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CIE iGCSE Co-Ordinated Science C4.1 Electrolysis Exam Style Questions Paper 2

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Question

Which gas is formed at the cathode during electrolysis?

A. carbon dioxide
B. chlorine
C. hydrogen
D. oxygen

▶️ Answer/Explanation
At the cathode (negative electrode), reduction occurs. In electrolysis of aqueous solutions or acids, hydrogen gas is often produced (if H⁺ ions are present). Chlorine and oxygen are formed at the anode. Carbon dioxide is not a typical electrolysis product. Hence, hydrogen is formed at the cathode.
✅ Answer: (C)

Question

Concentrated aqueous sodium chloride is electrolysed using inert electrodes.

What is the ionic half-equation for the reaction at the cathode?

A. \( 2\mathrm{Cl}^- \rightarrow \mathrm{Cl}_2 + 2e^- \)
B. \( 2\mathrm{H}^+ + 2e^- \rightarrow \mathrm{H}_2 \)
C. \( \mathrm{Na}^+ + e^- \rightarrow \mathrm{Na} \)
D. \( \mathrm{NaCl} \rightarrow \mathrm{Na}^+ + \mathrm{Cl}^- \)

▶️ Answer/Explanation
In the electrolysis of concentrated aqueous sodium chloride, the ions present are Na⁺, Cl⁻, H⁺, and OH⁻. At the cathode (negative electrode), positively charged ions gain electrons (reduction). Hydrogen ions (H⁺) are preferentially discharged over sodium ions (Na⁺) because hydrogen is less reactive than sodium. The half-equation for this reduction is \( 2\mathrm{H}^+ + 2e^- \rightarrow \mathrm{H}_2 \). Option A shows oxidation at the anode, not the cathode.
✅ Answer: (B)

Question

Which changes take place at the electrodes during electrolysis?

▶️ Answer/Explanation
During electrolysis, the anode is the positive electrode where negatively charged ions (anions) lose electrons (oxidation). The cathode is the negative electrode where positively charged ions (cations) gain electrons (reduction). This is because opposite charges attract, and electrons flow from the anode to the cathode in the external circuit.
✅ Answer: (B)

Question

Which statement describes what happens during electrolysis?

A. Covalent compounds produce more complex substances.
B. Covalent compounds produce simpler substances.
C. Ionic compounds produce more complex substances.
D. Ionic compounds produce simpler substances.

▶️ Answer/Explanation
Electrolysis is defined as the decomposition of an ionic compound (when molten or in solution) using an electric current — the compound is broken down into simpler substances at the electrodes.
Covalent compounds generally do not conduct electricity and cannot be electrolysed; only ionic compounds undergo electrolysis, and the products are always simpler than the starting compound.
✅ Answer: (D)

Question

During electrolysis, reactions occur at the cathode and anode. Which statements about the electrode reactions are correct?

  1. At the anode, anions gain electrons and are reduced.
  2. At the anode, anions lose electrons and are oxidised.
  3. At the cathode, cations gain electrons and are reduced.
  4. At the cathode, cations lose electrons and are oxidised.

A. 1 and 2
B. 1 and 4
C. 2 and 3
D. 3 and 4

▶️ Answer/Explanation
At the anode (positive electrode), anions lose electrons (oxidation). At the cathode (negative electrode), cations gain electrons (reduction). Statements 2 and 3 correctly describe these processes.
✅ Answer: (C)
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