Electrolysis
Electrolysis uses electricity to force a non-spontaneous reaction. Charge Q = current × time, and moles of electrons = Q / 96,485 C/mol. The moles of product follow from the electron count in the half-reaction.
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Analogy
Pushing a ball uphill: it will not go on its own, so you must supply energy.
Example
I = 2.00 A, t = 965 sQ = 1930 C → 0.0200 mol e⁻Cu²⁺ + 2e⁻ → Cu0.0100 mol Cu = 0.635 g
In practice
Aluminium metal is made by electrolysis of molten aluminium oxide, and copper is purified the same way.
Practice
Flashcards
- Faraday constant?96,485 C per mole of electrons.
Questions people ask
What is the difference between a galvanic cell and an electrolytic cell?
A galvanic cell runs a spontaneous reaction and produces current. An electrolytic cell uses an outside power supply to drive a non-spontaneous reaction. In both, oxidation is at the anode and reduction at the cathode, but the electrode signs are reversed.
Why is molten NaCl electrolysed differently from salt water?
In molten NaCl, only Na⁺ and Cl⁻ are present, so sodium metal forms at the cathode and chlorine at the anode. In salt water, water competes: it is reduced more easily than Na⁺, so hydrogen gas forms at the cathode instead of sodium.