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

Here you can learn chemical cells and fuel cells for OxfordAQA International GCSE Chemistry (9202) from the start. The guide is written against the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, and teaches both statements in section 3.9.3 (a and b). This qualification is not tiered, so every candidate needs all of it. Paper 1 and Paper 2 can each assess any part of the specification, so expect this unit on either.

Quick links: OxfordAQA chemistry hub · checklist to print · cells revision notes · cells practice set · diagnostics

Scope of section 3.9.3

Spec ref	You should be able to
3.9.3 a	State that a chemical cell produces a potential difference until its reactants are used up, and explain what that means for how long a cell lasts
3.9.3 b	Describe a fuel cell as a device that produces electricity from the reaction of a fuel with oxygen
3.9.3 b	Know that hydrogen–oxygen fuel cells use hydrogen as the fuel and are useful in cars and spacecraft
3.9.3 b	Explain why water is the only waste product, so fuel cells cause less pollution when in use
3.9.3 b	Compare burning hydrogen with using it in a fuel cell, from information you are given
3.9.3 b	Give the benefits and disadvantages of hydrogen fuel in terms of storage and use, and products of combustion

Section 6.2 of the specification lists the required practicals. None of them belongs to section 3.9.3, so this unit has no required practical of its own.

The specification also says: “Knowledge of the details of the reactions in fuel cells is not required.” You do not need electrode equations for the fuel cell. You do need the overall change, hydrogen plus oxygen giving water, because that is what makes the pollution argument work.

3.9.3 a: Chemical cells

A chemical cell contains reactants that react together and transfer energy as electricity. Its potential difference (pd) is measured in volts (V) with a voltmeter connected across the cell.

The key statement is short: a chemical cell produces a potential difference until the reactants are used up. Everything follows from that.

• The reactants are sealed inside the cell. Nothing new is added while it works.
• As long as some of each reactant is left, the reaction continues and the pd is kept up.
• Once one reactant runs out, the reaction stops. With no reaction there is no energy transfer, so the pd falls to zero. The cell is “flat”.

This links to section 3.9.1. The cell reaction is exothermic, but the energy is transferred mostly electrically rather than as heat.

Worked example 1: reading cell data

A student connects a new cell to a small lamp and records the pd across it every so often.

Time (hours)	0	2	4	6	8	9	10
pd (V)	1.52	1.50	1.47	1.41	1.24	0.66	0.04

(a) Describe the pattern. For the first 6 hours the pd falls only slightly, by 0.11 V. After about 8 hours it falls steeply, by 1.37 V between 6 hours and 10 hours, until it is almost zero.

(b) Explain the pattern. While plenty of reactants are left, the reaction keeps the pd almost steady. As the reactants are used up the reaction cannot be kept going, so the pd drops. At 10 hours at least one reactant is almost completely used up.

(c) Could the cell keep the lamp lit for a whole day? No. Nothing is added to the cell, so once its reactants are used up it cannot produce a pd.

3.9.3 b: Fuel cells

A fuel cell produces electricity through the reaction of a fuel with oxygen. The difference from an ordinary chemical cell is where the reactants come from.

	Chemical cell	Fuel cell
Reactants	Sealed inside when the cell is made	Fed in continuously from outside (fuel plus oxygen, often from air)
How long it works	Until the reactants are used up	As long as fuel and oxygen keep being supplied
What you do when it stops	Replace it	Refill the fuel tank

The hydrogen–oxygen fuel cell

A hydrogen–oxygen fuel cell uses hydrogen as its fuel. Hydrogen and oxygen react inside the cell and the overall change is:

hydrogen + oxygen → water

2H₂ + O₂ → 2H₂O

Hydrogen does not burn in the cell. There is no flame, and the energy is transferred as electricity.

Uses named in the specification: cars and spacecraft.

• Cars: the fuel cell supplies electricity to an electric motor. The driver refills a hydrogen tank.
• Spacecraft: a spacecraft carries hydrogen and oxygen and the fuel cell turns them into electricity. On the Apollo missions the water made by the fuel cells was used by the crew for drinking, so the “waste” was useful.

Why fuel cells cause less pollution when in use

Water is the only waste product from a hydrogen–oxygen fuel cell. Hydrogen contains no carbon and no sulfur, so the cell cannot make carbon dioxide, carbon monoxide, soot or sulfur dioxide. That is why the specification says fuel cells cause less pollution when in use.

Those last three words matter. Hydrogen is not found as a free gas in useful amounts; it has to be made. If it is made using fossil fuels, or using electricity from power stations that burn fossil fuels, pollution is released at the factory or power station instead of at the vehicle. One route you have met in this course is the electrolysis of sodium chloride solution, which produces hydrogen (see the electrolysis study guide). Electrolysis needs a large supply of electricity.

Worked example 2: water from a fuel cell

A fuel cell car uses 0.90 kg of hydrogen on a journey. Calculate the mass of water produced. (Relative atomic masses: H = 1, O = 16.)

Step 1. Equation: 2H₂ + O₂ → 2H₂O. The mole ratio of H₂ to H₂O is 2 : 2, which is 1 : 1.

