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In thermodynamics, the word exothermic "outside heating" describes a process or reaction that releases energy usually in the form of heat, but it can also release energy in form of light (e.g. explosions), sound, or electricity (e.g. a battery). Its etymology stems from the Greek prefix ex-, meaning “outside” and the Greek word thermein, meaning “to heat”. The term “exothermic” was first coined by Marcellin Berthelot. The opposite of an exothermic process is an endothermic process, one that absorbs energy in the form of heat.

The concept is frequently applied in physical sciences to chemical reactions, where chemical bond energy is converted to thermal energy (heat).


Exothermic refers to a transformation in which a system releases energy (heat) to the surroundings:

Q < 0

When the transformation occurs at constant pressure:

∆H < 0

and constant volume:

∆U < 0

In an adiabatic system (e.g. a system that does not give off heat to the surroundings), an exothermic process results in an increase in temperature.[1]

Exothermic processes

Some examples of exothermic processes are:[2]

  • Condensation of rain from water vapour
  • Combustion of fuels such as wood, coal and oil
  • Mixing water and strong acids
  • Mixing alkalis and acids
  • The setting of cement and concrete
  • Most polymerisation reactions such as the setting of epoxy resin

Implications for chemical reactions

Chemical exothermic reactions are generally more spontaneous than their counterparts, endothermic reactions. In a thermochemical reaction that is exothermic, the heat may be listed among the products of the reaction.

See also

  • Endergonic
  • Endergonic reaction
  • Exergonic
  • Exergonic reaction

External links


  1. Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.
  2. Exothermic - Endothermic examples

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