Exergonic

Exergonic (from the suffix exo-, derived for the Greek word ἔξω exō, "outside" and the suffix -ergonic, derived from the Greek word ἔργον ergon, "work") means "releasing energy in the form of work". By thermodynamic standards, work, a form of energy, is defined as moving from the system (the internal region) to the surroundings (the external region). Thus, an exergonic process, as contrasted with an endergonic process, is one that releases energy from the system, of which it is a part, to the surroundings. As a result, during an exergonic process, energy is released out of the system. If the transformation occurs at constant pressure and temperature, ∆G < 0. When Gibbs free energy is less than 0, the reaction is exergonic (releases energy).

All physical and chemical systems in the universe follow the second law of thermodynamics and proceed in a downhill, i.e., exergonic, direction. Thus, left to itself, any physical or chemical system will proceed, according to the second law of thermodynamics, in a direction that tends to lower the free energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously.