Thermodynamic Potentials

Abstract

In this chapter, we introduce equivalent descriptions of a thermodynamic system for different choices of independent state parameters. The same information about the system can be presented either in the form of the fundamental relation between the internal energy, entropy, volume and amount of substance or in the form of a relation between a certain extensive state function (thermodynamic potential or a Massieu function) and a unique set of independent state parameters (natural variables). In thermodynamics, usually three potentials are used: the enthalpy, Helmholtz free energy and Gibbs free energy (or free enthalpy), but we define also other potentials. A mathematical tool to change from one description to another is the Legendre transformation. As an example, we calculate the most important thermodynamic potentials for the ideal gas case. Then we present the free-energy minimum principle as an alternative formulation of the second law of thermodynamics for systems at constant temperature or at constant temperature and pressure. Next we give examples of application of thermodynamic potentials. Finally, we derive the conditions of intrinsic stability of a pure substance.