enthalpy of activation, Δ‡H°

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 kc°, where c° denotes the standard concentration (usually mol dm-3). \[k = \frac{k_{\text{B}}\ T}{h}\ \text{e}^{\frac{\Delta ^{\ddagger }S^{\,\unicode{x26ac}}}{R}}\ \mathrm{e}^{\frac{-\Delta ^{\ddagger }H^{\,\unicode{x26ac}}}{R\ T}}\] 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.
Green Book, 2nd ed., p. 56 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
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