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Electrolysis of Molten Compounds

Molten electrolysis: predict cathode/anode products from ions present (no water competing), then write correct half-equations and overall equations.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe electrolysis as evidence for the existence of ions which are held in a lattice when solid but which are free to move when molten or in solution
  • describe, in terms of the mobility of ions present and the electrode products, the electrolysis of molten sodium chloride, using inert electrodes
  • predict the likely products of the electrolysis of a molten binary ionic compound using inert electrodes
  • construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given relevant information

Molten electrolysis has no water competing for discharge. Start from the ions in the compound, then connect ion movement to the electrode products and balanced half-equations.

1. Definition

A. Molten

Molten means melted (heated until it becomes a liquid). In a molten ionic compound, ions are mobile, so it conducts electricity.

B. Electrolysis of a Molten Ionic Compound

Electrolysis of a molten ionic compound is the decomposition of the compound when a direct current passes through it, causing ions to be discharged at the electrodes.

2. Key Ideas

  • Only ions from the molten salt are present (no water ions), so prediction is straightforward.
  • Cations move to the cathode (negative) and are reduced (gain electrons).
  • Anions move to the anode (positive) and are oxidised (lose electrons).
  • Write a half-equation at each electrode, then combine if asked for the overall equation.
Particle comparison of a solid and molten ionic compoundIn the solid, alternating positive and negative ions occupy fixed lattice positions and cannot carry charge. In the molten liquid, the same ions are disordered and mobile; positive and negative ions can move in opposite directions towards electrodes.Solid ionic compoundions fixed in a regular lattice+−+−−+−++−+−Does not conductThe charged particles cannot movethrough the solid.Molten ionic compoundsame ions, now mobile and disordered+−+−−+−+Conducts electricityCations and anions carry charge bymoving in opposite directions.
Melting does not create ions: it frees ions already present in the ionic lattice so they can move, carry charge and reach the electrodes.
Recall: Redox at electrodes

Cathode = reduction (gain electrons). Anode = oxidation (lose electrons). Redox Reactions What Is Electrolysis?

3. Detailed Explanations

Quick Recall (molten electrolysis)
  • Molten = only the salt’s ions are present (no water ions competing).
  • Cations → cathode (reduction). Anions → anode (oxidation).
  • Halide ions form halogens at the anode: Cl₂ / Br₂ / I₂.
  • Oxide ions form oxygen at the anode: O₂.

A. Why the Compound Must Be Molten (or Aqueous)

Solid ionic compounds do not conduct because the ions are fixed in a lattice. Melting breaks the rigid structure so ions can move.

B. What Always Happens at Each Electrode (Molten Salt)

General patterns (mark-scheme safe):

  • Cathode (reduction of cation): Mⁿ⁺(l) + n e⁻ → M
  • Anode depends on the anion:
    • Halide anions: 2X-(l) → X₂ + 2e⁻
    • Oxide anions: 2O²⁻(l) → O₂(g) + 4e⁻

Use the product state stated or implied by the operating temperature; do not assume every deposited metal is liquid.

Laboratory Warning

Molten-salt electrolysis involves hot liquids that can cause severe burns, and may release toxic or irritant halogens. Treat these as teacher-demonstration or industrial processes with suitable heat protection and gas control, not student bench experiments.

4. Common Mistakes

  • Writing the electrode signs incorrectly (cathode is negative, anode is positive in electrolysis).
  • Writing water half-equations in molten electrolysis (there is no water).
  • Forgetting diatomic molecules: chlorine is Cl₂, bromine is Br₂, oxygen is O₂.
  • Missing state symbols (molten = (l), gas = (g)) when asked.

5. Exam Tips

Reliable three-step method (molten)
  1. Write the ions present. 2) Send cations to cathode and anions to anode. 3) Write half-equations with electrons, then check charge balance.
  • Use the word discharged in explanations (it is mark-scheme language).
  • If you see “molten”, stop thinking about H⁺ and OH⁻. Those only matter in aqueous electrolysis.

6. Worked Examples

Modelled example 1

Molten Sodium Chloride (NaCl(l))

Core

Problem

Molten sodium chloride is electrolysed using graphite electrodes. State both products and write the half-equations.
Study the worked solution

  1. List the molten ions

    Method

    Use Na⁺ and Cl⁻.

