 In dealing with chemical reactions and looking at thermodynamic calculations with chemical reactions we need a way to be able to account for the fact that when the bonds are either breaking or forming there can be a change in the energy of the system. And so consequently what we're going to do now we're going to introduce a new property and that is called the enthalpy of formation and also if it's an oxidation reaction it turns out to be enthalpy of combustion but we'll take a look at that in this segment and I'll show you how we do or handle this enthalpy of formation. So if you recall we've been looking at a number of different properties that are very useful for thermodynamic calculations so I'm going to do now and going to give a paragraph for each of the different main properties that we've been using such as internal energy and enthalpy and then what I'll do is I'll proceed to contrast these properties with what we need for this new enthalpy of formation. So we'll begin with internal energy. So for internal energy what we were dealing with was basically accounting for the kinetic energy of the molecular motion within the substance that we would be looking at and the higher the temperature of the usually it was a gas but it could also be for a liquid you'd have a molecular motion there but the higher the temperature the higher the internal energy and consequently this was a form of what we call sensible energy so that was the first treatment of internal energy. We also had internal energy associated with a phase change when a substance would go from either a liquid to a vapor or from a vapor back to a liquid or it could also account for when it's going from a solid to a liquid. So in this course when we look at internal energy for a phase change we would use the subscript fg to denote that and this is usually in this course what we're looking at it would be our working fluid either going through boiling so going from liquid to vapor or condensing from vapor back to liquid and this is what we refer to as being latent energy. Sensible energy is without a phase change it's just a temperature change when you get into the phase change process it becomes latent energy and so that was another way that we looked at the property data. The next thing that I'll do is I'll take a look at enthalpy. So when we looked at enthalpy we saw enthalpy applied to a fixed mass system and essentially what enthalpy is doing there is it accounts for both internal energy as well as boundary work and that was for a fixed mass constant pressure process so that was one way that we were applying the enthalpy for a fixed mass system. It also applied or covered the flow work term for a steady flow process that would be for an open control volume where we have mass crossing the control boundary. So enthalpy was another property that we've dealt with but what we're now faced with so we've looked at internal energy we've looked at enthalpy we now need a property that enables us to take into account the fact that when we have bonds breaking or forming there could be energy exchange so this is something that we have not looked at yet in the course and we need to be able to account for this when we're dealing with chemical reactions such as what we would have within a system undergoing oxidation or combustion reactions. So what we need is we need a form of enthalpy that can help us internal energy or enthalpy that accounts for the change of energy when bonds are broken and formed and in order to do that we introduce this new property or value called the enthalpy of formation and the symbol that we use for enthalpy of formation will be h just like normal enthalpy we're going to put an over bar on it and that is because we're going to give it in units per kilomole basis we are going to put a zero in the subscript position because that denotes that it is referenced to a standard reference state which is 25 degrees c in one atmosphere and finally we will give it a subscript f to denote formation so the different things that we have the over bar that's because it's on a per kilomole basis which we will find to be quite convenient when we're dealing with stoichiometry and chemical reactions and the balance the number of kilomoles either on the reactant or the product side the zero references to a standard reference state so that means that this property has been determined at a standard reference state and that standard state by convention is 25 degrees c and one atmosphere and finally the subscript f refers to enthalpy of formation so that is the enthalpy of formation and we'll be using that as we work through problems involving chemical reactions and combustion so when we're dealing with these problems we're going to be dealing with a lot of different components for species for example we get of oxygen nitrogen carbon dioxide carbon but no matter what we're doing we always need to reference the enthalpy for all of the components that we have in our system to a standard reference state so as an example let's take a look at the enthalpy of oxygen and let's say we want to look at it at 790 kelvin so what we would need to do given that we're not at the 25 degrees c reference state we would need to look in the back of our books and look for table dealing with ideal gas properties of oxygen o2 and what you'll find these tables will be tabulated in a per kilomole basis and you'll find the value for enthalpy at 790 kelvin and then what you would do is you would subtract off the value of enthalpy at the zero reference state which in this particular case would be the per kilomole basis at 298 kelvin so you would take off the value of 298 kelvin and that would enable you to determine the enthalpy at 790 k you could then tack that onto the value the heat of formation for enthalpy of oxygen if that's what you needed in a calculation we'll see that in an example in a minute but what you would get if you look in the tables in your book so those are the values for oxygen we would find 15504 kilojoules per kilomole so that would be the enthalpy with respect to the reference state of 25 25 degrees c and you would then use that in combination with your heat of formation so then what you would do is you would go and you'd find your enthalpy of formation tables which again will be in the back of your textbook and what they will list is they will provide enthalpy of formation at the base state so at 25 degrees c one atmosphere and they will be listed for for different substances that you might be dealing with and you'll also notice that there will be values for carbon hydrogen nitrogen or oxygen which it turns out that this is what all of the substances that we have enthalpy of formation are with respect to but interesting if if you look at the enthalpy of formation for these four substances carbon hydrogen nitrogen and oxygen if you look in your enthalpy of formation table you'll find that the heat of formation value for those four substances is zero and that's because everything is being formed from those and consequently they are given the reference state value of zero and and then other substances for example if I look in the table here we have things like carbon monoxide so we could have carbon monoxide in the gaseous state and you can get the heat of formation for that and that turns out to be minus one 10 530 kilojoules per kilomole now that would give me the heat of formation value at 25 degrees c and one atmosphere if I had this in an equation that we'll see in a later segment where we're dealing with the first law I would need to correct that value for the enthalpy at whatever particular temperature we would be dealing with for that substance and and so that's the correction that we'll see as I work an example in a later segment but anyways that is the enthalpy of formation what it does it takes into account chemical bond energy and the change of energy associated with either the breaking of the formation of those bonds and so we'll be dealing with this as we continue to do calculations involving combustion