exergonic

Exergonic describes a reaction or process that releases free energy to its surroundings. In thermodynamics, a process is exergonic when the Gibbs free energy change, ΔG, is negative (ΔG < 0) at the temperature and pressure of interest, indicating the reaction can proceed spontaneously without additional energy input.

The sign and magnitude of ΔG depend on ΔH and ΔS through the relation ΔG = ΔH − TΔS. Therefore, an exergonic process is not defined solely by heat release; it may be exothermic (negative ΔH) or endothermic (positive ΔH) if the entropy increase is large enough to make ΔG negative.

In biology, exergonic steps often drive endergonic ones by coupling reactions, such as the hydrolysis of ATP, which provides usable free energy for cellular processes. The standard free energy change for ATP hydrolysis is about −30 kJ/mol under physiological conditions, though it varies with conditions and concentrations.

Examples include cellular respiration and the spontaneous hydrolysis of high-energy bonds, as well as protein folding that proceeds without external energy input. Exergonic should be distinguished from exothermic; the former concerns free energy change, while the latter concerns heat exchanged with the surroundings.