Temperature Dependence of the Tafel Slope and Electrochemical Barrier Symmetry Factor, β, in Electrode Kinetics

The significance of the now well‐established situation that the Tafel slopes, , for simple charge‐transfer processes at electrodes are usually not represented with respect to variation with temperature, , by the conventional relation , where β is a constant‐valued electrochemical charge‐transfer barrier‐symmetry coefficient, is examined in the light of recent comments on this problem by Gileadi. Clear evidence is given that has the form for proton transfer at Hg in water and various other solvents, where and are "enthalpic" and "entropic" components of the overall β, corresponding to experimentally observable potential‐dependences of both the enthalpy and the entropy of activation, respectively. The frequent deviation from conventional behavior thus arises because the entropy of activation, as well as the energy of activation, can be potential‐dependent, a situation that, until recently, has been neglected in interpretations of electrode‐kinetic experiments. The origin of the conventional effect of potential on electrode reaction rates, through the change of electrode work function, Φ, with overpotential or electrode potential, , , is examined critically in relation to the potential‐dependent surface‐potential component, , in Φ, which can also be . Finally, an interesting compensation effect in the variation of with for proton transfer at Hg from a series of proton‐donors in several nonaqueous solvents is demonstrated and it is shown that is only near 0.5 when is zero.