Exergonic Definition

What Does Exergonic Mean?

Exothermic reactions are examples of exergonic reactions. A good example is the thermite reaction, which releases energy in the form of light and heat.
Exothermic reactions are examples of exergonic reactions. A good example is the thermite reaction, which releases energy in the form of light and heat. Andy Crawford & Tim Ridley, Getty Images

Exergonic Definition

Exergonic refers to a chemical reaction where the free energy of the system decreases and energy is released. A exergonic reaction occurs spontaneously because it typically does not require energy to proceed. The reactants lose energy (G decreases) and the Gibbs free energy (ΔG) is negative under conditions of constant pressure and temperature. Helmholtz energy is used for exergonic reactions which occur under conditions of constant temperature and volume.

ΔG = Gproducts - Greactants < 0

Note that even though an exergonic process occurs spontaneously, it may not proceed quickly enough to be observable. Examples of slow reactions include the rusting of iron in air or the disproportiation of hydrogen peroxide. Catalysts may be used to increase the rate of chemical reaction.

Example of Exergonic Reactions

In human biochemistry, catabolic reactions are exergonic. In these reactions, larger molecules are broken into smaller ones. Breaking chemical bonds releases their stored energy.

Chemical reactions that release heat (exothermic reactions) are all examples of exergonic reactions. The distinction between exergonic and exothermic is that the form of released energy (heat) is specified for an exothermic reaction, while an exergonic reaction might include the release of heat, light, or another type of energy. Examples of exothermic (and thus exergonic) reactions include:

  • reaction between hydrogen and oxygen to form water
  • reaction between sodium and chlorine to form salt
  • rusting of steel
  • any combustion reaction, such as burning of wood or gasoline
  • freezing of water into ice
  • reaction between water and a strong acid
  • nuclear fission