Eyringligningen

Eyringligningen, commonly known as the Eyring equation, is a formula in chemical kinetics derived from transition state theory. Introduced by Henry Eyring in 1935, it describes how the rate constant k of a reaction depends on temperature and the activation parameters of the transition state, providing a thermodynamic framework beyond the Arrhenius model.

In its standard form, k = (k_B T/h) exp(-ΔG‡/RT) = (k_B T/h) exp(ΔS‡/R) exp(-ΔH‡/RT), where k_B is Boltzmann’s constant, h is Planck’s constant, R is the gas constant, ΔH‡ is the activation enthalpy, ΔS‡ is the activation entropy, and ΔG‡ = ΔH‡ − TΔS‡ is the activation Gibbs energy. This expresses the temperature dependence of the rate constant through both enthalpic and entropic contributions of the transition state.

The equation provides a framework for analyzing how k varies with temperature and for extracting activation parameters from experimental rate constants. A common practice is to linearize the relation by plotting ln(k/T) versus 1/T, yielding a slope of −ΔH‡/R and an intercept of ln(k_B/h) + ΔS‡/R.

Assumptions include a quasi-equilibrium between reactants and the transition state and a transmission coefficient near unity; it usually presumes a single dominant pathway and neglects quantum tunneling or dynamic recrossing. Deviations can arise at very low temperatures or in systems with significant tunneling or non-classical effects.

The Eyring equation remains a foundational tool in chemical kinetics, used for solution- and enzyme-catalyzed reactions, and can be extended with a transmission coefficient κ to account for non-classical effects. It complements the Arrhenius approach by emphasizing thermodynamic contributions to reaction rates.