Why Are Some Electrode Reactions Fast

One mode may be referred to as the make-before-break mechanism. This involves the gradual creation of the new bonds, while the old bonds are gradually weakened, in a concerted manner. A nice example of such a process observed almost a century ago) is the spontaneous formation of adsorbed hydrogen atoms on nickel and some other metals in contact with molecular hydrogen in the gas phase, as represented by Eq. (19.46) 

No energy of activation was detected, although the energy of dissociation of molecular hydrogen is quite substantial (4.52 eV). 

The other mode involves reactions that advance in small steps from the initial to the final state. For example, the electrochemical reduction of oxygen is a slow process, even in the presence of the best of catalysts known today, as is well known in the field of fuel cells. Yet, this very same process takes place readily in living organisms, where it occurs in many small steps (assisted, of course, by enzymes acting as catalysts). 

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