Equilibrium Constant, Activities, and Reaction Gibbs Energy

Abstract

The concept of chemical equilibrium is intimately linked to that of Gibbs energy change accompanying the studied reaction. According to the value of the latter, one can forecast if, spontaneously, the reaction is possible (or not) in the sense in which it is considered in the given experimental conditions. Likewise, it permits to forecast the position of the equilibrium. Hence, applying the general properties of the quantity Gibbs energy to a chemical reaction leads to the law of the chemical equilibrium, also called “mass action law.” It stems from the notion of standard reaction Gibbs energy which is quantified with the aid of the introduction of a constant named the equilibrium constant.

In this chapter, it is demonstrated that the equilibrium constant is truly a constant only when it is calculated in terms of the activities of the reactants and products. This point is a very important one, because it is only at this price that the interactions between the particles of the system are taken into account. The introduction of the activities allows, indeed, to express the chemical potentials of the reactants and products and as a result the mass action law with the same formalism as that used to study the behavior of species in ideal solutions.

The theoretical developments of this chapter lead to the somewhat surprising result that, according to the chosen standard states of the reacting species, the values of the equilibrium constants of a given reaction vary! This is the reason why it is interesting to mention the relations existing between the different equilibrium constants. Finally, the changes of equilibrium constants with temperature and pressure are examined.