The Fugacity Quantity

Abstract

This chapter is devoted to the notion of fugacity. It is developed entirely under the standpoint of classical thermodynamics. The fugacity quantity has been introduced in order to describe the behavior of imperfect gases. It permits to express the molar Gibbs energy of a pure imperfect gas and, also, to express the chemical potential (the molar partial Gibbs energy) of a gas in a mixture of imperfect gases with a formalism analogous to that used in the case of perfect gases. For this reason, it can be said that the chemical potential which is an abstract notion can be, through the use of the fugacity, expressed in terms of a new function which is more easily identified with the physical reality than the chemical potential is. The chapter mentions the definition of the fugacity of a pure gas, the chemical potential of a perfect or real pure gas in terms of it, the fugacity of liquids and solids, the notion of fugacity coefficient of a real gas, a coming back to the notion of reference state, and the changes in fugacity with the temperature and the pressure. It also mentions the expressions of the chemical potential of a component of a mixture of perfect gases, the fugacities of real gases in mixtures, and their changes with pressure and temperature together with their determination. From another viewpoint, the values of the fugacity of a species present in different phases may assert or not the state of partition equilibrium.