Chemistry 401

Latimer Diagrams

This is a method of conveying large amounts of reduction potential information about a series of reductions for a given element.

Simple redox reactions:

VO₂⁺(aq) + 2 H⁺(aq) + e^– → VO²⁺(aq) + H₂O(l)     E^o_red = +1.000 V

VO²⁺(aq) + 2 H⁺(aq) + e^– → V³⁺(aq) + H₂O(l)     E^o_red = +0.337 V

V³⁺(aq) + e^– → V²⁺(aq)     E^o_red = –0.255 V

V²⁺(aq) + 2 e^– → V(s)     E^o_red = –1.13 V

More complex redox reactions (one of many possible examples)

V³⁺(aq) + 3 e^– → V(s)

Disproportionation (one of many possible examples)

2 V³⁺(aq) + H₂O(l) → V²⁺(aq) + VO²⁺(aq) + 2 H⁺(aq)

E°= –0.255 + –0.337 = –0.592 V

(disfavored, rather, the comproportionation reaction will occur)

Pourbaix diagrams

These are phase diagrams for the stability of various species as a function of both pH and reduction potential. Like any phase diagram, the Pourbaix diagram gives the predominant species at the given equilibrium conditions, in this case the pH and E^o.

pH < 3, only +7, +4, +2, and 0 oxidation states available

pH between 4 and 13, +7, +4, +3, +2, and 0 oxidation states

only above pH = 13 can +6 oxidation state be found

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) E°= –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.