Negative Apparent Electrochemical Enthalpy of Activation: The Reduction of Bromate at the Dropping Mercury Electrode in Alkaline Solutions

The reduction of bromate in alkaline solutions at a dropping mercury electrode has been studied as a function of temperature. Transfer coefficients and apparent enthalpy of activation were calculated. The apparent electrochemical enthalpy of activation was found to change sign in the linear Tafel region. The potential at which the value of ∆H^o# is zero is positive with respect to the half-wave-potential for the monovalent alkali cations, while it is negative to E^l/2 at the edge of the linear Tafel region for Ca₊₂. In order to correct for diffuse double-layer effects, different reduction mechanisms were postulated. The best results, in view of the temperature independence of the transfer coefficient, were obtained by postulating a scheme in which the negatively charged bromate ions are adsorbed on a layer of the positive ions of the supporting electrolyte, which dominates the outer Helmholtz plane (OHP) in the range of potentials where the reduction takes place. Other models did not yield strict independence of the transfer coefficient of temperature. However, the involvement of ion pair formation cannot be completely ruled out. The negative apparent electrochemical enthalpy of activation can be explained by a sufficiently negative enthalpy for the preceding adsorption equilibrium, which can lead to a negative apparent electrochemical enthalpy of activation for the overall process, although ∆H^o# for the rate-determining step proper is positive. The observation of a negative apparent electrochemical enthalpy of activation within the linear Tafel region can be used as an important mechanistic tool for elucidating the mechanism of electrode reactions.