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 e lectrode has been studied as a function of temperature. Transfer coeff icients and apparent enthalpy of activation were calculated. The appar ent electrochemical enthalpy of activation was found to change sign in the linear Tafel region. The potential at which the value of DELTAH-d egrees# is zero is positive with respect to the half-wave-potential fo r the monovalent alkali cations, while it is negative to E1/2, at the edge of the linear Tafel region for Ca+2. In order to correct for diff use double-layer effects, different reduction mechanisms were postulat ed. The best results, in view of the temperature independence of the t ransfer coefficient, were obtained by postulating a scheme in which th e negatively charged bromate ions are adsorbed on a layer of the posit ive ions of the supporting electrolyte, which dominates the outer Helm holtz 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 form ation cannot be completely ruled out. The negative apparent electroche mical enthalpy of activation can be explained by a sufficiently negati ve enthalpy for the preceding adsorption equilibrium, which can lead t o a negative apparent electrochemical enthalpy of activation for the o verall process, although DELTAH-degrees# for the rate-determining step proper is positive. The observation of a negative apparent electroche mical enthalpy of activation within the linear Tafel region can be use d as an important mechanistic tool for elucidating the mechanism of el ectrode reactions.