Entropy of activation

In chemical kinetics, the entropy of activation of a reaction is one of the two parameters (along with the enthalpy of activation) which are typically obtained from the temperature dependence of a reaction rate constant, when these data are analyzed using the Eyring equation. The standard entropy of activation is symbolized ΔS and equals the change in entropy when the reactants change from their initial state to the activated complex or transition state (Δ = change, S = entropy, ‡ = activation). It determines the preexponential factor A of the Arrhenius equation for temperature dependence of reaction rates. The relationship depends on the molecularity of the reaction: for reactions in solution and unimolecular gas reactions A = (ek_BT/h) exp(ΔS/R), while for bimolecular gas reactions A = (ek_BT/h) (R'T/p) exp(ΔS/R). In these equations e is the base of natural logarithms, h is the Planck constant, k_B is the Boltzmann constant and T the absolute temperature. R' is the ideal gas constant in units of (bar*L)/(mol*K). The factor is needed because of the pressure dependence of the reaction rate. R' = 8.3145 * 10^-2 (bar*L)/(mol*K).