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 a form of light (e.g. a spark, flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen). Its etymology stems from the prefix exo (derived from the Greek word ἔξω, exō, "outside") and the Greek word thermasi (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.
Exothermic (and endothermic) are terms usually used to describe two types of chemical reactions or systems found in nature:
Simply stated, after an exothermic reaction, more energy has been released to the surroundings than was absorbed to initiate and maintain the reaction. An example would be the burning of a candle, wherein the sum of calories produced by combustion (found by looking at radiant heating of the surroundings and visible light produced, including increase in temperature of the fuel (wax) itself, which with oxygen, have become hot CO2 and water vapor,) exceeds the number of calories absorbed initially in lighting the flame and in the flame maintaining itself. (i.e. some energy produced by combustion is reabsorbed and used in melting, then vaporizing the wax, etc. but is far outstripped by the energy produced in breaking carbon-hydrogen bonds and combination of oxygen with the resulting carbon and hydrogen).
Whereas, in an endothermic reaction, or system, energy is taken from the surroundings in the course of the reaction. An example of an endothermic reaction: a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, as the reaction cools the pouch and surroundings by absorbing heat from them. An endothermic system is seen in the production of wood: trees absorb radiant energy, from the sun, use it in endothermic reactions such as taking apart CO2 and H2O and combining the carbon and hydrogen generated to produce cellulose and other organic chemicals. These products, in the form of wood, say, may later be burned in a fireplace, exothermically, producing CO2 and water, and releasing energy in the form of heat and light to their surroundings, e.g., to a home's interior & chimney gasses.
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 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. Therefore in an exothermic reaction the energy needed for the reaction to occur is less than the total energy released.
Some examples of exothermic processes are:
- Condensation of rain from water vapor
- Combustion of fuels such as wood, coal and oil petroleum
- Mixing water and strong acids
- Mixing alkalis and acids
- The setting of cement and concrete
- Some 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.
Contrast between thermodynamic and biological terminology 
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.
See also 
|Look up exothermic in Wiktionary, the free dictionary.|
- http://chemistry.about.com/b/a/184556.htm Observe exothermic reactions in a simple experiment