In this screencast we're going to discuss how to choose a reference state when you're
doing an enthalpy calculation. So in order to calculate these enthalpies and energy balance
you need to set the reference state. In other words, the conditions where the enthalpy of
that particular component is zero. Then enthalpies of that component are calculated relative
to that reference state. In order to set a reference state you need to put a temperature,
a pressure, and a phase, and this is for every component that's in your system. How do we
set a reference state? The first thing is if you're going to use a table of enthalpies,
and if you're going to use that table to calculate your enthalpies and your energy balance you
have to use the reference state that's given by that table. Let's look at a couple of examples.
Let's say you want to find the enthalpy of an ideal gas, in particular those involved
with combustion at a certain temperature. You can either use heat capacities to calculate
it, or you can use a table that has all the enthalpies for you. So here's an enthalpy
table for the ideal gases, and if you take a look here we have a temperature that's 25
degrees C, we have our reference pressure of 1 atm, and notice that these are gases.
For example, if you stated that the reference was 25 degrees C, 1 atm, and N2 in it's gaseous
form, N2 gas, and you wanted to know what the enthalpy was at 600 degrees, all you would
have to do is look it up right there and you'd find that your enthalpy is 17.39 kJ/mol. Notice
by the way if you look at 25 degrees C all the enthalpies for all the different gases
are zero. Here's another example. If we look at part of a steam table the reference is
liquid water at the triple point. The triple point is the pressure, 0.00611 bar, temperature
which is 0.01, and note right here that the enthalpy for liquid water is zero, so the
reference for the steam table would be water, liquid, at the triple point. So if we set
that as our reference state we can look up, for example, the enthalpy of saturated steam
at 40 bar, right here, 2800.8 kJ/kg. One thing to note is that the enthalpy for saturated
steam at the triple point is not 0, that's the importance of setting a phase, as well
as the temperature and pressure as your reference. Now let's look at a case when you're using
heat capacity. So you don't have a table, and you are using heat capacities to determine
your enthalpies. Because of that you're not given a reference, so you can set one on your
own, remembering that the reference state that you set is where the enthalpy of that
particular component is 0, then you can measure other enthalpies relative to it. So let's
look at an example. We have a re-boiler, and we have some amount of benzene liquid coming
in, and some amount of toluene liquid coming in, and it goes through the re-boiler. We
have coming out benzene and toluene vapor, as well as benzene and toluene liquid, and
let's say that the liquid here comes in at 65 degrees C, and exits the re-boiler at 100
degrees C. We need to figure out how much heat we have to put into this system. The
first thing we're going to do is put together an enthalpy table and select a reference.
So again we can choose any reference we want, but look at what happens when we set as our
reference benzene liquid and toluene liquid at 65 degrees C, and 1 atm, again recalling
this is when the enthalpy will be zero, and we're going to assume that this system is
operating at 1 atm. So take a look at our table because H1 and H2 are at our reference
state, therefore they're going to equal 0, and that means that there's two less enthalpies
that you need to calculate. If you take a look at this flow chart it might help you
when your're trying to choose a reference. This flow chart is a summary of what I just
went over. However, what it doesn't take into account is what happens if you have a reaction.
There are two ways to calculate heat from a reaction. The first is what's known as the
heat of formation method, and the second is known as the heat of reaction method. Whichever
method you use determines what the reference state is. Let's start with the heat of formation
method. Here the reference state is each substance in its most stable state at 25 degrees C and
1 atm. So for example if we had carbon it would be carbon solid, H2 gas, O2 gas, etc.,
at 25 degrees C and 1 atm. What this does is it allows us to form our components, giving
us the heat of formation that's measured at 25 degrees C and 1 atm. Look at it this way,
let's say we have carbon dioxide, with the heat of formation method, therefore using
these references, so carbon and O2 have an enthalpy of zero at 25 degrees C and 1 atm.
Then we form the CO2 and this gives us the heat of formation of the CO2, and then finally
we have our CO2 at 25 degrees C and we can add to it any sensible heat, or heat caused
by a change in temperature from the reference, and note because it's CO2 we can go back to
our enthalpy table of ideal gases here and add to our heat of formation the enthalpy
caused by a raise in temperature to 400 degrees C, or something like that. However, if you
use the heat of reaction method then our references have to be the molecular species, again at
25 degrees C and 1 atm. So for example the enthalpy of CO2 with these references at 25
degrees C and 1 atm is zero, and then to get your specific enthalpy is just going to be
the sensible heat. In order to be able to calculate your enthalpies in an energy balance
you're going to need to set a reference state, and depending on what type of energy balance
you're doing, the reference state is either given too you, so that you can use the appropriate
table, or you choose one that is the most convenient for your system.