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