transition state theory

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⁡(Δ‡S⚬R) exp⁡(−Δ‡H⚬R T) where Δ‡S⚬, the
entropy of activation
, is the standard molar change of
entropy
when the
activated complex
is formed from reactants and Δ‡H⚬, the
enthalpy of activation
, is the corresponding standard molar change of
enthalpy
. The quantities Ea (the
energy of activation
) and Δ‡H⚬ are not quite the same, the relationship between them depending on the type of reaction. Also: k=kB Th exp⁡(−Δ‡G⚬R T) where Δ‡G⚬, 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 Δ‡S⚬, Δ‡H⚬ and Δ‡G⚬ 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]
Citation: 'transition state theory' in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.T06470 RIS BibTex EndNote
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Last revised: February 24, 2014