Equilibrium is a special set of conditions that are not at standard conditions. Equilibrium occurs when the driving force of the forward reaction is equal to the driving force of the reverse reaction resulting in no further net change in concentrations of products and reactants. This equilibrium condition is the place where ΔG = 0. The mass action law is related to the previously derived dependence of ΔG.

ΔG = ΔG^o + RT ln Q

At equilibrium, Q is the equilibrium constant (K_eq):

0 = ΔG^o + RT ln Q = ΔG^o + RT ln K_eq

ΔG^o = - RT ln K_eq

Note the sign in the equation. If the equilibrium constant is one, then the standard free energy is zero. If the equilibrium constant favors the products (K_eq > 1 ), then the standard free energy is negative which indicates that the reaction is spontaneous. If the equilibrium constant favors the reactants (K_eq < 1 ), then the standard free energy is positive which indicates that the reaction is not spontaneous.

Note that the standard free energy may be calculated from the equilibrium constant, or the equilibrium constant may be calculated from standard free energies. For example in the laboratory, one may measure concentrations of colored ions in solution using a spectrophotometer to determine an equilibrium constant and then calculate the standard free energy of the reaction from that equilibrium constant. Alternatively, data for standard free energies may be obtained from tables of measured in the laboratory and those values may be used to calculate the equilibrium constant for a reaction.