Chapter 7 - Electrode reactions

Abstract

Electrode reactions can be classified into two groups: one in which an electron transfer takes place across the electrode interface and the other in which an ion transfer takes place across the electrode interface. In either group of electrode reactions, the energy level of reacting particles in the electrode depends linearly on the electrode potential. Hence, the reaction affinity can be varied over a wide range by simply controlling the electrode potential. This is one of the characteristics of electrode reactions, in contrast with ordinary chemical reactions. From the flow of electric charge it follows that the cathodic transfer of metal ions requires the electrode to accept electrons from an external cell circuit, and that the anodic transfer of metal ions requires the electrode to donate electrons to an external cell circuit. No electron transfer, however, takes place across the electrode interface. In the regime of high reaction affinity, the rate of the forward or backward reaction is determined by the first leading elementary step. This means that the rate-determining step is not necessarily the same in the forward and backward reactions. Such an apparent alternation of the rate-determining step depending on the direction of reaction takes place without any change in the exchange reaction rate at equilibrium. The chapter concludes that for multistep reaction, the mechanism of single rate determining step is valid only in the affinity regime near the equilibrium; and that the mechanism of multiple rate determining steps predominates in the high affinity regime.