good tool to connect all the enthalpies connected in a particular reaction. So, in the later stages we will do some problems where we will use this gaseous law to find out the enthalpy changes of a particular reaction and connected to the standard reaction that enthalpies are already known and we can connect them all together and find out the enthalpy change for the full particular process. So, in this particular segment so far we have find out what is the connection between the enthalpy with the energy heat and the work and we find out enthalpy is nothing but can be shown as a expression of E plus PV. So, it is actually a nothing but a heat content of the system it is connected to the heat capacities and enthalpy of the system is a very important, but because this is actually giving us idea whether it is endothermic or exothermic reaction. For endothermic reaction delta H is greater than 0 that means the product enthalpy is higher than the reactant one whereas for exothermic reaction it is just the opposite delta H is less than 0 and the enthalpy of my product is actually lower than the enthalpy of my reactants. And we can also use the bond enthalpy to find out the bond energies of different systems and we can connect the bond enthalpies of different reactions with the standard reaction with the help of the Gaseous law. So, that will be here for this particular segment. Thank you.