# 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.

Chemistry (university) · Part 08: Acids, bases and electrochemistry · Card 9 of 9 · Last updated 2026-10-19

## Analogy

Pushing a ball uphill: it will not go on its own, so you must supply energy.

## Example

**Copper plating**

I = 2.00 A, t = 965 s
Q = 1930 C → 0.0200 mol e⁻
Cu²⁺ + 2e⁻ → Cu
0.0100 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.

## Related concepts

- [Galvanic cell](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-6)
- [Standard cell potential](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-7)
- [Nernst equation](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-8)

Web page: https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-9
Study this card in the app: https://studyvisually.com/app/learn?category=chemistry-university&card=chemistry-university-part-8-concept-9