Yet Another (and Very Important) Thermodynamic Function

There is one more thermodynamic function that we can define by the same means that we found both the enthalpy and the Helmholtz free energy.

This new thermodynamic is formed by subtracting off the ``thermal energy'' and the ``compressive energy'' from $U$:

$$\input{equations/G-def}$$ (12-5)

This should be a function expressing how much work could be extracted from a fluid at constant pressure and temperature. This function is called the Gibbs free energy--and it is a pleasure to introduce you to one of my dear friends--I am sure that you will find many topics of common interest.

So, if we have subtracted off the ``stored compressive energy'' and the ``stored thermal energy'' from the potential energy, then what is left? Suppose that the system that we are considering has some other ``internal degrees of freedom'' that can be changed at constant pressure and temperature, then this is the energy associated with those internal degrees of freedom.

Question: Can you identify some variables that describe how a system can change at constant pressure and temperature?