Exergonic reaction

An exergonic reaction (such as cellular respiration) is a reaction that loses energy during the process of the reaction. Activation energy (1) catalyzes the reaction to occur in a spontaneous manner. The progress of the reaction is shown by the line. The change of Gibbs free energy (ΔG) in an exergonic reaction (that takes place under constant pressure and temperature conditions) is a negative value because energy is lost (2).

An exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy),^[1] indicating a spontaneous reaction. For processes that take place under constant pressure and temperature conditions, the Gibbs free energy is used whereas the Helmholtz energy is used for processes that take place under constant volume and temperature conditions.

Symbolically, the release of free energy, G, in an exergonic reaction (at constant pressure and temperature) is denoted as

$\Delta G=G_{\rm{products}}-G_{\rm{reactants}}<0.\,$

Although exergonic reactions are said to occur spontaneously, this does not imply that the reaction will take place at an observable rate. For instance, the disproportionation of hydrogen peroxide is very slow in the absence of a suitable catalyst. It has been suggested that eager would be a more intuitive term in this context.^[2]

More generally, the terms exergonic and endergonic relate to the free energy change in any process, not just chemical reactions. An example of an exergonic reaction is cellular respiration.

By contrast, the terms exothermic and endothermic relate to the overall exchange of heat during a process.

See also

• Endergonic reaction

References

1. IUPAC Gold Book definition: exergonic (exoergic) reaction
2. Hamori, Eugene; James E. Muldrey (1984). "Use of the word "eager" instead of "spontaneous" for the description of exergonic reactions". Journal of Chemical Education 61 (8): 710. Bibcode:1984JChEd..61..710H. doi:10.1021/ed061p710.