Even though the thermodynamic potential for a reaction may be favorable, the reaction may proceed slowly. This is because an additional driving force, called the overpotential, is required to make the reaction proceed at a reasonable rate. The overpotential acts like an activation barrier to the kinetic process.
Zn metal does not react rapidly with neutral water, despite having a favorable potential:
Zn2+(aq) + 2 e– → Zn(s) Eo = –0.762 V (pH independent)
2 H+(aq) + 2 e– → H2(g) E = –0.0591×pH V
so for the reaction
Zn(s) + 2 H+(aq) → H2(g) + Zn2+(aq) E = –0.0591(7) – (–0.762) = +0.348 V
The overpotential prevents reaction from occurring.
At lower pH, the potential becomes more positive and reaction proceeds:
at pH = 2, E = –0.0591(2) – (–0.762) = +0.644 V
Commonly, two types of mechanisms are found: inner sphere and outer sphere
The reacting species are linked by a weak covalent bond during the electron transfer.
There is room on the Co(II) to accept another species and OH– is capable of bonding to two metals in a bridging situation, thus the Co(II)-OH-Co(III) bridge forms in the transition state.
Reacting species collide and during the collision the electron transfer takes place
The transition state has no definable bond formation between the two reactants.