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 $k c °$ , where $c °$ denotes the standard concentration (usually $1 mol dm −3$ ).

$k = k B T h e Δ ‡ S ° R e − Δ ‡ H ° R T$

The quantity $Δ ‡ S °$ is the standard entropy of activation, and care must be taken with standard states. In this equation $k B$ 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 $°$ on the symbol are often omitted. The symbol is frequently (but incorrectly) written $Δ H ‡$ , where the standard symbol is omitted and the $‡$ is placed after the $H$ .

Source:

PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 164

PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1113

Green Book, 2nd ed., p. 56

PAC, 1993, 65, 2291 (Nomenclature of kinetic methods of analysis (IUPAC Recommendations 1993)) on page 2294