Q1(d)(ii). Overvoltage and its applications

Answer in brief

Overvoltage is the extra potential, beyond the reversible electrode potential, that must be applied before an electrode reaction goes at a measurable rate: η = E_applied − E_rev.

Key points

• It arises from a slow step at the electrode and depends on the metal, surface and current density (Tafel: η = a + b log j).
• Hydrogen overvoltage is about 0.005 V on platinised Pt, but 0.64 V on Pb, 0.70 V on Zn and 0.78 V on Hg.
• Electroplating: zinc deposits from acidic ZnSO₄ because the hydrogen overvoltage on zinc is high.
• Lead–acid battery and chlor-alkali cell: high overvoltages let PbSO₄ be converted, and Cl₂ form instead of O₂, before water is electrolysed.
• Polarography: the dropping mercury electrode can be taken to about −2 V without evolving hydrogen.