Inthermodynamics, anexothermic process (fromAncient Greekέξω (éxō) 'outward' andθερμικός (thermikós) 'thermal')[1] is athermodynamic process orreaction that releasesenergy from the system to itssurroundings,[2] usually in the form ofheat, but also in a form oflight (e.g. a spark, flame, or flash),electricity (e.g. a battery), orsound (e.g. explosion heard when burning hydrogen). The termexothermic was first coined by 19th-century French chemistMarcellin Berthelot.[3]
The opposite of an exothermic process is anendothermic process, one that absorbs energy, usually in the form of heat.[2] The concept is frequently applied in thephysical sciences tochemical reactions where chemicalbond energy is converted tothermal energy (heat).
Exothermic and endothermic describe two types of chemical reactions or systems found in nature, as follows:
An exothermic reaction occurs when heat is released to the surroundings. According to theIUPAC, an exothermic reaction is "a reaction for which the overall standardenthalpy change ΔH⚬ is negative".[4] Some examples of exothermic process are fuelcombustion,condensation andnuclear fission,[5] which is used innuclear power plants to release large amounts of energy.[6]
In anendothermic reaction or system, energy is taken from the surroundings in the course of the reaction, usually driven by a favorableentropy increase in the system.[7] 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.[8]
Photosynthesis, the process that allows plants to convert carbon dioxide and water to sugar and oxygen, is an endothermic process: plants absorbradiant energy from the sun and use it in an endothermic, otherwise non-spontaneous process. The chemical energy stored can be freed by the inverse (spontaneous) process: combustion of sugar, which gives carbon dioxide, water and heat (radiant energy).[9]
Exothermic refers to a transformation in which a closed system releases energy (heat) to the surroundings, expressed by
When the transformation occurs atconstant pressure and without exchange ofelectrical energy, heatQ is equal to theenthalpy change, i.e.
While atconstant volume, according to thefirst law of thermodynamics it equalsinternal energy (U) change, i.e.
In anadiabatic system (i.e. a system that does not exchange heat with the surroundings), an otherwise exothermic process results in an increase in temperature of the system.[11]
In exothermic chemical reactions, the heat that is released by the reaction takes the form of electromagnetic energy orkinetic energy of molecules.[12] The transition ofelectrons from one quantumenergy level to another causes light to be released. This light is equivalent in energy to some of the stabilization energy of the energy for the chemical reaction, i.e. thebond energy. This light that is released can be absorbed by other molecules insolution to give rise to molecular translations and rotations, which gives rise to the classical understanding of heat. In an exothermic reaction, theactivation energy (energy needed to start the reaction) is less than the energy that is subsequently released, so there is a net release of energy.[13]
Some examples of exothermic processes are:[14]
Chemical exothermic reactions are generally more spontaneous than their counterparts,endothermic reactions.[16]
In a thermochemical reaction that is exothermic, the heat may be listed among the products of the reaction.
