A Generalized Statement of the Law of Mass Action

Abstract

When expressed in a generalized form in terms of thermodynamic activities, the law of mass action can be made to take into account the non-ideal behavior of reactants in solution in a way that is consistent with both equilibrium and non-equilibrium thermodynamics. Specifically, when combined with the principle of detailed balance, the generalized law of mass action predicts the thermodynamically exact result linking the forward and reverse rate constants, the equilibrium activities, and the equilibrium constant. For a reaction approaching equilibrium, it predicts in agreement with non-equilibrium thermodynamics a proportionality between the net reaction rate and the instantaneous Gibbs free energy of reaction. In the case of an irreversible reaction between ions in solution, the generalized law leads to the Brönsted-Bjerrum equation connecting the rate constant to the ionic strength. For reactions in the gas phase or in dilute ideal solution, the generalized law assumes the familiar form of the original Guldberg and Waage law of mass action (1864). Because of its universal character, the generalized law can be used as a unique starting point for a course in chemical kinetics.