In thermodynamics, the word endothermic ("within-heating") describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. It is a modern coinage formed from Greek roots (as is often the case with scientific terminology). The prefix endo- derives from the Greek word "endon" (ἔνδον) meaning "within," and the latter part of the word comes from the Greek word root "therm" (θερμ-) meaning "hot." Hence it refers to a reaction that needs heat. The opposite of an endothermic process is an exothermic process, one that releases energy in the form of heat. Thus in each term (endothermic & exothermic) the prefix refers to where heat goes as the reaction occurs. The term endothermic was coined by Marcellin Berthelot (25 October 1827 – 18 March 1907).

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

Implications for chemical reactions[link]

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.

Contrast between thermodynamic and biological terminology[link]

Note that because of historical accident, students encounter a source of possible 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[link]

• photosynthesis
• melting ice cubes
• general decomposition reactions
• forming a cation from an atom in the gas phase
• dissolving ammonium chloride in water

See also[link]

• Endergonic
• Exothermic
• Exergonic
• Exergonic reaction

References[link]

External links[link]

• Endothermic Definition - MSDS Hyper-Glossary