OxfordAQA International GCSE Chemistry: Chemical cells and fuel cells (9202) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

The questions below test section 3.9.3 Chemical cells and fuel cells, statements a and b, from the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. With no tiers, all of them suit every candidate, and either paper may assess this content. Some questions link to moles (section 3.6) and bond energies (section 3.9.2).

See also: cells study guide · cells revision notes · OxfordAQA chemistry hub · checklist to print · diagnostics

Data for all questions: H = 1, O = 16; molar gas volume at room temperature and pressure = 24 dm³.

Questions

1. A wall clock is powered by a chemical cell.

(a) State what the chemical cell produces that makes the clock work. [1] (b) State why the clock eventually stops, even though the cell is not damaged. [1]

2. A student records the potential difference (pd) of a cell in a clock over several days.

Time (days)	0	20	40	60	70	75
pd (V)	1.55	1.53	1.50	1.46	1.30	0.42

(a) Describe how the pd changes. Use numbers from the table. [2] (b) Explain the change in pd between 60 days and 75 days. [2]

3. This question is about fuel cells.

(a) State what a fuel cell reacts a fuel with to produce electricity. [1] (b) Name the only waste product of a hydrogen–oxygen fuel cell. [1] (c) Give one use of hydrogen–oxygen fuel cells. [1]

4. Explain why a hydrogen fuel cell in a car can keep producing electricity for much longer than a sealed chemical cell. [2]

5. An advert says: “Our fuel cell scooter causes no pollution.” Explain why a chemist would prefer the statement “causes less pollution when in use”. [3]

6. A small hydrogen fuel cell car carries 1.2 kg of hydrogen.

(a) Calculate the volume, in m³, that this hydrogen would occupy at room temperature and pressure. [3] (b) The tank has an internal volume of 0.036 m³. Calculate how many times smaller this is than your answer to (a). [1] (c) Use your answers to suggest two disadvantages of storing hydrogen in a vehicle. [2]

7. A fuel cell uses 0.24 kg of hydrogen during a test.

(a) Write a balanced symbol equation for the overall reaction in a hydrogen–oxygen fuel cell. [1] (b) Calculate the mass of water produced, in kg. [3]

8. Hydrogen can be burned in an adapted engine instead of being used in a fuel cell.

(a) Name the substance formed when hydrogen burns in oxygen. [1] (b) Explain why burning hydrogen produces no carbon dioxide. [1] (c) The exhaust from a hydrogen engine can contain oxides of nitrogen. Explain why, and give one effect of these gases on the environment. [3]

9. A delivery company must choose between two vans that both use hydrogen. Use this information.

	Van A: hydrogen engine	Van B: hydrogen fuel cell
Hydrogen used per 100 km	1.8 kg	1.1 kg
Gases released when running	Water vapour, oxides of nitrogen	Water vapour
Price of van	Lower	Higher
Time to refill the tank	5 minutes	5 minutes

Evaluate the two vans. Give a conclusion. [6]

10. Use these values: 1 g of hydrogen releases 140 kJ. A hydrogen engine car transfers 25% of this energy usefully. A fuel cell car transfers 55% usefully. Each car needs 700 kJ of useful energy per km.

(a) Calculate the total energy released by 2.0 kg of hydrogen. [1] (b) Calculate the useful energy each car gets from 2.0 kg of hydrogen. [2] (c) Calculate how far each car can travel on 2.0 kg of hydrogen. [2] (d) Calculate the mass of hydrogen the fuel cell car needs for a 300 km journey. Give your answer to 3 significant figures. [2]

11. The overall reaction in a hydrogen–oxygen fuel cell is 2H₂ + O₂ → 2H₂O. Use these bond energies: H–H 432 kJ/mol, O=O 494 kJ/mol, O–H 460 kJ/mol.

(a) Calculate the overall energy change for the reaction as written. [4] (b) Is the reaction exothermic or endothermic? Use your answer to (a) to justify your choice. [1]

12. A crewed spacecraft gets its electricity from hydrogen–oxygen fuel cells. In one day the fuel cells use 2.5 kg of hydrogen.

(a) Calculate the mass of water produced in one day. [3] (b) Calculate the mass of oxygen used in one day. [2] (c) Suggest two reasons why fuel cells suit a crewed spacecraft better than burning hydrogen on board. [2]

Answers

1. (a) A potential difference (voltage) [1]. (b) The reactants in the cell are used up, so it stops producing a potential difference [1]. Examiner insight: Answers such as “the cell runs out of electricity” or “runs out of energy” tend not to earn credit; name the reactants being used up.

2. (a) The pd falls only slightly over the first 60 days, by 0.09 V (from 1.55 V to 1.46 V) [1]. It then falls rapidly, by 1.04 V, to 0.42 V at 75 days [1]. (b) The reactants in the cell are running out [1], so the reaction cannot keep the pd up and the pd drops towards zero [1]. Examiner insight: “Use numbers from the table” means a description with no values quoted is unlikely to gain both marks; give at least one calculated change.

3. (a) Oxygen [1]. (b) Water [1]. (c) Cars or spacecraft [1]. Examiner insight: Name a use the specification gives, cars or spacecraft; a vague answer such as “transport” is less likely to be credited.

4. The fuel (hydrogen) and oxygen are supplied to the fuel cell from outside, from a tank and from the air [1]. So it keeps producing electricity as long as they are supplied, whereas a sealed cell stops when the reactants inside it are used up [1]. Examiner insight: The comparison earns the second mark; a description of the fuel cell alone does not explain why it lasts longer.

