exothermic reaction
A chemical or physical transformation in which energy is released in the form of heat. For instance, the energy which is expelled as heat when coal burns, uniting with oxygen in the process, is derived from chemical energy which was stored in the coal itself.
Another example of exothermic reaction is the formation of ammonia from its constituent elements, nitrogen and hydrogen; and the solution of hydrogen chloride gas in water to make hydrochloric acid.
Such processes are exothermic reactions, and are characteristic of many chemical reactions that reach completion at ordinary temperatures since they are accompanied by an evolution of heat.
The remaining chemical reactions absorb heat in the process of the reaction and are classified as endothermic reactions. Endothermic reactions are not limited to chemical processes. For instance when steam is condensed into water, 536 calories of heat are expelled for each gram of steam involved.
See Thermochemistry.
See exothermic.
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