Criteria for Reversible and Irreversible Reactions

If an electrode reaction is not entirely controlled by the kinetics but also by diffusion we can express the overvoltage r/ by two terms, namely 

These energies and potentials are illustrated in an energy diagram as given in Fig. 7.9. In the non-equilibrium case the anodic current can be derived from Eqs. (7.2c), (7.10) and (7.25) and one obtains 

In the case of slow reaction kinetics, quite high overvoltages are required to obtain a measurable current. Then Ain s determined by Eq. (7.11) and Aiff by /tim (EQ-(7.30b). We have then 

This equation looks similar to Eq. (7.33) which was derived for the diffusion-controlled case. Equation (7.42) differs from Eq. (7.33), however, insofar as here the half-wave potential depends on and, hence, on the rotation speed of the electrode. On the other hand, if the current is only diffusion controlled, Eq. (7.33) determines the current-potential curve. In this case, independent of and, hence, also independent of the rotation speed (Eq. (7.32)). 

According to these differences with respect to investigation of the ro- 

This method can lead to misinterpretations, however [2]. In the case of semiconductors, the latter method would even be useless because then a = 0 (see Section 7.3.4). 

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