In thermodynamics, the term exothermic ("outside heating") describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light (e.g. a spark, flame, or explosion), electricity (e.g. a battery), or sound (e.g. burning hydrogen). Its etymology stems from the Greek prefix ex- (meaning "outside") and the Greek word thermein (meaning "to heat"). The term exothermic was first coined by Marcellin Berthelot. The opposite of an exothermic process is an endothermic process, one that absorbs energy in the form of heat.
The concept is frequently applied in the physical sciences to chemical reactions, where chemical bond energy is converted to thermal energy (heat).
Exothermic refers to a transformation in which a system releases energy (heat) to the surroundings:
When the transformation occurs at constant pressure:
and constant volume:
In an adiabatic system (e.g. a system that does not give off heat to the surroundings), an exothermic process results in an increase in temperature.
In chemical reactions, the heat that is absorbed is in the form of electromagnetic energy. The loss of kinetic energy via reacting electrons causes light to be released. This light is equivalent in energy to the stabilization energy of the energy for the chemical reaction, i.e. the bond energy. This light that is released can be absorbed by other molecules in solution to give rise to molecular vibrations or rotations, which gives rise to the classical understanding of heat. In contrast, when endothermic reactions occur, energy is absorbed to place an electron in a higher energy state, such that the electron can associate with another atom to form another chemical complex. The loss of energy within solution is absorbed by the endothermic reaction and therefore is a loss of heat. This is the physical understanding of exothermic and endothermic reactions within solution.
Some examples of exothermic processes are:
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