Theoretical study of the kinetics of the homogeneous ee electrochemical reaction

Abstract

A theoretical study of a homogeneous 2 e reaction Q ahR ahV is presented, when the electrons are successively exchanged. Electrochemical equilibrium is assumed. General kinetic diagrams in the planes log[ k dif( k 1 k 2) −1/2]−Δ E 0 and log[( k 1/ k 2)( c Q/ c v)] 1/2−log[( k f K 1/2)( k 1 k 2) −1/2] are given ( k dif: diffusion-limited rate constant for the disproportionation reaction; k 1, k 2: self-exchange rate constant for each of the monoelectronic reactions; Δ E 0: difference between the redox potentials of the two steps; K: disproportionation equilibrium constant). The relative influence of the disproportionation and of the self-exchange reactions depend on the relative concentration of the species. The influence of the disproportionation is maximal for Δ E 0=0. Numerical evaluation shows that the disproportionation will often have to be taken into account even when the reactions are widely separated (Δ E 0≫0). The conditions in which an apparent direct reaction from Q to V or vice-versa takes place (Δ E 0≪0) are defined. It is shown that the reaction is then equivalent to a monoelectronic process, with an apparent rate constant which is a general function of the disproportionation rate constant k f, of k 1, k 2, Δ E 0 and of the ratio of the concentrations of Q and V. The reaction can be controlled by one of the self-exchange reactions and/or by the disproportionation.