Dear student,

Exergonic vs. Endergonic reactions: exergonic release more energy than they absorb. Endergonic reactions absorb more energy than they release.

In chemical thermodynamics, an endergonic reaction (also called an unfavorable reaction or a nonspontaneous reaction) is a chemical reaction in which the standard change in free energy is positive, and energy is absorbed. In layman's terms the total amount of energy is a loss (it takes more energy to start the reaction than what you get out of it) so the total energy is a negative net result.

Under constant temperature and constant pressure conditions, this means that the change in the standard Gibbs free energy would be positive

for the reaction at standard state (ie at standard pressure (1 bar), and standard concentrations (1 molar) of all the reagents).

An exergonic reaction is a chemical reaction where the change in the Gibbs free energy is negative,^[1] indicating a spontaneous reaction. Symbolically, the release of Gibbs free energy, G, in an exergonic reaction is denoted as

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 Gibbs free energy change in any process, not just chemical reactions. An example of an exergonic reaction is cellular respiration.

The terms exothermic and endothermic reactions relate to the enthalpy change of a process.