https://doi.org/10.1351/goldbook.E02142
The standard
enthalpy
of
activation
ΔH 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 moldm3). k=kB Th eΔSR eΔHR T The quantity ΔS 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]