Inchemical thermodynamics, anexergonic reaction is achemical reaction where the change in thefree energy is negative (there is a net release of free energy).[1] This indicates aspontaneous reaction if the system isclosed and initial and final temperatures are the same. For processes that take place in a closed system at constant pressure and temperature, theGibbs free energy is used, whereas theHelmholtz energy is 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,, in an exergonic reaction (at constant pressure and temperature) is denoted as
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 peroxide releases free energy but is very slow in the absence of a suitable catalyst. It has been suggested thateager would be a more intuitive term in this context.[2]
More generally, the termsexergonic andendergonic relate to thefree energy change in any process, not just chemical reactions. By contrast, the termsexothermic andendothermic relate to anenthalpy change in a closed system during a process, usually associated with the exchange ofheat.
