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Redox Chemistry

Topic 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)

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Electrolysis of a molten compound

A power supply forces chemical change using mobile ions.

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.

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 understandingWhy does solid sodium chloride fail to undergo the same electrolysis?Think it through, then reveal the answer
Its ions cannot move through the lattice to carry current. Melting frees the ions to move; it does not create the ions from neutral NaCl molecules.