    Reason

    Molten NaCl contains only its mobile constituent ions.

    Working

    Ions: Na + (l), Cl-(l).

  2. Reduce sodium ions

    Method

    Add one electron at the cathode.

    Reason

    Positive sodium ions gain electrons by reduction.

    Working

    Na + (l) + e⁻ → Na(l).

  3. Oxidise chloride ions

    Method

    Remove electrons from chloride at the anode.

    Reason

    Chlorine forms diatomic molecules.

    Working

    2Cl-(l) → Cl₂(g) + 2e⁻; products are sodium and chlorine.

Guided practice 2

Molten Magnesium Chloride (MgCl₂(l))

About 7 min

Problem

Molten magnesium chloride is electrolysed using graphite electrodes. State both products and write the half-equations.

Balance charges with electrons

Cathode equation
Anode equation

Hints

Hint 1: cation charge
Mg²⁺ must gain two electrons.
Hint 2: diatomic halogen
Chlorine leaves as Cl₂.
View solution step by step

  1. Reduce magnesium

    Method

    Send Mg²⁺ to the cathode and add two electrons.

    Reason

    Reduction must cancel the + 2 charge.

    Working

    Mg²⁺(l) + 2e⁻ → Mg(l).

  2. Oxidise chloride

    Method

    Form chlorine at the anode.

    Reason

    Two chloride ions release the same two electrons.

    Working

    2Cl-(l) → Cl₂(g) + 2e⁻; products are magnesium and chlorine.

Common misconception 3

Overall Equation (Combine Half-Equations)

Find and correct the mistake

Learner equation

A student combines the molten PbBr₂ half-equations but leaves 2e⁻ on both sides. Explain what must happen and write the overall equation.

Remove the internal electron transfer

Electrons in overall equation
Overall equation

View solution step by step

  1. Add the ionic half-equations

    Method

    Combine Pb²⁺ + 2e⁻ → Pb with 2Br⁻ → Br₂ + 2e⁻.

    Reason

    The electron counts are equal.

    Working

    Pb²⁺(l) + 2Br-(l) → Pb(l) + Br₂(g).

  2. Cancel electrons and restore formula

    Method

    Remove electrons and combine the molten ions as PbBr₂.

    Reason

    Electrons are transferred internally, not consumed or produced overall.

    Working

    PbBr₂(l) → Pb(l) + Br₂(g).

Challenge 4

Predict the Products: Molten Lead(II) Bromide (PbBr₂(l))

Minimal support

Halide transfer

Molten lead(II) bromide is electrolysed using graphite electrodes. Identify both products and write the half-equations.

Apply the molten two-ion method

Cathode product
Anode product

Hints

Hint 1: only two ions
Molten PbBr₂ contains Pb²⁺ and Br⁻.
Hint 2: halogen molecule
Bromine forms as Br₂.
View solution step by step

  1. Reduce lead ions

    Method

    Send Pb²⁺ to the cathode.

    Reason

    Cations gain electrons there.

    Working

    Pb²⁺(l) + 2e⁻ → Pb(l).

  2. Oxidise bromide ions

    Method

    Send bromide to the anode and form Br₂.

    Reason

    Anions lose electrons and halogens are diatomic.

    Working

    2Br-(l) → Br₂(g) + 2e⁻; products are lead and bromine.

7. Mind Stretchers

Mind stretcher 1: Spot the Mistake (Ion Direction)Extension

A student writes: “Cl⁻ goes to the cathode because it has a negative charge.” Explain the mistake and correct it.

Show Answer

The mistake is mixing up “negative ion” with “negative electrode”.

Correct rule: anions (negative ions) are attracted to the anode because the anode is positive.

Mind stretcher 2: Observation + Test (Chlorine)Extension

State one observation at the anode when molten sodium chloride is electrolysed, and one test for the gas produced.

Show Answer

Observation: bubbles of greenish-yellow gas at the anode.

Test: damp blue litmus turns red then is bleached white (chlorine).

8. Quiz

Quiz Time!

Ready to test your knowledge? Practice predicting products and writing half-equations for molten salts.

Go to Quiz Page