Asymmetry in Tafel plots

The asymmetry of the plots is related first to the asymmetry of reactants and prodncts. In the anodic oxidation the reactant is Cr(OH2)g and the product is Cr(OH2)6 +, hence and/p =f . On the other hand, in the cathodic reduction the reactant is Cr(OH2)6 + and the product is Cr(OH2)g, henceand/p = Xed- Th potential-energy curves of reactant and product are asymmetric for each reaction, and this asymmetry is reversed between electrochemical oxidation and reduction. With the data available in Appendix El, this contribution to the asymmetry of the cathodic reduction of the Cr(OH2)5 /Cr(OH2V+ couple system can be shown to be given by =/ox = 3.75x10 kJ mol A and /p=, = 2.35xl03klmol- A-2. 

The pre-exponential factor provides another source of asymmetry. The barrier for tunnelling is raised by the application of an anodic overpotential and lowered by applying a cathodic one. Where there is overpotential the barrier for tunnelling varies according to the expression  

However, these two contributions are not enough to account for the full asymmetry in the Tafel plots, although they can account for a factor of ca. 40 times faster rates for the anodic process when compared with the cathodic ones. Further corrections are needed to account for the experimental data. An additional contribution may be due to a difference in entropy change for the cathodic and anodic processes. 

2X10 cm sec and the double-layer corrected rate constant is 2X10 cm sec [45]. In this case the double-layer correction decreases the rate because the potential drop across the diffuse layer is negative. In view of aU the approximations and corrections involved, the calculations provide a reasonable estimate of the experimental rate. 

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