Exergonic reaction

An exergonic reaction (such ascellular respiration) is a reaction that releases free energy in the process of the reaction. The progress of the reaction is shown by the line.Activation energy(1) slows down the reaction. The change of Gibbs free energy (ΔG) in an exergonic reaction (that takes place at constant pressure and temperature) is negative because energy is lost (2).

Inchemical thermodynamics,anexergonic reactionis achemical reactionwhere the change in thefree energyis negative (there is a net release of free energy).This indicates aspontaneous reactionif the system isclosedand initial and final temperatures are the same. For processes that take place in a closed system at constant pressure and temperature, theGibbs free energyis used, whereas theHelmholtz energyis relevant for processes that take place at constant volume and temperature. Any reaction occurring at constant temperature without input of electrical or photon energy is exergonic, according to thesecond law of thermodynamics.An example iscellular respiration.

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

$\Delta G=G_{\rm {p}}-G_{\rm {r}}<0.\,$

Although exergonic reactions are said to occurspontaneously,this does not imply that the reaction will take place at an observablerate.For instance, thedisproportionation of hydrogen peroxidereleases free energy but is very slow in the absence of a suitable catalyst. It has been suggested thateagerwould be a more intuitive term in this context.

More generally, the termsexergonicandendergonicrelate to thefree energychange in any process, not just chemical reactions. By contrast, the termsexothermicandendothermicrelate to anenthalpychange in a closed system during a process, usually associated with the exchange ofheat.

See also

• Endergonic reaction