By John Brennan
Updated Aug 30, 2022
Gibbs free energy measures the capacity of a system to perform non‑mechanical work. When the Gibbs free energy of reactants exceeds that of the products, the reaction is exergonic—thermodynamically spontaneous and capable of releasing energy without external input.
While many exergonic reactions are exothermic and emit heat, the two concepts are distinct. An exergonic process may even absorb heat (endothermic) if the overall entropy change outweighs the enthalpy change. The sign of the free‑energy change, not the heat flow, determines spontaneity.
19th‑century chemists struggled with spontaneous endothermic reactions because they overlooked entropy, the measure of unavailable energy. A process becomes exergonic when the total entropy of the system plus surroundings increases, even if the system itself absorbs heat.
Reactions that produce a gas or involve evaporation typically exhibit large positive entropy changes, making them more exergonic at higher temperatures. Conversely, heat‑releasing reactions are usually more exergonic at lower temperatures. Reaction kinetics also matter; an exergonic reaction can still be kinetically sluggish and practically unobservable.
