enthalpy of activation, Δ‡H⚬

https://doi.org/10.1351/goldbook.E02142
The standard enthalpy of activation Δ‡H⚬ 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⚬, where c⚬ denotes the standard concentration (usually 1 moldm−3). k=kB Th eΔ‡S⚬R e−Δ‡H⚬R T The quantity Δ‡S⚬ is the standard entropy of activation, and care must be taken with standard states. In this equation kB 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 ∘ on the symbol are often omitted. The symbol is frequently (but incorrectly) written ΔH‡, where the standard symbol is omitted and the ‡ is placed after the H.
Sources:
Green Book, 2nd ed., p. 56 [Terms] [Book]
PAC, 1993, 65, 2291. (Nomenclature of kinetic methods of analysis (IUPAC Recommendations 1993)) on page 2294 [Terms] [Paper]
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1113 [Terms] [Paper]
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 164 [Terms] [Paper]
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. https://doi.org/10.1351/goldbook.E02142 RIS BibTex EndNote
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