Step 2. Mr of H₂ = 2 × 1 = 2. Moles of H₂ = 900 g ÷ 2 = 450 mol.

Step 3. Moles of H₂O = 450 mol.

Step 4. Mr of H₂O = (2 × 1) + 16 = 18. Mass of water = 450 × 18 = 8100 g = 8.1 kg.

The same journey also used 225 mol of oxygen (7.2 kg), taken from the air.

Hydrogen as a fuel: storage and use

The specification asks for the benefits and disadvantages of hydrogen fuel under two headings. Here is the first.

Benefits in use

• It releases a lot of energy per gram compared with common fuels, because each hydrogen molecule is so light.
• It can be used in a fuel cell to give electricity directly, with water as the only waste product.
• A fuel cell vehicle is refilled with fuel rather than waiting for cells to be replaced.

Disadvantages in storage and use

• Hydrogen is a gas at room temperature and pressure with a very low density. A useful mass takes up a huge volume.
• To fit it into a vehicle it must be compressed to a high pressure, so tanks must be strong and heavy, or it must be cooled to a liquid at a very low temperature, which uses energy.
• Hydrogen is highly flammable. A leak can form an explosive mixture with air. Its small molecules escape easily through tiny gaps.
• There are far fewer hydrogen filling stations than petrol stations, and making the hydrogen can itself cause pollution.

Worked example 3: why storage is a problem

A car’s tank holds 5.0 kg of hydrogen. (H = 1. At room temperature and pressure, 1 mol of gas fills 24 dm³.)

(a) Calculate the volume of this hydrogen at room temperature and pressure.

Moles of H₂ = 5000 g ÷ 2 = 2500 mol.

Volume = 2500 × 24 = 60 000 dm³ = 60 m³ (1 m³ = 1000 dm³).

(b) The tank has an internal volume of 0.125 m³. How many times smaller is this than the volume in (a)?

60 ÷ 0.125 = 480. The gas has to be squeezed into a space 480 times smaller, which is why a high-pressure tank is needed.

Products of combustion

The second heading is products of combustion. Hydrogen can also be burned, for example in an engine adapted to use it.

hydrogen + oxygen → water (exothermic)

Benefit: because hydrogen contains no carbon or sulfur, burning it releases no carbon dioxide, so it does not add to the increase in carbon dioxide that may cause climate change. It also gives no carbon monoxide, no soot particulates and no sulfur dioxide.

Disadvantage: an engine burns hydrogen in air, not pure oxygen, and combustion happens at a high temperature. Section 3.10 of the specification says oxides of nitrogen are formed at high temperatures. So a hydrogen engine can release oxides of nitrogen, which cause acid rain. A fuel cell does not burn the hydrogen, so this problem does not arise in the same way and water is still the only waste product.

Comparing combustion of hydrogen with fuel cells

Questions on this statement give you information, such as a data table, and ask you to compare or evaluate.

Method in steps

1. Read the table and pick out each property that differs between the two options.
2. For each, write one sentence that states both sides: “the fuel cell … whereas the engine …”.
3. Add any chemistry the data hints at: water as the only product; oxides of nitrogen at high temperature; hydrogen storage.
4. Finish with a judgement that refers back to the data.

Worked example 4: using supplied data

A company tests two buses that both use hydrogen. Use these values: 1 g of hydrogen releases 142 kJ. The hydrogen engine bus transfers 30% of this energy usefully. The fuel cell bus (fuel cell plus electric motor) transfers 50% usefully. Each bus needs 710 kJ of useful energy per kilometre.

(a) Calculate the useful energy each bus gets from 1.0 kg of hydrogen.

Energy released = 1000 g × 142 kJ/g = 142 000 kJ.

Engine bus: 0.30 × 142 000 = 42 600 kJ. Fuel cell bus: 0.50 × 142 000 = 71 000 kJ.

(b) Calculate how far each bus can travel on 1.0 kg of hydrogen.

Engine bus: 42 600 ÷ 710 = 60 km. Fuel cell bus: 71 000 ÷ 710 = 100 km.

(c) Use this and your own knowledge to compare the two buses.

On these figures the fuel cell bus goes further on the same mass of hydrogen, so it needs less hydrogen to be stored and made. Both buses produce water and no carbon dioxide when running. The engine burns hydrogen in air at high temperature, so it may also release oxides of nitrogen, whereas the fuel cell’s only waste product is water. Both still share the storage problems of hydrogen gas. On this information the fuel cell bus is the better choice.

Common errors

• Saying a chemical cell stops because it “runs out of electricity”. It stops because a reactant is used up.
• Saying a fuel cell burns hydrogen. It reacts hydrogen with oxygen without combustion.
• Writing “fuel cells cause no pollution”. The specification says less pollution when in use, and making the hydrogen may pollute.
• Forgetting that a hydrogen engine burns the fuel in air, so oxides of nitrogen can form at the high temperature.
• Listing facts about only one option in a “compare” question.
• Converting kilograms to grams wrongly, or forgetting that hydrogen gas is H₂ with Mr = 2, not 1.

Where to go next

Next, do the self-test in the revision notes, followed by the practice questions. For moles and gas volumes, see the quantitative chemistry study guide.

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.