Electrolysis and Fuel Cells: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Electrolysis revision notes

Tier note: questions and parts marked (0620 Extended, 5070 required) — constructing ionic half-equations — go beyond 0620 Core. All other questions are answerable by a 0620 Core candidate.

---

Section A

1. Define electrolysis and explain why the substance must be molten or in solution. [3]

2. State what is meant by anode, cathode, anion and cation. [4]

Section B

3. Molten lead(II) bromide is electrolysed with inert electrodes.

(a) (0620 Extended, 5070 required) Write the half equation at each electrode and state the products. [4] (b) Explain why lead is formed and not hydrogen. [1]

4. Concentrated aqueous sodium chloride is electrolysed.

(a) Name the products formed at each electrode. [2] (b) (0620 Extended, 5070 required) Write the half equation at the cathode and explain why hydrogen is formed rather than sodium. [3] (c) State the test and result for the gas formed at the anode. [2]

5. A spoon is to be electroplated with silver.

(a) State what should be used as the anode, the cathode and the electrolyte. [3] (b) Give two reasons for electroplating. [2]

6. In a hydrogen–oxygen fuel cell:

(a) Write the overall equation. [1] (b) Give two advantages and two disadvantages compared with a petrol engine, the syllabus comparator for this topic. [4]

7. Copper(II) sulfate solution is electrolysed twice, once with inert (carbon) electrodes and once with copper electrodes.

(a) State the products formed at each electrode using inert electrodes. [2]

(b) Explain how the result at each electrode differs when copper electrodes are used instead, and why the electrolyte’s colour and concentration stay roughly constant throughout. [3]

8. Compare a hydrogen fuel cell with a petrol engine, rather than with a battery.

(a) State one substance produced by a petrol engine but not by a fuel cell, other than water. [1]

(b) Explain, in terms of energy losses, why a fuel cell is generally more efficient than a petrol engine. [2]

9. Dilute sulfuric acid is electrolysed using inert electrodes.

(a) Name the gas formed at each electrode, and give the test and result that confirms each. [4] (b) State the ratio in which the two gases are collected, by volume. [1]

---

Answers

1. The breaking down of an ionic compound using electricity [1]. The ions must be free to move to carry the current [1]; in a solid lattice the ions are held in fixed positions [1].

2. Anode — the positive electrode [1]. Cathode — the negative electrode [1]. Anion — a negatively charged ion, attracted to the anode [1]. Cation — a positively charged ion, attracted to the cathode [1].

3. (a) Cathode: Pb²⁺ + 2e⁻ → Pb [1] — lead forms, a silvery liquid [1]. Anode: 2Br⁻ → Br₂ + 2e⁻ [1] — bromine, an orange-brown vapour [1]. (b) There is no water present, so no hydrogen ions are available [1].

4. (a) Hydrogen at the cathode [1] and chlorine at the anode [1]. (b) 2H⁺ + 2e⁻ → H₂ [1]. Sodium is more reactive than hydrogen [1], so the hydrogen ion is discharged in preference, being lower in the reactivity series [1]. (c) Hold damp blue litmus paper in the gas [1]; it turns red and then is bleached white [1].

5. (a) Anode: pure silver [1]. Cathode: the spoon [1]. Electrolyte: a solution of a silver salt, e.g. silver nitrate [1]. (b) To improve appearance [1] and to protect the underlying metal from corrosion [1].

6. (a) 2H₂ + O₂ → 2H₂O [1]. (b) Advantages: the only product is water, so there are no polluting emissions at point of use, unlike a petrol engine’s exhaust gases [1]; energy conversion is more efficient, since chemical energy is converted directly to electrical energy without the heat-to-motion step that wastes energy in a petrol engine [1]. Disadvantages: hydrogen is difficult and hazardous to store and transport, being a highly flammable gas, whereas petrol is comparatively easy to store and distribute [1]; the hydrogen is usually manufactured from fossil fuels or by electrolysis using electricity, so the process is not carbon-free overall, and refuelling infrastructure for hydrogen is far less widespread than for petrol [1].

7. (a) Cathode: copper deposits (Cu²⁺ + 2e⁻ → Cu) [1]. Anode: oxygen gas forms [1]. (b) With copper electrodes, copper still deposits at the cathode, but at the anode the copper dissolves into solution instead of oxygen being released [1]. Because the mass of copper leaving the anode roughly equals the mass depositing at the cathode, the concentration of Cu²⁺ ions, and therefore the blue colour, stays approximately constant [1] — unlike with inert electrodes, where Cu²⁺ ions are steadily removed from solution as oxygen escapes instead of being replaced [1].

8. (a) Any one: carbon dioxide, carbon monoxide, or oxides of nitrogen (NOₓ) [1]. (b) A petrol engine burns fuel and converts heat to mechanical motion, losing a large fraction of the energy as waste heat at each stage [1]; a fuel cell converts chemical energy directly into electrical energy, without an intermediate heat-to-motion step, so proportionally less energy is lost [1].

9. (a) Cathode: hydrogen [1] — a lit splint gives a squeaky pop [1]. Anode: oxygen [1] — a glowing splint relights [1]. (b) 2 : 1 by volume, hydrogen to oxygen [1].

---

Where marks are usually lost

• Balancing electrons incorrectly in half equations.
• Saying sodium is produced at the cathode from aqueous NaCl.
• Getting the anode and cathode the wrong way round in electroplating.
• Claiming fuel cells are entirely pollution-free.
• Forgetting that copper electrodes change the anode’s product (dissolving rather than releasing oxygen) but not the cathode’s — copper still deposits there either way.
• Comparing a fuel cell only against a battery, when a question specifically asks about a petrol engine — the relevant differences (combustion products, efficiency, refuelling infrastructure) are different in each comparison.

Questions 7 and 8 draw on the copper-electrode special case and the fuel-cell-vs-petrol-engine comparison in the Electrolysis revision notes, material the earlier questions on this page don’t reach.