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 with temperature. It was developed almost simultaneously in 1935 by Henry Eyring, Meredith Gwynne Evans and Michael Polanyi. This equation follows from the transition state theory (a.k.a. activated-complex theory) and (if one assumes constant enthalpy of activation and constant entropy of activation) is similar to the empirical Arrhenius equation, although the Arrhenius equation is empirical, and the Eyring equation has a statistical mechanical justification.

General form

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

${\displaystyle \ k={\frac {\kappa k_{\mathrm {B} }T}{h}}\mathrm {e} ^{-{\frac {\Delta ^{\ddagger }G^{\ominus }}{RT}}}}$

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:

${\displaystyle k={\frac {\kappa k_{\mathrm {B} }T}{h}}\mathrm {e} ^{\frac {\Delta ^{\ddagger }S^{\ominus }}{R}}\mathrm {e} ^{-{\frac {\Delta ^{\ddagger }H^{\ominus }}{RT}}}}$

One can put this equation in the following form:

${\displaystyle \ln {\frac {k}{T}}={\frac {-\Delta ^{\ddagger }H^{\ominus }}{R}}\cdot {\frac {1}{T}}+\ln {\frac {\kappa k_{\mathrm {B} }}{h}}+{\frac {\Delta ^{\ddagger }S^{\ominus }}{R}}}$

where:

• ${\displaystyle \ k}$ = reaction rate constant
• ${\displaystyle \ T}$ = absolute temperature
• ${\displaystyle \ \Delta ^{\ddagger }H^{\ominus }}$ = enthalpy of activation
• ${\displaystyle \ R}$ = gas constant
• ${\displaystyle \ k_{\mathrm {B} }}$ = Boltzmann constant
• ${\displaystyle \ h}$ = Planck's constant
• ${\displaystyle \ \Delta ^{\ddagger }S^{\ominus }}$ = entropy of activation

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 ${\displaystyle \ \ln(k/T)}$ versus ${\displaystyle \ 1/T}$ gives a straight line with slope ${\displaystyle \ -\Delta ^{\ddagger }H^{\ominus }/R}$ from which the enthalpy of activation can be derived and with intercept ${\displaystyle \ \ln(\kappa k_{\mathrm {B} }/h)+\Delta ^{\ddagger }S^{\ominus }/R}$ from which the entropy of activation is derived.

Accuracy

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

References

• 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.
• Eyring, H.; Polanyi M. (1931). "Über Einfache Gasreaktionen". Z. Phys. Chem. B. 12: 279–311.
• Laidler, K.J.; King M.C. (1983). "The development of Transition-State Theory". J. Phys. Chem. 87 (15): 2657–2664. doi:10.1021/j100238a002.
• 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
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