5. When the scooter runs, water is the only waste product, so it releases no carbon dioxide or other pollutants at that point [1]. But hydrogen has to be manufactured [1], and if fossil fuels or electricity from fossil-fuel power stations are used to make it, pollution is released where the hydrogen is made [1]. Examiner insight: Strong answers link to how the hydrogen is produced; repeating “only water is made” earns one point only.

6. (a) Moles of H₂ = 1200 ÷ 2 = 600 mol [1]. Volume = 600 × 24 = 14 400 dm³ [1]. In m³: 14 400 ÷ 1000 = 14.4 m³ [1]. (b) 14.4 ÷ 0.036 = 400 times smaller [1]. (c) Hydrogen must be compressed to a very high pressure to fit, so the tank must be strong and heavy [1]. A leak of a highly flammable gas stored under pressure could form an explosive mixture with air [1]. Examiner insight: Dividing 1.2 kg by 1 instead of 2 is an easy slip; hydrogen gas is H₂, so check the Mr before finding moles.

7. (a) 2H₂ + O₂ → 2H₂O [1]. (b) Moles of H₂ = 240 ÷ 2 = 120 mol [1]. Moles of H₂O = 120 mol (ratio 1 : 1) [1]. Mass = 120 × 18 = 2160 g = 2.16 kg [1]. Examiner insight: Keep units in each line; an answer of “2160” when the question asks for kg can lose the final mark.

8. (a) Water [1]. (b) Hydrogen contains no carbon, so no carbon dioxide can form [1]. (c) The hydrogen burns in air, which contains nitrogen [1]. The combustion happens at a high temperature, at which oxides of nitrogen form [1]. Oxides of nitrogen cause acid rain [1]. Examiner insight: No detail of how oxides of nitrogen form is needed, but “high temperature” must be stated.

9. Van B uses less hydrogen per 100 km (1.1 kg) than van A (1.8 kg) [1], so less hydrogen has to be made, bought and stored [1]. Van A releases oxides of nitrogen as well as water vapour, whereas van B releases only water vapour [1], so van B causes less acid rain [1]. Van A is cheaper to buy, but both take the same time to refill, so refilling does not separate them [1]. Conclusion: van B is the better choice, because the lower hydrogen use and cleaner exhaust outweigh its higher price [1]. Examiner insight: A conclusion with no reason, or one that ignores the price data, rarely gains the final mark; tie the judgement back to the table.

10. (a) 2000 × 140 = 280 000 kJ [1]. (b) Engine car: 0.25 × 280 000 = 70 000 kJ [1]. Fuel cell car: 0.55 × 280 000 = 154 000 kJ [1]. (c) Engine car: 70 000 ÷ 700 = 100 km [1]. Fuel cell car: 154 000 ÷ 700 = 220 km [1]. (d) Useful energy needed = 300 × 700 = 210 000 kJ; energy released needed = 210 000 ÷ 0.55 = 381 818 kJ [1]. Mass = 381 818 ÷ 140 = 2727 g = 2.73 kg [1]. Examiner insight: Dividing by the percentage efficiency rather than multiplying by it in (d) is easy to miss; check that a less efficient device needs more fuel, not less.

11. (a) Energy to break bonds = (2 × 432) + 494 = 1358 kJ [1]. Energy released forming bonds = 4 × 460 = 1840 kJ [1]. Energy change = 1358 − 1840 [1] = −482 kJ [1]. (b) Exothermic: the 1840 kJ given out as the new bonds form exceeds the 1358 kJ taken in to break the old ones, so the overall change is negative [1]. Examiner insight: Each water molecule has two O–H bonds, so 2H₂O contains four; using two O–H bonds is a frequent error and gives a positive answer that should look wrong.

12. (a) Moles of H₂ = 2500 ÷ 2 = 1250 mol [1]. Moles of H₂O = 1250 mol [1]. Mass = 1250 × 18 = 22 500 g = 22.5 kg [1]. (b) Moles of O₂ = 1250 ÷ 2 = 625 mol [1]. Mass = 625 × 32 = 20 000 g = 20.0 kg [1]. (c) Any two of these, one mark each, up to [2]: the only waste product is water, which the crew can use (for example for drinking); there is no flame or high-temperature combustion inside the spacecraft, so no oxides of nitrogen form in the cabin air; the electricity is produced directly to run equipment. Examiner insight: “Suggest” questions reward reasons that fit the context; points about filling stations do not apply to a spacecraft.

Where marks are usually lost

• Writing that a chemical cell stops because it “runs out of electricity” instead of “the reactants are used up”.
• Saying a fuel cell “burns” hydrogen.
• Claiming no pollution at all, instead of “less pollution when in use”.
• Forgetting the link between high temperature, nitrogen in air and oxides of nitrogen in a hydrogen engine.
• Using Mr = 1 for hydrogen gas, or not converting kg to g first.
• Mixing up dm³ and m³ when finding gas volumes (1 m³ = 1000 dm³).
• In evaluations, writing about only one option, or giving a conclusion that ignores the data provided.
• In bond energy questions, miscounting O–H bonds in 2H₂O, or subtracting the wrong way round.

Next steps

• Revision notes for a fast recap and self-test.
• Study guide for full explanations and worked examples.
• Quantitative chemistry practice for more mole questions.
• All free 10-minute diagnostics.
• OxfordAQA chemistry hub and checklist to print.
• Book a free trial class.

Official syllabus

Every item here follows the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA: section 3.9.3, Chemical cells and fuel cells.