Calculation of the Tafel Slope

Equipped with the assumption of quasi-equilibrium, we can now proceed to calculate the Tafel slopes and some other kinetic parameters for a few very simple cases, to show how such calculations are made. In Chapter 7 we shall discuss the kinetics of several reactions that either have been important in the development of the theory of electrode kinetics or are of current practical importance. 

Consider again the chlorine evolution reaction and let us assume first that step (6.6) is rate-determining 

This equation is applicable at high overpotentials, where the reverse reaction can be ignored. Also, it is assumed in writing this equation (and all following kinetic 

Considering Eq. (6.11), we have good reason to assume that the partial coverage 6 is very small, since the intermediate is formed in the rate-determining step and is removed by the following step, which is much faster. Taking 0 1 and hence (1-0) 1, we can rewrite Eq. (6.11), to a very good approximation, as  

For the overall reaction we can then write j = nF feiCb.a-exp = joexp 

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It is important to remember that some assumptions have been made in the derivation of Fig.l. First, the equations given there are applicable only if the electron transfer is the rate determining step in the partial corrosion reactions. This is important with respect to the calculation of the Tafel slope (RT/anF) or the interpretation of an experimental one. It is further assumed that during the polarization of the test electrode (corroding piece of metal) the composition of the solution in the vicinity of the electrode remains... 

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