Endothermic process

In thermochemistry, an endothermic process (from Greek ἔνδον (endon) 'within', and θερμ- (therm) 'hot, warm') is any thermodynamic process with an increase in the enthalpy H (or internal energy U) of the system.[1] In such a process, a closed system usually absorbs thermal energy from its surroundings, which is heat transfer into the system. Thus, an endothermic reaction generally leads to an increase in the temperature of the system and a decrease in that of the surroundings. It may be a chemical process, such as dissolving ammonium nitrate (NH4NO3) in water (H2O), or a physical process, such as the melting of ice cubes.

The term was coined by 19th-century French chemist Marcellin Berthelot. The opposite of an endothermic process is an exothermic process, one that releases or "gives out" energy, usually in the form of heat and sometimes as electrical energy. Thus in each term (endothermic and exothermic) the prefix refers to where heat (or electrical energy) goes as the process occurs.

In chemistry

Due to bonds breaking and forming during various processes (changes in state, chemical reactions), there is usually a change in energy. If the energy of the forming bonds is greater than the energy of the breaking bonds, then energy is released. This is known as an exothermic reaction. However, if more energy is needed to break the bonds than the energy being released, energy is taken up. Therefore, it is an endothermic reaction.[2]


Whether a process can occur spontaneously depends not only on the enthalpy change but also on the entropy change (S) and absolute temperature T. If a process is a spontaneous process at a certain temperature, the products have a lower Gibbs free energy G = HTS than the reactants (an exergonic process),[1] even if the enthalpy of the products is higher. Thus, an endothermic process usually requires a favorable entropy increase (S > 0) in the system that overcomes the unfavorable increase in enthalpy so that still G < 0. While endothermic phase transitions into more disordered states of higher entropy, e.g. melting and vaporization, are common, spontaneous chemical processes at moderate temperatures are rarely endothermic. The enthalpy increase H ≫ 0 in a hypothetical strongly endothermic process usually results in G = ∆HTS > 0, which means that the process will not occur (unless driven by electrical or photon energy). An example of an endothermic and exergonic process is


Distinction between endothermic and endotherm

The terms "endothermic" and "endotherm" are both derived from Greek ἔνδον endon "within" and θέρμη thermē "heat", but depending on context, they can have very different meanings.

In physics, thermodynamics applies to processes involving a system and its surroundings, and the term "endothermic" is used to describe a reaction where energy is taken "(with)in" by the system (vs. an "exothermic" reaction, which releases energy "outwards").

In biology, thermoregulation is the ability of an organism to maintain its body temperature, and the term "endotherm" refers to an organism that can do so from "within" by using the heat released by its internal bodily functions (vs. an "ectotherm", which relies on external, environmental heat sources) to maintain an adequate temperature.


  1. Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).Principle of Modern Chemistry, Brooks Cole. p. 617. ISBN 978-1305079113
  2. "Exothermic & Endothermic Reactions | Energy Foundations for High School Chemistry". highschoolenergy.acs.org. Retrieved 2021-04-11.
  3. Austin, Patrick (January 1996). "Tritium: The environmental, health, budgetary, and strategic effects of the Department of Energy's decision to produce tritium". Institute for Energy and Environmental Research. Retrieved 2010-09-15.
  4. Qian, Y.-Z.; Vogel, P.; Wasserburg, G. J. (1998). "Diverse Supernova Sources for the r-Process". Astrophysical Journal 494 (1): 285–296. arXiv:astro-ph/9706120. Bibcode:1998ApJ...494..285Q. doi:10.1086/305198.
  5. "Messing with Mass". WGBH. 2005. Retrieved 2020-05-28.
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