Electrolysis of Molten Zinc Chloride

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★★ Requires a more experienced teacher
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Electrochemistry

Alternative titles:

Summary

When molten zinc chloride is electrolyzed, zinc ions are reduced to liquid zinc metal at the cathode, while chloride ions are oxidized to chlorine gas at the anode. This process demonstrates the extraction of a metal from its molten salt.

Procedure

Heat zinc chloride until it melts; no reaction occurs until it is in liquid form.
Insert an inert cathode (negative electrode) and an inert anode (positive electrode), ensuring they do not touch.
Pass an electric current through the molten salt.
At the cathode, observe the formation of molten zinc, which collects as small beads below the electrode.
At the anode, observe the release of chlorine gas, which is pale and can be confirmed by its bleaching effect on damp litmus paper.

Links

Electrolysis of Molten Compounds. GCSE Chemistry. - Revise Chemistry with Mr B:

📄 The electrolysis of molten ionic compounds - Chemguide: https://www.chemguide.co.uk/14to16/electrolysis/melts.html

Variations

Use carbon vs platinum electrodes and observe whether electrode material affects results.
Investigate current or voltage changes on the rate of zinc and chlorine production.

Safety Precautions

Wear safety glasses, lab coat, and gloves; molten salts are very hot and can cause severe burns.
Handle chlorine gas with extreme care; it is toxic and must be produced only in a fume hood.
Ensure electrodes and apparatus are heat resistant and securely clamped.

Questions to Consider

Why does electrolysis not occur until the zinc chloride is molten? (Because ions are only free to move and conduct electricity in the liquid state.)
What evidence confirms that chlorine is produced at the anode? (It bleaches litmus paper and has a distinct smell.)
Why is zinc formed at the cathode? (Because Zn²⁺ ions gain electrons and are reduced to zinc metal.)
How does the energy required to melt zinc chloride compare with that of sodium chloride? (Zinc chloride melts more easily than sodium chloride, making it simpler to demonstrate in the lab.)
What type of reaction occurs at the anode and cathode? (Oxidation at the anode, reduction at the cathode — a redox process overall.)