The purpose of this chapter is to explain the origin of the spontaneity of physical and chemical
change. We examine two simple processes and show how to define, measure, and use
a property, the entropy, to discuss spontaneous changes quantitatively. The chapter also
introduces a major subsidiary thermodynamic property, the Gibbs energy, which lets us
express the spontaneity of a process in terms of the properties of a system. The Gibbs
energy also enables us to predict the maximum non-expansion work that a process can do.
As we began to see in Chapter 2, one application of thermodynamics is to find relations
between properties that might not be thought to be related. Several relations of this kind can
be established by making use of the fact that the Gibbs energy is a state function. We also
see how to derive expressions for the variation of the Gibbs energy with temperature and
pressure and how to formulate expressions that are valid for real gases. These expressions
will prove useful later when we discuss the effect of temperature and pressure on equilibrium
constants