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IGCSE · Practice questions · Electrolysis of molten compounds

Follow the ions.

Six original Cambridge-style questions on electrolysis of molten compounds: key terms, products, observations, charge transfer and half-equations.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

PANCAKE: positive anode, negative cathode.

01
Recall
[2 marks]

Define electrolysis.

  • The decomposition (breakdown) of an ionic compound, when molten or in aqueous solution, ✓
  • by the passage of an electric current. ✓
02
Recall
[3 marks]

Molten lead(II) bromide is electrolysed using graphite electrodes. Name the product at each electrode and describe what is seen at the anode.

  • Cathode: lead. ✓
  • Anode: bromine. ✓
  • A red-brown gas (vapour) is seen at the anode. ✓
03
Explain
[2 marks]

Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.

  • In the solid, the Na+ and Cl− ions are held in fixed positions in the lattice and cannot move. ✓
  • In the molten state the ions are free to move and carry the charge. ✓
04
Recall
[2 marks]

Predict the products at the cathode and at the anode when molten potassium chloride is electrolysed.

  • Cathode: potassium. ✓
  • Anode: chlorine. ✓
05
Analysis
[3 marks]

(Extended) Write the half-equations for the reactions at each electrode during the electrolysis of molten sodium chloride, and state which electrode reaction is oxidation.

  • Cathode: Na+ + e− → Na. ✓
  • Anode: 2Cl− → Cl2 + 2e−. ✓
  • The anode reaction is oxidation, because the chloride ions lose electrons. ✓
06
Explain
[3 marks]

(Extended) Describe how electric charge is transferred during the electrolysis of molten lead(II) bromide.

  • Electrons flow through the external circuit (the wires) from the anode, through the power supply, to the cathode. ✓
  • In the molten electrolyte the charge is carried by ions: Pb2+ moves to the cathode and Br− moves to the anode. ✓
  • At the cathode Pb2+ ions gain electrons; at the anode Br− ions lose electrons. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.