of a chemical process. In these expressions, a reaction quotient appears, which
is a construction involving the reactants and products of the reaction. For gas-
phase reactions, the reaction quotient includes the partial pressures or fugaci-
ties of the species. By defining activity, we can expand the reaction quotient to
include solids and liquids (although their activities are close enough to 1 that
their influence on Qcan be ignored) and solutions. For solutions, the molal
concentration of solutes is the convenient variable for Q.
At equilibrium,Qhas a value that is characteristic of the chemical reaction,
because there is a characteristic change in the Gibbs free energy for any par-
ticular chemical reaction. This characteristic value ofQis called the equilib-
rium constant,K. Equilibrium constants are convenient measures of the extent
of the reaction at the minimum Gibbs free energy, that is, at equilibrium.
Equilibrium constants can change with changes in conditions of a system, but
the mathematics of thermodynamics gives us tools to model those changes.
5.7 Summary 137