PSV: Explain why one must choose a reference point when calculating state variables

State Variables and Reference States

So, now we know that as long as we know "where we are" in the state of the material, we know that the energy and enthalpy are fixed values, but what are those values?!

This is a trick question. It is, in fact, impossible to measure the energy or enthalpy exactly, but it is easy to measure changes in the energy or enthalpy (as you can see from the energy balance equation: hold kinetic and potential energy fixed, apply no work, and add a known amount of heat).

OUTCOME:

Explain why one must choose a reference point

So if we don't know the actual value, but only differences, what do we do?!

Since we are doing balances, we are really only interested in differences of differences! Just like in the case of changing a temperature difference from Celsius to Kelvin, where the offset (reference state!) was canceled out, here we can choose things such that the reference state chosen is immaterial! To get the change in enthalpy in a balance, we define both enethalpies relative to the same reference state and then that reference state cancels out!

(H1 - Href) - (H2 - Href) = H1 - H2

In all of our dealings with these state variables (energy and enthalpy), we will use a reference state (set equal to zero) in order to do the calculation).

NOTE

As we will see in our next lecture, when we choose a reference state, we can set the enthalpy (energy) of that reference to any arbitrary value. For simplicity, we will set these arbitrary values to zero.