What is exothermic energy

Exothermic reaction

As exothermic is a chemical reaction in which energy, e.g. B. in the form of heat, is released into the environment. The opposite is the endothermic reaction. In the case of an exothermic reaction, the so-called enthalpy of reaction $ \ Delta_ \ mathrm {R} H $ is negative.[1] The enthalpy $ H $ is the sum of the internal energy of a system and the product of pressure and volume. It is the heat content of a system at constant pressure.[2][3]

The term exothermic is not with the term exergon to be confused (see: Demarcation).

Typical exothermic reactions are:

  • Fire (combustion) as well
  • Setting (= hardening) of concrete.

The mixing of substances (heat of the mixture) or the adsorption and absorption of substances on activated carbon or zeolites, for example, are often exothermic, albeit to a much lesser extent.

$ \ Delta H $ denotes the difference between the enthalpies of the final ($ H_2 $) and starting materials ($ H_1 $), i.e. the recorded Energy, applies to exothermic reactions $ \ Delta H = H_ {2} - H_ {1} <0 $.

In physics, too, a nuclear reaction that releases energy is called exothermic. An exothermic nuclear fusion is the burning of hydrogen, for example, as it happens in the sun.


The starting materials are initially in a metastable state. By briefly supplying a certain amount of energy, the activation energy (activation enthalpy), the system is lifted into the unstable state. Activation sets the reaction in motion and runs independently without any additional energy input. In the overall balance, the chemical system gives off energy to the environment; it is called the enthalpy of reaction. The products are now in a stable condition. For the stability of systems see also system properties.

An example of adding activation energy is igniting a fire by rubbing a match on the friction surface or, in the case of gases, with an electric spark.

$ \! \ H: = \ text {enthalpy} $
$ \! \ \ Delta ^ {\ ddagger} H: = \ text {Activation enthalpy} $
$ \! \ \ Delta_ \ mathrm {R} H: = \ text {enthalpy of reaction} $

left: initial state of the starting materials: metastable
middle: transition state of the activated complex: unstable
right: final state of the products: stable

Example: Carbon burns with the oxygen in the air, generating heat to form carbon dioxide: $ \ mathrm {C + O_2 \ \ xrightarrow {\ bigtriangledown} \ CO_2 + W \ ddot arme} $.

The enthalpy of reaction (enthalpy difference) $ \ Delta H $ of this reaction is negative. It can be calculated from the standard enthalpies of formation.

If the activation energy is very low, the reaction can be set in motion without additional external energy input. The necessary activation energy is withdrawn from the environment. The reaction takes place exergonically (spontaneously).


If one speaks of an exothermic process, no statement is made about whether a reaction takes place voluntarily. A distinction is made here between exergonic and endergonic reactions.

Individual evidence

  1. ↑ PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics [IUPAC Recommendations, 1996]), page 165.
  2. ^ PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry [IUPAC-IFCC Recommendations, 1996]), page 972.
  3. ^ PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms [Recommendations, 1990]), page 2187.