OXYGEN OVERPOTENTIAL AND MECHANISM

(1) During the electrolysis of dil. H₂SO₄ > dil. HNO₃ etc. O₂ gas is evolved at the anode and this anodic process also involves definite Overpotential which is known as ηo₂ , unlike ηH₂, ηo₂ fluctuate and is difficult to measure ηo₂ at constant current density.

(2) Generally, ηo₂ is measured over Pt electrodes using dil. H₂SO₄ saturated with oxygen gas.

(3) The ηo₂ varies linearly with log i as given by the following equation.

ηo₂ = a' + b' log i ( in this respect, it resembles ηH₂)

(4) ηo₂ is independent of the PH of medium ( resembles ηH₂)

Note :-

          Under highly alkaline condenser ηH₂

depends on PH.

5) As ηo₂ is independent of PH of medium and it taking they same for different medium like dil. H₂SO₄ ,dil NaOH etc. On Pt, it's inferred that the source for OH⁻ is only water and it's concentration of merely a constant.

6) Mechanism :-

From the primary stealth of H₂O molecules, OH⁻ deposits at the anode and the proton get OH⁻ from the water molecules present at the secondary stealth and the proton is accepted by solvent.

As OH⁻ gets transferred between two H₂O molecules, the transition state is symmetric and hence,

∝ = 0.5 for ηo₂ also (iiir to ηH₂)

Anode :-

OH⁻ →e⁻ OH →OH →H - O ↓- O - H

                                   1/2 O₂ + H₂O

Merits :-

1) The anodic surface is coated with the respective metal oxide and rendered passive.

2) ηo₂ also depends on the nature of the electrode.

                         Metal forming M- H bond (like Pt) will form weaker M - O bond at the anode. Therefore OH⁻ deposition at the anode will be lower. Hence on these metals, ηo₂ will be high.

This is expected from this theory and that is found to be true for Pt.

iiiy metals having higher ηH₂ will have lower ηo₂ and this prediction has been found to be true. This is taken as the merit of this theory.