OxfordAQA International GCSE Chemistry: Chemical cells and fuel cells (9202) – Revision Notes

These revision notes condense section 3.9.3 Chemical cells and fuel cells of the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. Both statements, a and b, are included. There are no tiers, and Paper 1 or Paper 2 may test this section. Go to the chemical cells and fuel cells study guide when you need the longer explanations.

Also useful: OxfordAQA chemistry hub · checklist to print · cells practice set · diagnostics

Key terms

Term	Meaning
Chemical cell	Contains reactants that react to transfer energy electrically; produces a potential difference until the reactants are used up
Potential difference (pd)	The voltage a cell produces, measured in volts (V) with a voltmeter across the cell
Fuel cell	Produces electricity through the reaction of a fuel with oxygen; fuel and oxygen are supplied from outside
Hydrogen–oxygen fuel cell	A fuel cell that uses hydrogen as its fuel; water is the only waste product

No required practical in section 6.2 belongs to this unit.

3.9.3 a: Chemical cells in three lines

1. Reactants are sealed inside the cell; nothing is added while it works.
2. While reactants remain, the reaction keeps the pd up.
3. When a reactant is used up, the reaction stops and the pd falls to zero.

Reading a pd–time graph or table. A typical cell shows a long, nearly flat stretch, then a steep drop near the end. The flat part is while plenty of reactant is left. The steep drop is when a reactant is running out. Describe the pattern with numbers from the data, then explain it with “reactants used up”.

Energy link (3.9.1). The cell reaction is exothermic. Energy is transferred from the chemicals to the surroundings, mainly by electricity rather than by heating.

3.9.3 b: Fuel cells

Fuel cell versus chemical cell

	Chemical cell	Fuel cell
Where the reactants are	Inside, from the start	Fed in from outside
How long the pd lasts	Until reactants are used up	As long as fuel and oxygen are supplied
When it stops	Replace it	Refuel it

The hydrogen–oxygen fuel cell

• Fuel: hydrogen. Other reactant: oxygen (often from air).
• Overall change: hydrogen + oxygen → water; 2H₂ + O₂ → 2H₂O.
• No flame. The hydrogen is not burned; the energy is transferred as electricity.
• Uses in the specification: cars and spacecraft. On the Apollo missions the crew drank the water made by the fuel cells.
• Only waste product: water, so fuel cells cause less pollution when in use.
• The specification states: “Knowledge of the details of the reactions in fuel cells is not required.” You do not need electrode equations.

“When in use” – the catch

The vehicle itself releases only water. But hydrogen has to be manufactured. If fossil fuels, or electricity from fossil-fuel power stations, are used to make it, pollution is released where the hydrogen is made. One route in this course: electrolysis of sodium chloride solution gives hydrogen, and it needs a lot of electricity.

Benefits and disadvantages of hydrogen fuel

The specification groups these under two headings. Learn both lists.

Storage and use

Benefits	Disadvantages
Releases a lot of energy per gram	It is a gas with a very low density, so a useful mass has a huge volume
Gives electricity directly in a fuel cell	Must be stored at high pressure in strong, heavy tanks, or cooled to a liquid at a very low temperature
A fuel cell vehicle is refuelled rather than having cells replaced	Highly flammable; leaks can form explosive mixtures with air
	Few hydrogen filling stations
	Making the hydrogen may use fossil fuels

Products of combustion

Burning hydrogen (for example in an engine)	Hydrogen in a fuel cell
Hydrogen + oxygen → water, exothermic	Water is the only waste product
No carbon dioxide, carbon monoxide, soot or sulfur dioxide, because hydrogen contains no carbon or sulfur	Same: no carbon or sulfur products
Burns in air at high temperature, so oxides of nitrogen can form (they cause acid rain)	No combustion, so this problem does not arise in the same way

Link to section 3.10: the specification says oxides of nitrogen are formed at high temperatures, and that sulfur dioxide and oxides of nitrogen cause acid rain.

Calculations you may meet

What you are asked	Method
Mass of water from a mass of hydrogen	Moles H₂ = mass ÷ 2; moles H₂O = moles H₂ (ratio 2 : 2); mass = moles × 18
Volume of hydrogen gas at room temperature and pressure	Moles × 24 dm³
Useful energy from a fuel	Energy released × fraction transferred usefully (from the data)
Percentage transferred usefully	(useful energy ÷ total energy) × 100

Shortcut to check your answer: the mass of water formed is always 9 times the mass of hydrogen used (18 ÷ 2 = 9).

