Some Comparisons Between the Energetics of Electrochemical and Homogeneous Electron-Transfer Reactions

Abstract

Abstract : Some conceptual relationships between the kinetics of corresponding electrochemical and homogeneous redox processes are discussed and applied to experimental data for suitable outer-sphere reactions in order to illustrate the utility of electrochemical kinetics for gaining some fundamental insights into the energetics of electron-transfer processes. It is pointed out that electrochemical kinetics and thermodynamics measurements as a function of temperature and electrode potential yield direct information on the shapes of the potential-energy surfaces and free energy barriers for individual redox couples. Comparisons between kinetics parameters for corresponding electrochemical and homogeneous exchange reactions show reasonable agreement with the predictions of the conventional 'weak overlap' model for several aquo redox couples, but exhibit substantial disagreement for couples containing amine and related ligands. These latter discrepancies may arise from the closer approach of the amine reactants to the electrode surface compared with the strongly solvated aquo complexes. A comparison is also made between the effects of varying the thermodynamic driving force upon the kinetics of related electrochemical and homogeneous reactions. It is shown that the apparent discrepancies seen between the predictions of the harmonic oscillator model and experimental data for some highly exoergic homogeneous reactions may be related to the anomalously small dependence of the rate constant upon overpotential observed for the electrooxidation of aquo complexes. This behavior seems most likely to be due to a marked asymmetry of the reactant and product free energy barriers for these half reactions.