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Anexergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with anendergonic process.^[1] Constant pressure, constant temperature reactions are exergonicif and only if theGibbs free energy change is negative (∆G < 0). "Exergonic" (from the prefix exo-, derived for the Greek word ἔξωexō, "outside" and the suffix -ergonic, derived from the Greek word ἔργονergon, "work") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from thesystem (the internal region) to thesurroundings (the external region) during a given process.

All physical and chemical systems in the universe follow thesecond 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 thesecond law of thermodynamics, in a direction that tends to lower thefree energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously.

Achemical reaction is also exergonic when spontaneous. Thus in this type of reactions the Gibbs free energy decreases. Theentropy is included in any change of the Gibbs free energy. This differs from anexothermic reaction or anendothermic reaction where the entropy is not included. The Gibbs free energy is calculated with theGibbs–Helmholtz equation:

${\displaystyle \mathrm{\Delta }G=\mathrm{\Delta }H-T\cdot \mathrm{\Delta }S}$

where:

T = temperature inkelvins (K)

ΔG = change in the Gibbs free energy

ΔS = change in entropy (at 298 K) as ΔS = Σ{S(Product)} − Σ{S(Reagent)}

ΔH = change in enthalpy (at 298 K) as ΔH = Σ{H(Product)} − Σ{H(Reagent)}

A chemical reaction progresses spontaneously only when the Gibbs free energy decreases, in that case the ΔG is negative. In exergonic reactions the ΔG is negative and inendergonic reactions the ΔG is positive:

exergon

${\displaystyle {\mathrm{\Delta }}_{\mathrm{R}}G>0}$ endergon

where:

${\displaystyle {\mathrm{\Delta }}_{\mathrm{R}}G}$ equals the change in the Gibbs free energy after completion of a chemical reaction.

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