Exothermic reaction
A reaction with negative standard enthalpy change, releasing heat.
Last updated
Such reactions usually release stored energy as heat.[1] In thermochemistry, a reaction where the standard enthalpy change ΔH⚬ is less than zero is called exothermic.[1] Such reactions typically release stored energy in the form of heat.[1] An exothermic reaction is distinct from an exergonic reaction, which IUPAC defines as one where the overall standard Gibbs energy change ΔG is negative.[2]
A strongly exothermic reaction is usually also exergonic because the enthalpy change contributes significantly to the Gibbs energy change.[2] The enthalpy change for an exothermic reaction is negative, calculated as ΔH = H(products) − H(reactants).[3] For example, the reaction 2H₂(g) + O₂(g) → 2H₂O(g) has an enthalpy change of −483.6 kJ/mol.[3]
Quick Facts
- Field
- Thermochemistry
- Opposite
- Endothermic reaction
Facts from the source article.
Key facts and history
The IUPAC definition specifies that an exothermic reaction has a negative standard enthalpy change.[1] A strongly exothermic reaction will usually also be exergonic, because ΔH makes a major contribution to ΔG.[2] The term exothermic is often confused with exergonic reaction, which IUPAC defines by a negative standard Gibbs energy change ΔG.[2] By definition, the enthalpy change in an exothermic reaction has a negative value: Δ = H products − H reactants.[3]
An example is the combustion of hydrogen: 2H₂(g) + O₂(g) → 2H₂O(g) with Δ = −483.6 kJ/mol.[3]
References
- Wikipedia, "Exothermic reaction" (CC BY-SA 4.0), which cites: "The IUPAC Compendium of Chemical Terminology", IUPAC, 2014; Laidler, "A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)", Pure and Applied Chemistry, 1996, p. 149–192 (original not read by us) en.wikipedia.org
- Wikipedia, "Exothermic reaction" (CC BY-SA 4.0), which cites: Laidler, "A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)", Pure and Applied Chemistry, 1996, p. 149–192 (original not read by us) en.wikipedia.org
- Wikipedia, "Exothermic reaction" (CC BY-SA 4.0), which cites: "Enthalpy (Chapter 5)" (original not read by us) en.wikipedia.org
Each numbered sentence above was checked against the source it cites before publication. Sentences citing Wikipedia adapt text from the English Wikipedia article Exothermic reaction (contributors), used under CC BY-SA 4.0; this entry is available under the same licence. Other sources were used for facts only; no text or images are copied from them.
More in Physical Chemistry And Thermodynamics
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Exothermic reaction (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced