https://doi.org/10.1351/goldbook.T06470
A theory of the rates of
elementary reactions
which assumes a special type of equilibrium, having an
equilibrium constant
K, to exist between reactants and activated complexes. According to this theory the
rate constant
is given by: k=kB Th K where kB is the
Boltzmann constant
and h is the
Planck constant
. The
rate constant
can also be expressed as: k=kB Th exp(ΔSR) exp(ΔHR T) where ΔS, the
entropy of activation
, is the standard molar change of
entropy
when the
activated complex
is formed from reactants and ΔH, the
enthalpy of activation
, is the corresponding standard molar change of
enthalpy
. The quantities Ea (the
energy of activation
) and ΔH are not quite the same, the relationship between them depending on the type of reaction. Also: k=kB Th exp(ΔGR T) where ΔG, known as the
Gibbs energy of activation
, is the standard molar Gibbs energy change for the conversion of reactants into
activated complex
. A plot of standard molar Gibbs energy against a
reaction coordinate
is known as a Gibbs-
energy profile
; such plots, unlike
potential-energy profiles
, are temperature-dependent. In principle the equations above must be multiplied by a
transmission coefficient
, κ, which is the
probability
that an
activated complex
forms a particular set of products rather than reverting to reactants or forming alternative products. It is to be emphasized that ΔS, ΔH and ΔG occurring in the former three equations are not ordinary thermodynamic quantities, since one degree of freedom in the
activated complex
is ignored. Transition-state theory has also been known as absolute rate theory, and as activated-complex theory, but these terms are no longer recommended.
Source:
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 190 [Terms] [Paper]