02716 IUPAC Gold Book - half life 10.1351/goldbook.H02716 H02716 current 1 For a given reaction the half life \(t_{1/2}\) of a reactant is the time required for its concentration to reach a value that is the arithmetic mean of its initial and final (equilibrium) values. For a reactant that is entirely consumed it is the time taken for the reactant concentration to fall to one half its initial value: The half life of a reaction has meaning only in special cases: For a first-order reaction, the half life of the reactant may be called the half life of the reaction. For a reaction involving more than one reactant, with the concentrations of the reactants in their stoichiometric ratios, the half life of each reactant is the same, and may be called the half life of the reaction. If the concentrations of reactants are not in their stoichiometric ratios, there are different half lives for different reactants, and one cannot speak of the half life of the reaction. lifetime https://goldbook.iupac.org//terms/view/L03515 reactant https://goldbook.iupac.org//terms/view/R05163 stoichiometric https://goldbook.iupac.org//terms/view/S06021 Green Book, 2nd ed., p. 55 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf) PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1119 (https://doi.org/10.1351/pac199466051077) PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 167 (https://doi.org/10.1351/pac199668010149) PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 975 (https://doi.org/10.1351/pac199668040957) https://goldbook.iupac.org/terms/view/H02716/html https://goldbook.iupac.org/terms/view/H02716/json https://goldbook.iupac.org/terms/view/H02716/plain Citation: 'half life' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.H02716 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. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . 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. 2026-05-18T06:47:58+00:00