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.