TEMPERATURE-DEPENDENCE OF SYMMETRY FACTORS AND THE SIGNIFICANCE OF EXPERIMENTAL ACTIVATION-ENERGIES

The temperature dependence of Tafel slopes and symmetry factors is discussed. The meaning and significance of Arrhenius log j vs. 1/T plots when beta = beta(H), beta = beta(H) + beta(s)T and beta = beta(s)T are analyzed. When the enthalpy DELTAH(not-equal) and entropy, DELTAS(not-equal) of activation are formally independent of temperature and beta = beta(H) or beta = beta(H) + beta(s)T, these plots are linear and yield DELTAH(not-equal) at a given electrode potential. When beta = beta(s)T, DELTAS(not-equal) is the principal factor controlling the reaction rate. Normally, DELTAS(not-equal) varies with temperature and no linear log j vs. 1/ T plots are possible. In this case log j vs. T, and not Arrhenius plots, are meaningful and should be used. The general case is considered when both DELTAH(not-equal) and DELTAS(not-equal) are functions of T. By separating temperature-dependent and temperature-independent terms in DELTAH(not-equal) and DELTAS(not-equal), the effective temperature-independent enthalpy and the effective temperature-dependent entropy of activation are defined. With such separation all three cases of beta can be accounted for.