The mechanism of electron transfer at an electrode

5 The relation between current and reaction rate the exchange current 

In this chapter the mechanisms of electrode reactions are explained for the most simple case of an electron transfer without chemical transformation, i.e. without formation or breaking chemical bonds. Other more complex cases are also referred to. Comparison with electron transfer reactions in homogeneous solution are made. 

In a system involving reagents and products at equilibrium, the rates of the reactions in each direction are equal. Equilibrium can thus be seen as a limiting case, and any kinetic model must give the correct equilibrium expression. For reactions at an electrode, half-reactions the equilibrium expression is the Nernst equation. 

We consider the case of an oxidation of reduction at an electrode without chemical transformation, an example being 

Steps 2-5 are included in the charge transfer rate constant, ka or kc, and include adsorption of the reagent on the electrode surface, which in the case of a soluble product will afterwards de-adsorb. Steps 2-4 can be seen as a type of pre-equilibrium before the electron transfer. During the electron transfer itself all positions of the atoms are frozen, obeying the Franck-Condon principle (adiabatic process). 

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