Exothermic [ek-soh-thur-mik]

Exothermic

[ek-soh-thur-mik]

In thermodynamics, the word exothermic "outside heating" describes a process or reaction that releases energy usually in the form of heat, but it can also release energy in form of light (e.g. explosions), sound, or electricity (e.g. a battery). Its etymology stems from the Greek prefix exo-, 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 physical sciences to chemical reactions, where chemical bond energy is converted to thermal energy (heat).

Overview

Exothermic refers to a transformation in which a system releases energy (heat) to the surroundings:

Q < 0

When the transformation occurs at constant pressure:

∆H < 0

and constant volume:

∆U < 0

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 released 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.

Examples

Some examples of exothermic processes are:

Condensation of rain from water vapour
Combustion of fuels such as wood, coal and oil
Mixing water and strong acids
Mixing alkalis and acids
The setting of cement and concrete
Most polymerisation reactions such as the setting of epoxy resin
Thermite reaction

Implications for chemical reactions

Chemical exothermic reactions are generally more spontaneous than their counterparts, endothermic reactions. In a thermochemical reaction that is exothermic, the heat may be listed among the products of the reaction.