Hydrogen–Oxygen Fuel Cells

Fuel cells carry few marks per paper (usually 2 to 4), but they are near-guaranteed marks because the syllabus demands so little: one equation, one product, and a balanced evaluation against petrol engines. The marks slip away when candidates write a vague “it’s eco-friendly” instead of the specific product-based reasoning the mark scheme lists.

The Core fact

A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity, and the only chemical product is water:

2H2 + O2 → 2H2O

That single sentence plus the equation is the entire Core requirement. Notice what a fuel cell is not: it is not a battery storing charge, and it is not an engine burning fuel for heat. Hydrogen and oxygen react inside the cell and the energy released is transferred electrically rather than as heat. The same reaction burned in air is strongly exothermic. The energy bookkeeping behind that is covered in Chemical Energetics.

The Supplement evaluation: fuel cell vs petrol engine

Extended papers ask you to weigh a hydrogen fuel cell against a gasoline (petrol) engine for powering a car. The mark scheme rewards specific, product-based points on both sides.

Advantages of the fuel cell	Disadvantages of the fuel cell
Water is the only product at the point of use	Hydrogen is difficult and expensive to store: a low-density, flammable gas needing high-pressure tanks
No CO2 released, so no contribution to climate change in use	Few hydrogen filling stations exist
No CO (toxic) or unburned hydrocarbons	Making the hydrogen may burn fossil fuels, shifting the emissions elsewhere
More efficient energy transfer than combustion	Fuel cells and hydrogen are currently expensive

Two phrasing details separate full marks from half marks. First, “at the point of use”: the cell itself releases no CO2, but hydrogen production might. Examiners credit candidates who show they know the difference. Second, compare like with like: the petrol engine’s products are carbon dioxide (greenhouse gas), carbon monoxide (toxic) and water, so name the products on both sides rather than writing “petrol pollutes”.

An “evaluate” or “suggest advantages and disadvantages” command wants at least one point from each column. One-sided answers cap themselves at half the marks no matter how detailed they are, the same balance rule that governs the 6-mark extended response technique.

Where the electricity comes from

0620 does not require electrode half-equations for the fuel cell, but the concept link is worth having: hydrogen is oxidised and oxygen is reduced, and the electrons transferred between them are routed through an external circuit instead of being exchanged directly. A fuel cell is electrolysis run in reverse: electrolysing dilute sulfuric acid splits water into hydrogen and oxygen using electricity, while the fuel cell recombines hydrogen and oxygen to release electricity. Linking the two earns the “explain the connection” mark when a question pairs them, and the electrolysis half sits in Electrolysis of Aqueous Solutions.

Worked exam question

Hydrogen–oxygen fuel cells can be used to power cars. (a) Write the balanced equation for the overall reaction in the cell. [2] (b) State one advantage and one disadvantage of using a hydrogen–oxygen fuel cell rather than a gasoline engine in a car. [2] (c) A student claims fuel-cell cars cause no pollution at all. Comment on this claim. [1]

Model answer: (a) 2H2 + O2 → 2H2O (1 for correct formulae, 1 for balancing). (b) Advantage: water is the only product, so no carbon dioxide is released while driving (1). Disadvantage: hydrogen is difficult to store because it is a flammable gas of very low density (1). (c) The cell itself produces only water, but manufacturing the hydrogen may burn fossil fuels and release carbon dioxide, so pollution can still occur elsewhere (1).

Mark-by-mark: (a) loses the second mark for H2 + O → H2O or unbalanced coefficients (oxygen is diatomic). (b) needs one point per side; two advantages score one mark, not two. (c) is the discriminator: the mark scheme wants the point-of-use vs production distinction stated explicitly.

The mistakes that cost marks

1. “It’s better for the environment” with no product named. The creditable version is “water is the only product, so no CO2 is released”.
2. Unbalanced equations: writing H2 + O2 → H2O. Balance it: 2H2 + O2 → 2H2O.
3. One-sided evaluations. “Give advantages and disadvantages” means both, every time.
4. Calling the fuel cell a battery or saying it “burns” hydrogen. It converts chemical energy directly to electrical energy without combustion.

How examiners want it phrased

Student wording	Mark-scheme wording
”Fuel cells are clean"	"Water is the only product, so no carbon dioxide is released at the point of use"
"Hydrogen is dangerous to keep"	"Hydrogen is a flammable, low-density gas that must be stored at high pressure"
"Petrol is bad for the planet"	"Burning gasoline releases carbon dioxide, a greenhouse gas, and toxic carbon monoxide"
"Hydrogen isn’t really green"	"Producing the hydrogen may use fossil fuels, releasing carbon dioxide during manufacture”

Specific products, named gases, and the point-of-use distinction: that is the whole game. The rest of the topic’s electrode chemistry is mapped on the Electrochemistry pillar. If your evaluations keep coming back marked 2 out of 4, a free 1-hour trial lesson with one of our Chemistry specialists will show you the balanced-answer structure on real past-paper questions.

Test yourself

All three are Supplement level. Answer in mark-scheme sentences, then click to check.

Q1 (2 marks). State what a hydrogen–oxygen fuel cell is used to produce, and name its only chemical product.

Show answer

• It produces electricity / electrical energy [1] • The only chemical product is water [1]

Q2 (2 marks). Explain why a hydrogen–oxygen fuel cell can be described as the reverse of the electrolysis of dilute sulfuric acid.

Show answer

• Electrolysis uses electricity to split water into hydrogen and oxygen [1] • The fuel cell combines hydrogen and oxygen to form water and release electricity [1]

Q3 (2 marks). A government is deciding whether to subsidise hydrogen fuel-cell cars. Explain two barriers that currently limit their use.

Show answer

• Hydrogen is difficult and expensive to store: it is a low-density, flammable gas that needs high-pressure tanks [1] • There are few hydrogen filling stations / producing the hydrogen may itself burn fossil fuels, releasing carbon dioxide [1]