Thermodynamic Background

The law of mass action states that the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants.

At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. The ratio of the rate constant of the forward reaction to the reverse reaction rate constant expresses the equilibrium constant.

Here, Gibbs energy is plotted against the extent of the reaction. The slope of this plot defines the reaction Gibbs energy, Δ_rG.

Δ_rG also quantifies the difference between the chemical potentials of the products and reactants for a given composition of the reaction mixture, predicting the reaction’s spontaneity.

If the chemical potential of the reactants is greater than that of the products, Δ_rG is less than zero, and the forward reaction is spontaneous. If the chemical potential of the products is greater than that of the reactants, Δ_rG is greater than zero, and the reverse reaction is spontaneous.

When the chemical potential of the reactants equals that of the products, Δ_rG is zero, and the reaction reaches equilibrium.