An expression for the rate of electrode reactions

For any type of electrode reaction in solution, the Arrhenius expression relates the activation enthalpy, A//, with the rate constant, k  

A being the pre-exponential factor. In an electron transfer reaction, the rearrangement of the ionic atmosphere is a fundamental step, and thus it is useful to include the activation entropy AS. The reorientation and rearrangement causes the separation between the energy levels to be different in the activated complex than in the initial state. If we write the pre-exponential factor, A, as 

Consider a half-reaction of first order occurring at an inert metallic electrode  

This is the formulation of electrode kinetics first derived by Butler and Volmer4. The observed current for kinetic control of the electrode reaction is proportional to the difference between the rate of the oxidation and reduction reactions at the electrode surface and is given by 

In many cases electrode processes involving the transfer of more than one electron take place in consecutive steps. The symmetry of the activation barrier referred to above relates to the rate-determining step. For example, a two-electron transfer involving a pre-equilibrium for the first electron transfer and the second electron transfer as the ratedetermining step leads to (cm) = 1 + 0.5 = 1.5. From this we might calculate or = 0.75, which is not a reflection of the position of the activated complex on the reaction coordinate. Thus extreme care must be 

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Fundamentals of kinetics and mechanism of electrode reactions 4.4 An expression for the rate of electrode reactions... 

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