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3.2.8—Electrolytic cells

Syllabus
First assessment 2025
Objective
3.2.8
Level
HL

Molten-Salt Electrolysis

In molten salt there is no water: metal ions are reduced to metal at the cathode and anions are oxidized at the anode. For molten chloride, chloride forms chlorine gas.

M(n+)+ne−→Matcathode;2X−→X2+2e−atanodeM^(n+) + ne− → M at cathode; 2X− → X2 + 2e− at anode

Molten MgCl₂ contains only Mg²⁺ and Cl⁻: Mg²⁺ + 2e⁻ → Mg at the cathode and 2Cl⁻ → Cl₂ + 2e⁻ at the anode. The melt conducts by ion migration; do not introduce H₂, O₂ or water-based competition into a molten-salt question.

Deducing Molten-Electrolysis Products

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

Deduce the products of the electrolysis of molten cobalt(II) bromide, CoBr2(l)\mathrm{CoBr}_{2}(\mathrm{l}).

Product at anode:
Product at cathode:

Electron Transfer Reactions Summary

Retrieve the route: assign oxidation states, balance half-equations, predict displacement, label cells, trace electrons and ions, follow organic redox pathways, calculate potentials and choose electrolysis products.

Check electron loss/gain, anode/cathode versus polarity, spontaneous sign, salt-bridge direction, ions present, organic functional-group direction and object-cathode placement.