Entropic and enthalpic components of the symmetry factor for electrochemical proton transfer from various proton donors over a wide temperature range

Abstract

In order to provide new information on the potential-modulated activation process in the elementary proton- and electron-transfer step of proton discharge in the hydrogen evolution reaction (h.e.r.), the enthalpic (β H ) and entropic ( Tβ S ) components of the overall charge-transfer symmetry factor, β, have been evaluated. These data are obtained from polarization curves for the h.e.r. at Hg from a series of oxonium, ammonium and molecular acid type proton donors in various solvents over a wide range of temperature through the m.p. of Hg. β S and β H define the form of the Tafel slope b as a function of temperature: b = RT/( β H + Tβ S ) F. β S and β H are not related to dielectric polarizability of the solvent but may depend on its proton affinity. However, clear linear relations are demonstrated between β S and the log of the electrochemical Arrhenius factor containing the entropy of activation, and between β H NH + and Δ H ≠. These relations have different slopes for proton discharge from OH + proton donors. Also Δ H ≠ and δ S ≠ are linearly related for the series of proton donors in the way familiar in “compensation effect” plots between Δ H and Δ S quantities in condensed media. β S and β H usually change discontinuously, for a given system, through the m.p. of Hg.