WordNet

(of a chemical reaction or compound) occurring or formed with the liberation of heat (同)exothermal, heat-releasing
a rise in the temperature of the body; frequently a symptom of infection (同)febrility, febricity, pyrexia, feverishness
intense nervous anticipation; "in a fever of resentment"
produced by or producing fever (同)pyrogenous, pyrogenetic
causing fever
a compound that gives off heat during its formation and absorbs heat during its decomposition

PrepTutorEJDIC

(化学変化で)放熱の,発熱の
〈U〉《しばしばa fever》(身体の異常な)『熱』,発熱 / 〈U〉『熱病』 / 〈U〉〈C〉(…に対する)『興奮』,熱狂《+『for』+『名』》
発熱;熱病

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/08/12 14:31:22」(JST)

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Explosions are some of the most violent exothermic reactions.

In thermodynamics, the term exothermic process (exo- : "outside") 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 Greek prefix έξω (exō, which means "outwards") and the Greek word θερμικός (thermikόs, which means "thermal").^[1] 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 as in chemical bond energy that will be converted to thermal energy (heat).

Exothermic (and endothermic) describe two types of chemical reactions or systems found in nature, as follows.

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).

On the other hand, in an endothermic reaction or system, energy is taken from the surroundings in the course of the reaction. An example of an endothermic reaction is a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, and 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 and chimney gasses.

Overview

Exothermic refers to a transformation in which a system releases energy (heat) to the surroundings, expressed by

Q < 0.

When the transformation occurs at constant pressure, one has for the enthalpy^{[clarification needed]}

∆H < 0,

and constant volume, one has for the internal energy^{[clarification needed]}^{[dubious – discuss]}

∆U < 0.

In an adiabatic system (i.e. a system that does not exchange heat with the surroundings), an exothermic process results in an increase in temperature of the system.^[2]

In exothermic chemical reactions, the heat that is released by the reaction takes the form of electromagnetic energy. The transition of electrons from one quantum energy level to another 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 a chemical complex. Net energy is absorbed by an endothermic reaction. In an exothermic reaction, the energy needed to start the reaction is less than the energy that is subsequently released, so there is a net release of energy. This is the physical understanding of exothermic and endothermic reactions.

Examples

An exothermic thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.

Some examples of exothermic processes are:^[3]

Combustion of fuels such as wood, coal and oil petroleum
Thermite reaction
Reaction of alkali metals and other highly electropositive metals with water
Condensation of rain from water vapor
Mixing water and strong acids or strong bases
Mixing acids and bases
The setting of cement and concrete
Some polymerisation reactions such as the setting of epoxy resin

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

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.

References

Notes

^ "Gate for the Greek language" on-line dictionary
^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.
^ Exothermic - Endothermic examples

External links

http://chemistry.about.com/b/a/184556.htm Observe exothermic reactions in a simple experiment

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