Microscopic Reversibility and the Anodic Dissolution of Metals

The law of microscopic reversibility stipulates that reactions taking place in the forward and backward direction must follow the same path or, more precisely, must cross the same Gibbs energy of activation barrier. Admittedly, this is not a universal law, and there can be situations under which it does not apply, but such situations are rare, so that it can be assumed to apply, unless proven otherwise. We shall not go into the fine details of this law, except to note that it is implicitly assumed to apply whenever one writes the common relationship. 

we recall that the three constants in the above equation can be written as 

the standard Gibbs energy of a reaction is equal to the difference between the standard Gibbs energies of activation of the forward and backward reactions, which only applies when the reaction crosses the same energy barrier in both directions, namely, when the law of microscopic reversibility applies. 

The purpose of introducing the concept of microscopic reversibility is to provide further evidence, confirming that charge must be transferred across the interface by metal cations, not by electrons. To prove this, consider the anodic process of metal dissolution, written commonly as 

oxidation in electrode kinetics is, by definition, a process in which electrons are transferred from a species in solution to the electrode. But viewing Eq. (19.26) we note that there is no such species in solution. The electrons shown on the right-hand side of this equation are in the metal and the only other species in solution (except water as the solvent) is the cation in its stable hydrated and oxidized form. Moreover, following the argument leading to Eqs. (19.21) and (19.22) above, Eq. (19.26) should be rewritten as 

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