IUPAC > Gold Book > alphabetical index > T > transition state theory


transition state theory

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 = k B T h K ‡
where k B is the Boltzmann constant and h is the Planck constant. The rate constant can also be expressed as:
k = k B T h 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 E a (the energy of activation) and Δ ‡ H ° are not quite the same, the relationship between them depending on the type of reaction. Also:
k = k B T h 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.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 190
Interactive Link Maps
First LevelSecond LevelThird Level
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.
Last update: 2014-02-24; version: 2.3.3.
DOI of this term: doi:10.1351/goldbook.T06470.
Original PDF version: http://www.iupac.org/goldbook/T06470.pdf. The PDF version is out of date and is provided for reference purposes only. For some entries, the PDF version may be unavailable.
Current PDF version | Version for print | History of this term