Term: encounter-controlled rate
https://doi.org/10.1351/goldbook.E02089

Definition:
A rate of reaction corresponding to the rate of encounter of the reacting molecular entities. This is also known as 'diffusion-controlled rate' since rates of encounter are themselves controlled by diffusion rates (which in turn depend on the viscosity of the medium and the dimensions of the reactant molecular entities). For a bimolecular reaction between solutes in water at 25°C an encounter-controlled rate is calculated to have a second-order rate constant of about 10E10 dm3 mol-1 s-1.

Related Terms:
1) microscopic diffusion control (http://doi.org/10.1351/goldbook.M03912).
2) rate of reaction (http://doi.org/10.1351/goldbook.R05156).
3) molecular entities (http://doi.org/10.1351/goldbook.M03986).
4) medium (http://doi.org/10.1351/goldbook.M03814).
5) bimolecular (http://doi.org/10.1351/goldbook.M03989).
6) rate constant (http://doi.org/10.1351/goldbook.O04322).
7) encounter (http://doi.org/10.1351/goldbook.E02086).
8) diffusion-controlled rate (http://doi.org/10.1351/goldbook.D01721).
9) diffusion (http://doi.org/10.1351/goldbook.D01716).
10) viscosity (http://doi.org/10.1351/goldbook.V06627).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1112 (https://doi.org/10.1351/pac199466051077)

Citation: 'encounter-controlled rate' 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.E02089

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