CHEMICAL-REACTIONS IN PHASES AT DIFFERENT ELECTRIC POTENTIALS

When some reactants or products in a chemical reaction are ions that are in a phase at a different electric potential, the equilibrium constant K for the reaction is affected by the difference in the electric potentials. The equilibrium expression can be derived by using the Gibbs energy G(T, P, n,) or a transformed Gibbs energy G'(T, P, n, phi(i)) defined by subtracting electrical work terms from the Gibbs energy. This terminology is used because such a change in natural variables is referred to as a Legendre transform. The thermodynamics of a simple reaction involving ionic species in two phases at different electric potentials is discussed using the Gibbs energy G and the transformed Gibbs energy G'. In the first case, the thermodynamic behavior is discussed in terms of standard molar enthalpies and entropies of species, and in the second case the thermodynamic behavior is discussed in terms of standard molar transformed enthalpies and standard molar transformed entropies of species. The use of the transformed reaction Gibbs energy leads to the apparent equilibrium constant K' that would be obtained if all the species were in the same phase. The advantages and disadvantages of these different treatments are discussed, as well as problems with the current electrochemical nomenclature.