Endothermic

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This article is about the physical effect. For self-maintained thermal homeostasis, see Endotherm.

In thermodynamics, the term endothermic describes a process or reaction in which the system absorbs energy from its surroundings in the form of heat. The term was coined by Marcellin Berthelot from the Greek roots endo-, derived from the word "endon" (ἔνδον) meaning "within" and the root "therm" (θερμ-) meaning "hot." The intended sense is that of a reaction that depends on taking in heat if it is to proceed. The opposite of an endothermic process is an exothermic process, one that releases, "gives out" energy in the form of heat. Thus in each term (endothermic & exothermic) the prefix refers to where heat goes as the reaction occurs.

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


Endothermic (and exothermic) analysis only accounts for the enthalpy (∆H) change of a reaction. The full energy analysis of a reaction is the Gibbs free energy (∆G), which includes an entropy (∆S) and temperature term in addition to the enthalpy. A reaction could be a Spontaneous process at a certain temperature if the products have a lower total free energy (an exergonic reaction) even if the enthalpy of the products is higher. Entropy and enthalpy are different terms, so the change in entropic energy could overcome an opposite change in enthalpic energy.

Contrast between thermodynamic and biological terminology[edit]

Because of a historical accident, students encounter a possible source of confusion between the terminology of physics and biology. Whereas the thermodynamic terms "exothermic" and "endothermic" respectively refer to processes that give out heat energy and processes that absorb heat energy, in biology the sense is effectively inverted. The metabolic terms "ectothermic" and "endothermic" respectively refer to organisms that rely largely on external heat to achieve a full working temperature, and to organisms that produce heat from within as a major factor in controlling their bodily temperature.

Examples[edit]

7
3
Li
 
n  →  4
2
He
 
3
1
T
 
n

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