The effect of the double layer on the heat of activation of simple electrode reactions

Abstract

Summary Apparent heats of activation have been measured for the electroreduction of hydrogen ion at mercury in 0.1 M NaCl, KCl, and CsCl, and for periodate ion (H 3 IO 6 2− ) in 0.2 M NaOH and KOH. For the first reaction the heat of activation was independent of solution composition, whereas for the second it decreased when the cation in the solution was changed from Na + to K + . In both cases the apparent transfer coefficient depended on the nature of the cation, and for the second also on temperature. It is concluded that the reaction site for both reactions is in the inner layer. The dependence of the heat of activation on the nature of a non-reacting ion in the double layer is attributed to specific ionic interactions which alter the local potential at the reaction site. The fact that the heat of activation of the hydrogen reaction is independent of solution composition indicates either that alkali metal cation adsorption is absent or that it does not depend on the nature of the cation.