The extra voltage needed beyond the thermodynamic value to drive an electrode reaction at useful speed.
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Overpotential is the extra voltage beyond the thermodynamic equilibrium potential that must be applied to drive an electrode reaction at a useful rate. It arises from kinetic barriers at the electrode surface and is large for some reactions, such as hydrogen evolution on mercury or oxygen evolution on many metals. Overpotential determines which species are actually discharged during electrolysis, even when their thermodynamic potentials suggest otherwise. It is one reason chlorine rather than oxygen is evolved from concentrated brine. Minimizing overpotential is a key goal in designing efficient electrolyzers and fuel cells.
Key ideas: Overpotential is the extra voltage beyond the thermodynamic value needed to drive a reaction at a given rate, eta = E applied - E equilibrium. Sources: activation (slow electron transfer), concentration polarization, and resistance. Tafel equation: eta = a + b log i. O2 evolution on most electrodes needs 0.4 to 0.8 V extra; H2 on Hg is huge (about 1 V), on Pt tiny.
Common mistakes and connections: Mistake: ignoring overpotential when predicting electrolysis products: it explains why Cl2 beats O2 at the anode of brine, and why Zn can be plated from aqueous solution despite H+ having a higher E. Lowers energy efficiency of electrolyzers and fuel cells; catalysts are used to reduce it.
Tafel lines for hydrogen evolution: overpotential grows by about 0.1 V per tenfold current density, and mercury needs far more than nickel. The interactive lesson shows this as a live animation or chart.
Hydrogen evolution on mercury has a very large overpotential, which is why sodium can be deposited as an amalgam instead of hydrogen in the mercury cell process.
The extra voltage needed beyond the thermodynamic value to drive an electrode reaction at useful speed.
Overpotential is a advanced topic in Acids, Bases & Electrochemistry. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Acids, Bases & Electrochemistry first: Debye-Huckel Theory, Ionic Strength, Activity and Activity Coefficients.
The next topics in Acids, Bases & Electrochemistry are Pourbaix Diagram and Latimer and Frost Diagrams.
Yes. The interactive lesson includes a self-check question on Overpotential, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Overpotential is topic 89 of 105 in Acids, Bases & Electrochemistry.