The Gibbs free energy change in a chemical reaction, , determines the spontaneity of the reaction. This is to be distinguished from the rate of reaction. When is negative, the reaction is exergonic and potentially spontaneous. When is positive, the reaction is endergonic and does not occur spontaneously. The instantaneous sum of the Gibbs free energies of the reactants and products is designated . Equilibrium is attained when is at a minimum, as a function of the extent of reaction .