Term: quasi-equilibrium
https://doi.org/10.1351/goldbook.Q04999

Definition:
In conventional transition-state theory it is assumed that activated complexes are formed in a state of equilibrium with the reactants. They are not in classical equilibrium with the reactants; if they were, addition of more activated complexes to the system would cause the equilibrium to shift in favour of the reactants. This would not occur for an activated complexes, and the term quasi-equilibrium is used to denote this special type of equilibrium.

Related Terms:
1) transition-state theory (http://doi.org/10.1351/goldbook.T06470).
2) activated complex (http://doi.org/10.1351/goldbook.A00092).

Source: PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 179 (https://doi.org/10.1351/pac199668010149)

Citation: 'quasi-equilibrium' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.Q04999

License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.

Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.