Eyring equation

The Eyring equation also known as Eyring-Polanyi equation in chemical kinetics relates the reaction rate to temperature. It was developed almost simultaneously in 1935 by Henry Eyring, M.G. Evans and Michael Polanyi. This equation follows from the transition state theory (aka, activated-complex theory) and contrary to the empirical Arrhenius equation this model is theoretical and based on statistic thermodynamics.

The general form of the Eyring-Polanyi equation resemble somewhat to the Arrhenius equation:

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

where ΔG^‡ is the Gibbs free energy of activation, k_B is Boltzmann's constant, and h is Planck's constant.

It can be rewritten as:

$k=\left({\frac {k_{B}T}{h}}\right)exp\left({\frac {\Delta S^{\ddagger }}{R}}\right)exp\left(-{\frac {\Delta H^{\ddagger }}{RT}}\right)$

To find the linear form of the Eyring-Polanyi equation:

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

where:

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

A certain chemical reaction is performed at different temperatures and the reaction rate is determined. The plot of $\ \ln(k/T)$ versus $\ 1/T$ gives a straight line with slope $\ -\Delta H^{\ddagger }/R$ from which the enthalpy of activation can be derived and with intercept $\ \ln(k_{b}/h)+\Delta S^{\ddagger }/R$ from which the entropy of activation is derived.

References

• Evans, M.G. (1935). Trans. Faraday Soc. 31: 875. {{cite journal}}: Missing or empty |title= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: date and year (link)

• Eyring, H. (1935). J. Chem. Phys. 3: 107. {{cite journal}}: Missing or empty |title= (help)CS1 maint: date and year (link)

• Eyring, H. (1931). Z. Phys. Chem. Abt. B. 12: 279. {{cite journal}}: Missing or empty |title= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: date and year (link)

• Laidler, K.J. (1983). "The development of Transition-State Theory". J. Phys. Chem. 87: 2657–2664. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: date and year (link)

• Polanyi, J.C. (1987). "Some concepts in reaction dynamics. Science". 236 (4802): 680–690. {{cite journal}}: Cite journal requires |journal= (help)CS1 maint: date and year (link)

External links

• Eyring equation at the University of Regensburg