This chapter has several goals. One is to increase your understanding of the chemical potential. Another is to describe the changes that arise in basic thermodynamic laws when particles may enter or leave “the system.” A third goal is to study additional properties of the Helmholtz free energy, which first appeared in chapter 7. And a fourth goal is to introduce another free energy, the Gibbs free energy, and to explore its properties. Clearly, a lot is going on, but the section structure should enable you to maintain your bearings.

Generalities about an open system

In this section, we examine changes that arise in basic thermodynamic laws when particles may enter (or leave) what one calls “the system.” What might be examples of such entering or leaving? The migration of ions in an aqueous solution provides an example. So does the motion of electrons from one metal to another at a thermocouple junction. When one studies the equilibrium between argon in the gaseous phase and argon adsorbed on glass (as we did in section 7.4), one may take the adsorbed atoms to constitute “the system.” Then atoms may enter the system from the vapor phase and also leave it. The coexistence of two phases–solid, liquid, or vapor–provides a final example in which one may take a specific phase as “the system.” Atoms or molecules may enter that phase or leave it.