ABSTRACT
This chapter introduces the Helmholtz and Gibbs energies, formerly called free energy and free enthalpy. Transformations are named monothermal if the processing system exchanges heat only with one exterior source, the temperature Text of which being constant. The practical importance of this notion is immense. As a result, the Helmholtz energy can only decrease or remain constant. This is the case of systems at constant temperature and volume, such as a system enclosed in a rigid container, held at constant temperature. Formerly, it was named free enthalpy. Its symbol is G. Under the conditions required for the judicious use of function G, the criterion of equilibrium of a process is that it has attained its minimum value. Like the Helmholtz energy, the Gibbs function is also a potential function of thermodynamics.
