Eyring equation

The Eyring equation (occasionally also known as Eyring–Polanyi equation) is an equation used in chemical kinetics to describe changes in the rate of a chemical reaction against temperature. It was developed almost simultaneously in 1935 by Henry Eyring, Meredith Gwynne Evans and Michael Polanyi. The equation follows from the transition state theory, also known as activated-complex theory. If one assumes a constant enthalpy of activation and constant entropy of activation, the Eyring equation is similar to the empirical Arrhenius equation, despite the Arrhenius equation being empirical and the Eyring equation based on statistical mechanical justification.

General formEdit

The general form of the Eyring–Polanyi equation somewhat resembles the Arrhenius equation:

 

where ΔG is the Gibbs energy of activation, κ is the transmission coefficient, kB is Boltzmann's constant, and h is Planck's constant. The transmission coefficient is often assumed to be equal to one as it reflects what fraction of the flux through the transition state proceeds to the product without recrossing the transition state, so a transmission coefficient equal to one means that the fundamental no-recrossing assumption of transition state theory holds perfectly.

It can be rewritten as:[1]

 

One can put this equation in the following form:

 

where:

If one assumes constant enthalpy of activation, constant entropy of activation, and constant transmission coefficient, this equation can be used as follows: A certain chemical reaction is performed at different temperatures and the reaction rate is determined. The plot of   versus   gives a straight line with slope   from which the enthalpy of activation can be derived and with intercept   from which the entropy of activation is derived.

AccuracyEdit

Transition state theory requires a value of the transmission coefficient, called   in that theory. This value is often taken to be unity (i.e., the species passing through the transition state   always proceed directly to products AB and never revert to reactants A and B). To avoid specifying a value of  , the rate constant can be compared to the value of the rate constant at some fixed reference temperature (i.e.,  ) which eliminates the   factor in the resulting expression if one assumes that the transmission coefficient is independent of temperature.

Error propagation formulasEdit

Error propagation formulas for   and   have been published. [2]

NotesEdit

  1. ^ Espenson, James H. (1981). Chemical Kinetics and Reaction Mechanisms. McGraw-Hill. p. 117. ISBN 0-07-019667-2.
  2. ^ Morse, Paige M.; Spencer, Michael D.; Wilson, Scott R.; Girolami, Gregory S. (1994). "A Static Agostic α-CH-M Interaction Observable by NMR Spectroscopy: Synthesis of the Chromium(II) Alkyl [Cr2(CH2SiMe3)6]2- and Its Conversion to the Unusual "Windowpane" Bis(metallacycle) Complex [Cr(κ2C,C'-CH2SiMe2CH2)2]2-". Organometallics. 13: 1646. doi:10.1021/om00017a023.

ReferencesEdit

  • Evans, M.G.; Polanyi M. (1935). "Some applications of the transition state method to the calculation of reaction velocities, especially in solution". Trans. Faraday Soc. 31: 875–894. doi:10.1039/tf9353100875.
  • Chapman, S. and Cowling, T.G. (1991). "The Mathematical Theory of Non-uniform Gases: An Account of the Kinetic Theory of Viscosity, Thermal Conduction and Diffusion in Gases" (3rd Edition). Cambridge University Press, ISBN 9780521408448

External linksEdit