A theory of the rates of elementary reactions which assumes a special type of equilibrium, having an equilibrium constant, to exist between reactants and activated complexes.
According to this theory the rate constant is given by:
where
is the Boltzmann constant and
is the Planck constant. The rate constant can also be expressed as:
where
, the entropy of activation, is the standard molar change of entropy when the activated complex is formed from reactants and
, the enthalpy of activation, is the corresponding standard molar change of enthalpy. The quantities
(the energy of activation) and
are not quite the same, the relationship between them depending on the type of reaction.
Also:
where
, 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,
, 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
,
and
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