Role of the Solvent in Kinetics of Heterogeneous Electron and Ion Transfer Reactions

Solvent effects studied in simple heterogeneous redox reactions, and for amalgam formation reactions have been reviewed. It is emphasized that on the basis of a regression analysis of the data, solvent effects relating to the pre- exponential factor of the electron transfer rate constant can be separated from those for the exponential term. Analysis of data in non-Debye solvents such as the alcohols shows that the parameters relating to the second dielectric relaxation process are more important than those for the first in determining the magnitude of the rate constant. Examination of solvent effects for amalgam formation reactions reveals that they are quite different from those observed in electron transfer processes. As a result, it is concluded that the rate determining step in the amalgam formation reaction is ion transfer, not electron transfer, and that this rate limiting step is often located in the inner part of the double layer. Data recently obtained for the electroreduction of CuI in the nitrile solvents are presented and discussed with respect to solvent trends found for other systems.