Title: enthalpy of activation
Long Title: IUPAC Gold Book - enthalpy of activation
DOI: 10.1351/goldbook.E02142
Status: current
Index: quantity

Definition
The standard enthalpy of activation \(\Delta ^{\ddagger}H^{\,\unicode{x26ac}}\) is the enthalpy change that appears in the thermodynamic form of the rate equation obtained from conventional transition state theory. This equation is only correct for a first order reaction, for which the rate constant has the dimension reciprocal time. For a second order reaction, for which the rate constant has the dimension (reciprocal time) × (reciprocal concentration), the left hand side should be read as \(k\ c^{\,\unicode{x26ac}}\), where \(c^{\,\unicode{x26ac}}\) denotes the standard concentration (usually \(\pu{1 mol dm-3}\)). \[k = \frac{k_{\rm{B}}\ T}{h}\ \rm{e}^{\frac{\Delta ^{\ddagger }S^{\,\unicode{x26ac}}}{R}}\ \mathrm{e}^{\frac{-\Delta ^{\ddagger }H^{\,\unicode{x26ac}}}{R\ T}}\] The quantity \(\Delta ^{\ddagger}S^{\,\unicode{x26ac}}\) is the standard entropy of activation, and care must be taken with standard states. In this equation \(k_{\rm{B}}\) is the Boltzmann constant, \(T\) the absolute temperature, \(h\) the Planck constant, and \(R\) the gas constant. The enthalpy of activation is approximately equal to the activation energy; the conversion of one into the other depends on the molecularity. The enthalpy of activation is always the standard quantity, although the word standard and the superscript \(^{\circ}\) on the symbol are often omitted. The symbol is frequently (but incorrectly) written \(\Delta H^{\ddagger }\), where the standard symbol is omitted and the \(\ddagger \) is placed after the \(H\).

Related Terms
- Boltzmann constant: https://dev.goldbook.iupac.org/terms/view/B00695
- Planck constant: https://dev.goldbook.iupac.org/terms/view/P04685
- activation: https://dev.goldbook.iupac.org/terms/view/A00093
- activation energy: https://dev.goldbook.iupac.org/terms/view/A00102
- enthalpy: https://dev.goldbook.iupac.org/terms/view/E02141
- entropy of activation: https://dev.goldbook.iupac.org/terms/view/E02150
- gas constant: https://dev.goldbook.iupac.org/terms/view/G02579
- molecularity: https://dev.goldbook.iupac.org/terms/view/M03989
- rate constant: https://dev.goldbook.iupac.org/terms/view/R05138
- standard concentration: https://dev.goldbook.iupac.org/terms/view/S05909
- transition state theory: https://dev.goldbook.iupac.org/terms/view/T06470

Sources
- Green Book, 2nd ed., p. 56 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
- PAC, 1993, 65, 2291. 'Nomenclature of kinetic methods of analysis (IUPAC Recommendations 1993)' on page 2294 (https://doi.org/10.1351/pac199365102291)
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1113 (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 164 (https://doi.org/10.1351/pac199668010149)

Other Outputs
- html: https://dev.goldbook.iupac.org/terms/view/E02142/html
- json: https://dev.goldbook.iupac.org/terms/view/E02142/json
- xml: https://dev.goldbook.iupac.org/terms/view/E02142/xml

Citation: Citation: 'enthalpy of activation' 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.E02142
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Accessed: 2026-06-13T04:29:04+00:00