Thermodynamics of the transition state and the application to interfacial reactions

Abstract

According to transition-state theory, when the equilibrium hypothesis is valid, activated complexes formed from reactants may be treated thermodynamically in the same way as other species present in very small amounts. Inclusion of these complexes in the Gibbs–Duhem equation for the solution enables an expression to be derived for the dependence of the equilibrium concentration of activated complexes on the solution composition. When the transmission coefficient of the reaction is insensitive to the reaction environment this expression leads directly to the effect of solution composition on the reaction rate. For bulk solutions, this approach is entirely equivalent to that published recently based on the Kirkwood–Buff theory of solutions. For reactions at interfaces the Gibbs adsorption equation plays the same role as the Gibbs–Duhem equation for reactions in bulk and similar arguments apply. These arguments are used to obtain a general expression for the transfer coefficient β in the Butler–Volmer equation of electrode kinetics and for the dependence of β on solution composition. A similar treatment can also be applied to the effects on reaction rates of macromolecules which interact with reactants. Finally, the situation in which adsorbed material is not in equilibrium with that in the bulk solution immediately adjacent to the interface is considered and general guidelines for dealing with this type of situation are forwarded.