Transfer, Activation, or Kinetic Control

As we have seen in Section 3.1.2.3, activation energy E is experimentally derived. To illustrate the effect of an applied electric potential on the rate of reaction we used G, the free energy of activation, in Fig. 3.4 instead of E. Consider now the reversible step  

When an electrochemical reaction is carried out, AG can be influenced by the imposed electric field, which is a source of potential energy. See Fig. 3.4, where we apply a cathodic field and the value of AG is reduced and that of AG increased. The fraction of the total electrode potential E affecting AG is a that affecting AG% is (1 — a) , where a is the transfer coefficient. Translated into energy per mole these potentials become ocE nF and (1 — (x)E nF, where F is the Faraday constant. The velocities of the forward and backward electrochemical reactions are  

FIGURE 3.4. Effect of applied field on the free energy of activation. 

When values of E are inserted in Eqs. (3.70) and (3.71), attention must be given to its sign. Here the potential E is negative and the addition of xE nF represents a reduction in the free energy of activation for the forward reaction (Fig. 3.4). 

Reaction velocity r, in moles per unit area of electrode in unit time, can be converted directly to current density i by multiplying by nF  

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