Free energy functions

We can determine the reversibility of the process by looking at the total entropy change.

However, total entropy change is not the system property. We need information on the surrounding and that’s not very convenient thermodynamically. We need some system properties that can determine the reversibility. The free energy, such as Gibbs free energy and Helmholtz free energy, is the system property. Under a certain limited condition, their sign can be the criteria for the reversibility equivalent to the total entropy change. We derive at two important free energy functions, the Helmholtz free energy and the Gibbs free energy.