Method in steps: mass of water

1. Write the balanced equation: 2H₂ + O₂ → 2H₂O.
2. Convert the mass of hydrogen to grams, then divide by 2 to get moles.
3. Use the 1 : 1 ratio to get moles of water.
4. Multiply by 18. Convert back to kg if the question used kg.

Method in steps: compare or evaluate from data

1. Pick out every property in the data that differs between the options.
2. Write a sentence for each property that mentions both options.
3. Bring in chemistry: water is the only product; oxides of nitrogen at high temperature; storage of a gas.
4. End with a judgement that uses the data.

Must-know distinctions

• Cell stops because a reactant is used up, not because it “runs out of charge” or “electricity”.
• Fuel cell = fuel reacts with oxygen without burning. Combustion = fuel burns with a flame.
• Less pollution when in use is not the same as no pollution.
• Hydrogen gas is H₂ (Mr = 2). Using Mr = 1 halves the moles.
• Volume at room temperature and pressure uses 24 dm³ per mole of gas; 1 m³ = 1000 dm³.

Quick self-test

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

1. State why a chemical cell stops producing a potential difference.
2. What does a fuel cell react a fuel with?
3. Name the fuel in a hydrogen–oxygen fuel cell and its only waste product.
4. Give the two uses of hydrogen fuel cells named in the specification.
5. Give one reason why a fuel cell can keep producing electricity for much longer than a chemical cell of a similar size.
6. Calculate the mass of water produced when 3.0 kg of hydrogen is used in a fuel cell.
7. Calculate the volume of 0.40 kg of hydrogen at room temperature and pressure.
8. A fuel cell reacts 64 g of oxygen. What mass of hydrogen reacts with it?
9. A fuel cell transfers 120 kJ usefully out of 200 kJ released. What percentage is transferred usefully?
10. Explain why a hydrogen engine may release a pollutant that a hydrogen fuel cell does not.
11. Give two disadvantages of hydrogen in terms of storage.
12. Why does the specification say fuel cells cause less pollution “when in use”?

Answers

1. One of its reactants has been used up, so the reaction stops.
2. Oxygen.
3. Hydrogen; water.
4. Cars and spacecraft.
5. Its fuel and oxygen are supplied continuously from outside, so it works as long as they keep being supplied.
6. Moles H₂ = 3000 ÷ 2 = 1500 mol; moles H₂O = 1500 mol; mass = 1500 × 18 = 27 000 g = 27 kg.
7. Moles H₂ = 400 ÷ 2 = 200 mol; volume = 200 × 24 = 4800 dm³ (4.8 m³).
8. Moles O₂ = 64 ÷ 32 = 2.0 mol; moles H₂ = 2 × 2.0 = 4.0 mol; mass = 4.0 × 2 = 8.0 g.
9. (120 ÷ 200) × 100 = 60%.
10. The engine burns hydrogen in air at a high temperature, so oxides of nitrogen can form; the fuel cell does not burn the hydrogen and its only waste product is water.
11. Any two: it is a gas with a very low density, so it takes up a large volume; it must be kept at high pressure in strong, heavy tanks (or cooled to a liquid); it is highly flammable and leaks can explode.
12. Only water is released at the vehicle, but making the hydrogen may use fossil fuels or electricity from them, so pollution can be released elsewhere.

Where marks are usually lost

• Writing that a cell “runs out of electricity”. The answer needs “reactants are used up”.
• Describing a fuel cell as burning hydrogen. It reacts hydrogen with oxygen without combustion.
• Claiming fuel cells produce “no pollution” with no qualification. Say “less pollution when in use” and explain why hydrogen production may pollute.
• Leaving out oxides of nitrogen when comparing a hydrogen engine with a fuel cell, or forgetting to say they form because of the high temperature in air.
• Writing facts about one option only in a “compare” question. Each point should mention both.
• Ignoring the data given. If the question provides figures, quote them in the answer.
• Using Mr = 1 for hydrogen gas, or forgetting to convert kg to g before finding moles.
• Giving a volume in dm³ when the question asked for m³, or the reverse.
• Listing a “benefit” such as “hydrogen is renewable” with no explanation. Say how it is made and from what.

Next steps

Work through the practice questions for longer data and evaluation questions. Revisit the quantitative chemistry revision notes if mole calculations feel shaky, and the electrolysis revision notes for how hydrogen can be made from brine. Then try a free diagnostic.

Official syllabus

Source document: the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards (OxfordAQA). This page covers section 3.9.3, Chemical cells and fuel cells.