Definitions of an Activity

Abstract

A way to express the chemical potential of a compound consists in using the quantity named activity.

It will be seen that the handling of activities necessarily requires the choice of thermodynamic standard states, choice which is fully arbitrary. But, actually, some choices are quasi-systematically done rather than others, according to whether the studied species is either in gaseous state or into a solution and, also, according to the studied system. Hence, it is not surprising that a great part of the considerations concerning the activities is devoted to the standard states and to their choice.

In this chapter, the examination of the definitions of an activity is carried out, since an activity of a compound can be defined according to two ways, one through its fugacity and the other directly without involving the notion of fugacity. The consequences of the arbitrary character of the choice of the standard states are considered later. Finally, some general properties of the activities are mentioned. It is the case of their changes with the temperature and pressure.