Term: mass action, law of
https://doi.org/10.1351/goldbook.08184

Definition:
Statement that the velocity of a reaction depends on the active mass, i.e., the concentrations of the reactants.

Note: First recognized in 1864 as the kinetic law of mass action by Guldberg and Waage, who first introduced the concept of dynamic equilibrium, but incorrectly assumed that the rates could be deduced from the stoichiometric equation. Only after the work of Horstmann and van’t Hoff a mathematical derivation of the reaction rates considering the order of the reaction involved was correctly made.

Example: For an association reaction (1) and its reverse (2) <span title="equation">\[\ce{A + B -> AB} \quad (1)\]</span>  <span title="equation">\[\ce{AB -> A + B} \quad (2)\]</span> the forward velocity is <span title="equation">\(\nu_{1} = k_{1}[\ce{A}][\ce{B}]\)</span>, with <span title="symbol">\(k_{1}\)</span> being the rate constant for the association reaction. For the dissociation reaction 2, the velocity is <span title="equation">\(\nu_{2} = k_{2}[\ce{AB}]\)</span>. This is valid only for elementary reactions. Furthermore, the law of mass action states that when a reversible chemical reaction reaches equilibrium at a given temperature, the forward rate is the same as the backward rate. Therefore, the concentrations of the chemicals involved bear a constant relation to each other, described by the equilibrium constant, i.e., for <span title="equation">\[\ce{A + B \leftrightarrows AB}\]</span> in equilibrium, <span title="equation">\(\nu_{1} = k_{1}[\ce{A}][\ce{B}] = \nu_{2} = k_{2}[\ce{AB}]\)</span> and one form of the equilibrium constant for the above chemical reaction is the ratio <span title="equation">\[K_{c} = \frac{[\ce{AB}]}{[\ce{A}][\ce{B}]} = \frac{k_{1}}{k_{2}}\]</span>

Example: Array

Source: PAC, 2022, 94, 353. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)' on page 460 (https://doi.org/10.1515/pac-2018-1010)

Citation: 'mass action, law of' 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.08184

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.