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K324 / 2027
Redox Chemistry

Lesson 3 of 6

Electrolysis of a molten compound

A power supply forces chemical change using mobile ions.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Pure only
  • I can explain electrolysis.Syllabus

    Syllabus K324, 7.2(a). Explain electrolysis

  • I can use electrolysis as ionic evidence.Syllabus

    Syllabus K324, 7.2(b). Use electrolysis as ionic evidence

  • I can explain molten NaCl electrolysis.Syllabus

    Syllabus K324, 7.2(c). Explain molten NaCl electrolysis

  • I can predict molten binary products.Syllabus

    Syllabus K324, 7.2(d). Predict molten binary products

  • I can construct electrode equations.Syllabus

    Syllabus K324, 7.2(g). Construct electrode equations

Electrolysis of a molten compound

A power supply forces chemical change using mobile ions.

Make a guess. You are not marked.

Two graphite electrodes connected to a battery are pushed into solid lead(II) bromide. Does a current flow?

  1. Yes, because lead is a metal and conducts
  2. Yes, because the solid is made of ions
  3. No, not until it melts
Show the answer

No, not until it melts

The ions are held in fixed positions in the solid lattice. They can carry charge only when melting lets them move.

Electrolysis passes electric current through a molten or dissolved ionic compound, the electrolyte, causing chemical changes at electrodes. In the solid, ions are fixed in a lattice; once molten or dissolved they can move. This supports the ionic model. Electrons move through metal wires; ions carry charge through the electrolyte.

Molten sodium chloride with inert electrodes
  1. Negative cathode

    Na+ ions move towards it and gain electrons: Na+ + e- -> Na.

  2. Positive anode

    Cl- ions move towards it and lose electrons: 2Cl- -> Cl2 + 2e-.

  3. Products

    Sodium metal and chlorine gas form. At the operating temperature sodium is liquid; chlorine bubbles at the anode.

Label a molten electrolysis cell

Two graphite electrodes dip into molten sodium chloride that is being heated. The left electrode joins the positive terminal of the d.c. supply and is the anode; chloride ions move to it and chlorine gas bubbles off. The right electrode joins the negative terminal and is the cathode; sodium ions move to it and sodium metal forms. Electrons flow through the wires from the anode to the supply and from the supply to the cathode.

Label the d.c. supply, the anode (+) and cathode (-), what each electrode is made of, the electrolyte and its state, and the product at each electrode. Keep heating so the sodium chloride stays molten: the solid does not conduct, because its ions cannot move.

For a molten binary ionic compound, there are only its constituent cations and anions to discharge. The metal forms at the cathode and the non-metal at the anode, with inert electrodes. For molten lead(II) bromide: Pb2+ + 2e- -> Pb and 2Br- -> Br2 + 2e-. Balance atoms and charge in each half-equation.

Check your understanding

Why does solid sodium chloride not undergo electrolysis, when molten sodium chloride does?

  1. Its electrons are held by the ions, so no delocalised electrons can carry the current.
  2. Solid sodium chloride is made of neutral molecules, and melting splits them into ions.
  3. Its ions are held in the lattice and cannot move to the electrodes to carry current.
Show the answer

Its ions are held in the lattice and cannot move to the electrodes to carry current.

Solid NaCl already contains Na+ and Cl- ions, but they are fixed in place. Melting lets the ions move, so they can carry charge and be discharged at the electrodes.

Check your understanding

Molten calcium bromide, CaBr2, is electrolysed with inert electrodes. What forms at each electrode?

  1. Calcium at the cathode and oxygen at the anode
  2. Calcium at the cathode and bromine at the anode
  3. Hydrogen at the cathode and bromine at the anode
  4. Bromine at the cathode and calcium at the anode
Show the answer

Calcium at the cathode and bromine at the anode

The melt contains only Ca2+ and Br- ions. Ca2+ moves to the cathode and is reduced: Ca2+ + 2e- -> Ca. Br- moves to the anode and is oxidised: 2Br- -> Br2 + 2e-.