Understanding Lewis activities

The notion of activity is of a purely thermodynamic nature. It is intimately linked to the law of mass action and more generally to that of equilibrium constant. It was conceived and introduced in the realm of chemistry at the beginning of twentieth century by Lewis (Proc Am Acad Arts Sci 37:49‒69, 1901; Proc Am Acad Arts Sci 43:259‒293, 1907; J Am Chem Soc 30:668‒683, 1908) and Lewis and Randall (Thermodynamics and the free energy of chemical substances. McGraw-Hill Book Company, London, 1923). Its introduction permits us to overcome insuperable theoretical and practical difficulties encountered in chemistry, physics and biochemistry with great ease. Before discussing the notion of activity, it is necessary to introduce ideal and non-ideal chemical systems, the different scales of unities permitting one to express their composition and the concept of chemical potential. Here, we seek to locate the notion of activity within the framework of classical thermodynamics. We shall see that its introduction in chemistry enables us to generalize the use of the Gibbs energy function (formerly, and still in German, called the free enthalpy function) for introducing the concept of chemical equilibrium. This is carried out by the introduction of the pivotal function known as chemical potential. Then, we set out the interest that represents the handling of activities. It, purely and simply, permits one to formulate the mass action law when the studied chemical systems are not ideal. This is the vast majority of cases! This is followed by introducing the notions of fugacity and activity from a general viewpoint. We then confine ourselves to the study of the activity of gases, those of non-electrolytes in liquid solutions, the activities of electrolytes, the determination of activities of non-electrolytes and of electrolytes, calculation of activities of electrolytes essentially with the aid of Debye–Hückel relations and that of activity coefficients and activities, their relations with another thermodynamic function, that is to say the excess Gibbs energy, the determination of pH of an aqueous solution, the general principles of calculations involving activities in solutions, and finally, the determination of thermodynamic equilibrium constants of polyfunctional compounds.