Proton- and ion-transfer reactions

Abstract

We consider the transfer of an ion or proton from the solution to the surface of a metal electrode; often this is accompanied by a simultaneous discharge of the transferring particle, such as by a fast electron transfer. The particle on the surface may be an adsorbate as in the reaction: . . .Cl - (sol) ⇋ Clad + e- (metal) . . . (9.1) In this case the discharge can be partial; that is, the adsorbate can carry a partial charge, as discussed in Chapter 4. Alternatively the particle can be incorporated into the electrode as in the deposition of a metal ion on an electrode of the same composition, or in the formation of an alloy. An example of the latter is the formation of an amalgam such as: . . . Zn2++2e- ⇋ Zn(Hg) . . . (9.2) The reverse process is the transfer of a particle from the electrode surface to the solution; often the particle on the surface is uncharged or partially charged, and is ionized during the transfer. Ion- and proton-transfer reactions are almost always preceded or followed by other reaction steps. We first consider only the chargetransfer step itself. Ions and protons are much heavier than electrons. While electrons can easily tunnel through layers of solution 5 to 10 Å thick, protons can tunnel only over short distances, up to about 0.5 Å, and ions do not tunnel at all at room temperature. The transfer of an ion from the solution to a metal surface can be viewed as the breaking up of the solvation cage and subsequent deposition, the reverse process as the jumping of an ion from the surface into a preformed favorable solvent configuration. In simple cases the transfer of an ion obeys a slightly modified form of the Butler-Volmer equation. Consider the transfer of an ion from the solution to the electrode. As the ion approaches the electrode surface, it loses a part of its solvation sphere, and it displaces solvent molecules from the surface; consequently its Gibbs energy increases at first.