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.
Implications for chemical reactions
Chemical endothermic reactions need heat to be performed. In a thermochemical reaction that is endothermic, the heat is placed on the reactants' side (heat is necessary for and absorbed during the reaction). The heat absorbed during the chemical reaction results in a decrease in temperature of its surroundings. This principle is employed in instant cold compresses (ice packs) which are used in first-aid.
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
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.
- Cracking of alkanes
- Melting ice
- Vaporising Rubbing Alcohol
- Thermal decomposition reactions
- Forming a cation from an atom in the gas phase
- Dissolving ammonium chloride in water
- High-energy neutrons can also produce tritium from lithium-7 in an endothermic reaction, consuming 2.466 MeV. This was discovered when the 1954 Castle Bravo nuclear test produced an unexpectedly high yield.
- Hisham Zerriffi (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.
- Endothermic Definition - MSDS Hyper-Glossary