C4.1 Electrolysis – Student Notes & Practice

IGCSE Chemistry • Inert electrodes: platinum/graphite • Berkeley colours

Student Information

Definition

Electrolysis is the decomposition (breaking down) of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

Remember: only ions conduct in the electrolyte; electrons move through the wires.

Parts of an Electrolytic Cell

ComponentDescriptionCharge
AnodePositive electrode where oxidation occurs (loss of electrons).+
CathodeNegative electrode where reduction occurs (gain of electrons).–
ElectrolyteMolten or aqueous ionic substance that is electrolysed.—

Cations → Cathode (gain e⁻) Anions → Anode (lose e⁻)

Key Concepts & Rules

Discharge Rules for Aqueous Solutions

At the cathode:

  • If the metal is less reactive than hydrogen (e.g., copper, silver), the metal is produced
  • If the metal is more reactive than hydrogen (e.g., sodium, potassium), hydrogen gas is produced

At the anode:

  • If halide ions are present (Cl⁻, Br⁻, I⁻), the halogen is produced
  • If no halide ions are present, oxygen gas is produced from OH⁻ ions
Electrode Reactions

At the cathode (reduction):

  • Metal ions gain electrons: M⁺ + e⁻ → M
  • Hydrogen ions gain electrons: 2H⁺ + 2e⁻ → H₂

At the anode (oxidation):

  • Halide ions lose electrons: 2X⁻ → X₂ + 2e⁻
  • Hydroxide ions lose electrons: 4OH⁻ → O₂ + 2H₂O + 4e⁻
Electrolysis Applications
  • Electroplating: Coating objects with a thin layer of metal
  • Extraction of metals: Obtaining reactive metals like aluminum from their ores
  • Purification of metals: Refining copper
  • Production of chemicals: Chlorine, sodium hydroxide, hydrogen

(a) Molten Lead(II) Bromide, PbBr₂

ElectrodeIon DischargedProductObservation
Cathode (–)Pb²⁺Lead (Pb)Grey molten metal forms at the bottom
Anode (+)Br⁻Bromine gas (Br₂)Reddish‑brown vapour

Overall: PbBr₂(l) → Pb(l) + Br₂(g)

(b) Concentrated Aqueous Sodium Chloride (Brine)

ElectrodeIon DischargedProductObservation
Cathode (–)H⁺ (from water)Hydrogen (H₂)Colourless gas pops with lighted splint
Anode (+)Cl⁻Chlorine (Cl₂)Pale green, choking smell
In solutionNa⁺ + OH⁻ remainNaOH solutionAlkaline solution left

Overall: 2NaCl(aq)+2H₂O(l) → 2NaOH(aq)+Cl₂(g)+H₂(g)

(c) Dilute Sulfuric Acid

ElectrodeIon DischargedProductObservation
Cathode (–)H⁺Hydrogen (H₂)Colourless gas, pops with lighted splint
Anode (+)OH⁻ (from water)Oxygen (O₂)Relights a glowing splint

Overall: 2H₂O(l) → 2H₂(g) + O₂(g)

Practice A – Multiple Choice Questions

Pick ONE answer for each question, then click Check MCQs.

1. What type of compound can be electrolysed?
2. During electrolysis, oxidation occurs at the…
3. What is produced at the cathode when molten lead(II) bromide is electrolysed?
4. Which gas is produced at the anode during electrolysis of concentrated sodium chloride solution?
5. During electrolysis of dilute sulfuric acid, which observation is made at the cathode?
6. In electrolysis, which particles carry the current through the electrolyte?
7. What happens to bromide ions at the anode during electrolysis of molten lead(II) bromide?
8. Which of the following is NOT a product of electrolysis of brine?
9. In the electrolysis of copper(II) sulfate solution using copper electrodes, what happens at the anode?
10. Which factor does NOT affect the amount of product formed during electrolysis?
11. What is the main industrial use of the electrolysis of brine?
12. Which metal is commonly extracted using electrolysis?
13. In electroplating, the object to be plated is connected to the...
14. During electrolysis of dilute sulfuric acid, the volume ratio of hydrogen to oxygen gas produced is:
15. Which of the following is NOT an application of electrolysis?
16. In the electrolysis of copper(II) sulfate using inert electrodes, what is produced at the cathode?
17. What happens to the concentration of copper(II) sulfate solution during electrolysis using copper electrodes?
18. Which of the following statements about inert electrodes is correct?
19. During the electrolysis of molten aluminum oxide, cryolite is added to:
20. Which gas is tested using a glowing splint?
21. In the reactivity series, which metal would be discharged in preference to hydrogen during electrolysis?

Practice B – Short Answer Questions

Type your answers, then click Auto‑mark Short Answers. Marking awards credit for key ideas/keywords.

  1. 1. Define electrolysis.

  2. 2. What is an "inert electrode"? Give one example.

  3. 3. Name the ions present in molten lead(II) bromide and state what happens to each during electrolysis.

  4. 4. State the products at the electrodes when concentrated sodium chloride solution is electrolysed.

  5. 5. Write the ionic half‑equations for the electrolysis of dilute sulfuric acid.

  6. 6. Explain why hydrogen gas is produced instead of sodium during electrolysis of concentrated sodium chloride solution.

  7. 7. How can you test for oxygen and hydrogen gases formed during electrolysis?

Teacher Notes