Term: transition state theory
https://doi.org/10.1351/goldbook.T06470

Definition:
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: \[k=\frac{k_{\text{B}}\ T}{h}\ K^{\ddagger }\] where kB is the Boltzmann constant and h is the Planck constant. The rate constant can also be expressed as: \[k=\frac{k_{\text{B}}\ T}{h}\ \exp (\frac{\Delta ^{\ddagger }S^{\,\unicode{x26ac}}}{R})\ \exp (- \frac{\Delta ^{\ddagger }H^{\,\unicode{x26ac}}}{R\ T})\] 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: \[k=\frac{k_{\text{B}}\ T}{h}\ \exp (- \frac{\Delta ^{\ddagger }G^{\,\unicode{x26ac}}}{R\ T})\] 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.

Related Terms:
1) elementary reactions (http://doi.org/10.1351/goldbook.E02035).
2) energy of activation (http://doi.org/10.1351/goldbook.E02108).
3) gibbs energy of activation (http://doi.org/10.1351/goldbook.G02631).
4) potential-energy profiles (http://doi.org/10.1351/goldbook.P04779).
5) equilibrium constant (http://doi.org/10.1351/goldbook.E02177).
6) rate constant (http://doi.org/10.1351/goldbook.O04322).
7) boltzmann constant (http://doi.org/10.1351/goldbook.B00695).
8) planck constant (http://doi.org/10.1351/goldbook.P04685).
9) entropy of activation (http://doi.org/10.1351/goldbook.E02150).
10) entropy (http://doi.org/10.1351/goldbook.E02149).
11) activated complex (http://doi.org/10.1351/goldbook.A00092).
12) enthalpy of activation (http://doi.org/10.1351/goldbook.E02142).
13) enthalpy (http://doi.org/10.1351/goldbook.E02141).
14) reaction coordinate (http://doi.org/10.1351/goldbook.R05168).
15) energy profile (http://doi.org/10.1351/goldbook.E02112).
16) transmission coefficient (http://doi.org/10.1351/goldbook.T06479).
17) probability (http://doi.org/10.1351/goldbook.P04855).

Source: PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 190 (https://doi.org/10.1351/pac199668010149)

Citation: 'transition state theory' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.T06470

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