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This screencast is about reference states. Reference state is where the enthalpy of any
substance equals 0. When you are setting a reference state you have to include the temperature,
the pressure, and the phase of whatever substance you are talking about. There are two categories
of reference states. The first is when you want to use a table of enthalpy data and this
is clearly the easiest way to find the enthalpies. What are some examples of that? First of all
you can find tables of ideal gas enthalpies and what these usually include are gases that
are involved in combustion. So you have O2, N2, H2O vapor, CO2, CO and air by itself.
If you want to use that table then your reference state has to be 25 degrees C, gas and 1 atm.
How would you use this table? You know that at 25 degrees the enthalpy is 0 so let's say
you want to find the enthalpy of O2 at 300 degrees C. All you have to do is look it up
in the table under 300 degrees C and that is your enthalpy. As you can see using an
enthalpy table is pretty simple. Let's say now you want to use the steam tables. If you
want to use those then your reference state is H2O liquid at the triple point. That is
where your enthalpy is going to be 0. When you are using the steam tables you generally
have to use it as a function of both temperature and pressure when you are using ideal gases
it is just a function of temperature. Another thing is that if you want to look up heats
of solution or mixing. If you do that you will find that the reference to use that table
is 25 degrees C, 1 atm, and then HCl in it's gas form, NaOH in it's solid form and H2SO4
in its liquid form. What you look up here are these heats of solution or mixing as a
function of "r" where "r" are the moles of solvent divided by the moles of solute. Now
let's say you have a reaction. Here it depends on the two methods that you are going to use
to solve for the Q depending on reaction. First method is the heat of reaction method
and if you are using that to solve for your Q then your reference state has to be 25 degrees
C, 1 atm and the molecular species. The reason for this reference is that is where heats
of formation and reaction are measured. Let's say instead that you have to use or you want
to use the heat of formation method. Here your reference state again is 25 degrees C
and 1 atm however, here you have to use elemental species. For example what you are using is
C in its solid form, O2 gas H2 gas etc. The reason that you use these is in the heat of
formation method you have to take things in their elemental form and create the molecule
which is that heat of formation. Finally, what if you have no table. Then you use any
reference but you probably want to use a reference that is convenient. What do we mean by convenient?
If you have for example benzene vapor and toluene vapor that comes in at 50 degrees
C and 1 atm and coming out you have both benzene and toluene vapor at 20 degrees C as well
as benzene and toluene liquid at 20 degrees C. Convenient reference states would be benzene
vapor, toluene vapor at 50 degrees C because that means that the enthalpy of those streams
coming in is 0. Or you could use benzene vapor and toluene vapor at 20 degrees C which means
that the vapor coming out at 20 degrees C has an enthalpy of 0. Finally you could use
benzene liquid and toluene liquid at 20 degrees C which means that the liquid at 20 degrees
C have an enthalpy of 0. Be careful that you include a reference state for every substance.
You can't just say vapor at 50 degrees C. As well as making sure you have the phase
of each on of those substances.