CHM 401

 

Kinetic Considerations

Overpotentials

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


Electron Transfer Mechanisms

Commonly, two types of mechanisms are found: inner sphere and outer sphere


Inner 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.


Outer sphere

Reacting species collide and during the collision the electron transfer takes place

The transition state has no definable bond formation between the two reactants.