Fuel cell — two ideas, both credited
A hydrogen-oxygen fuel cell reacts hydrogen with oxygen to produce electricity, with water as the only chemical product — mark schemes credit those two ideas separately. The useful output is electricity, not heat. Water is the only product because the fuel is hydrogen: no carbon atom exists in the reactants, so no carbon dioxide, carbon monoxide or soot can form. Hydrogen and oxygen are fed in continuously from outside, so the cell works while the supply lasts.
Fuel cell reaction — hydrogen + oxygen → water
Word equation: hydrogen + oxygen water. Symbol equation: . Both reactants contain only hydrogen and oxygen atoms, so water really is the only chemical product. With Cambridge values H = 1 and O = 16, , and ; reacting masses scale from these by simple proportion using the equation coefficients.
(Extended) Fuel cell vs petrol engine — both sides
Advantages: at the point of use only water — no carbon dioxide (greenhouse gas), carbon monoxide or particulates; chemical energy becomes electrical directly, with no combustion/heat stage, so more is used usefully; quieter, less vibration; quicker to refuel than a battery car. Disadvantages: hydrogen needs high-pressure or liquefied storage and is flammable; few refuelling stations; making hydrogen often uses fossil-fuel electricity; cells and tanks cost more.
Drawn from real examiner reports.
Fuel cell ≠ burning hydrogen
Combustion is an uncontrolled reaction with a flame, releasing energy mainly as heat and light; a fuel cell is a controlled electrochemical reaction at separate electrodes, releasing energy mainly as electricity, with no flame. Both make water, but "it burns hydrogen to produce heat" describes a combustion engine and misses the required word "electricity".
"Produces electricity" is half an answer
The C4.2 statement is marked as two separate ideas, so stopping at "it produces electricity" without naming water throws away a mark. So does the vague swap: "it doesn't pollute" or "it's clean" instead of the chemical fact. The word only in "water is the only chemical product" is part of the credited answer — drop it and the statement stops ruling out other products.
Heat is not a chemical product
Candidates asked to name the product often add something extra. Carbon dioxide is impossible: neither nor contains a carbon atom, so there is no source for it. Heat (and electricity) are energy outputs, not chemical products — listing heat as a product loses the mark. The only chemical product of the reaction inside the cell is water.
A fuel cell is not a battery
A fuel cell is supplied continuously with fuel (hydrogen) and oxidant (oxygen) from outside the cell, and keeps producing electricity for as long as that supply continues. A battery stores its reactants sealed inside and stops once they are used up. Do not describe a fuel cell as "storing" hydrogen the way a battery stores charge, and never say it "runs down".
"Water only" ≠ zero emissions overall
The mark-scheme fact is that water is the only chemical product of the reaction inside the cell. That is not a claim that the whole fuel cycle is pollution-free: the hydrogen itself has to be made, often using electricity from a fossil-fuelled power station, so carbon dioxide may be released elsewhere. Write "no emissions at the point of use", not "emission-free".
Fuels store chemical energy, not kinetic
The efficiency advantage only makes sense if the energy store is named correctly. The energy in hydrogen — as in petroleum or diesel — is a chemical store, not a kinetic one; candidates often mislabel it. The fuel cell converts that chemical store directly to electrical energy, whereas a petrol engine goes chemical → thermal → kinetic, losing energy at the extra step.
Flagged Jun 2022 P31 Q6e
(Extended) Name both vehicles in every point
"In comparison with gasoline/petrol engines" is a comparison command. Write each point with both vehicles in the same sentence: "a fuel cell vehicle releases only water, whereas a petrol engine releases carbon dioxide". A fact about fuel cells alone does not make the comparison.
Match the points to the mark tariff
A 2-mark "advantages and disadvantages" question wants one clear advantage and one clear disadvantage, not four vague ones. Keep the points distinct too — "no pollution" and "environmentally friendly" are the same idea, and examiners credit distinct ideas, not repeated phrasing.
Quote the full two-clause statement
Reproduce the syllabus wording rather than a paraphrase: "a hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product". Both clauses are credited separately, so writing the whole sentence secures both marks.
(Extended) Answer both sides of the comparison
"Describe the advantages and disadvantages" needs at least one of each. A one-sided answer cannot reach full marks however correct each sentence is. Read the command word before writing, and check your answer has both halves before moving on.
Cambridge 0654 spec reference: Section C4 "Electrochemistry", sub-topic C4.2 (Core + Extended/Supplement). This leaf covers the Core statement that a hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity with water as the only chemical product, and the Extended (Supplement) statement describing the advantages and disadvantages of hydrogen-oxygen fuel cells compared with gasoline/petrol engines in vehicles.
Scope note: this is a topic newly added for the 2025-2027 syllabus. Core C4.2 is entirely qualitative -- the Core statement is simply that a fuel cell produces electricity with water as the only chemical product. Every numeric worked example below (reacting masses, gas volumes, percentage composition) is a stoichiometry calculation and is therefore (Extended) only, consistent with 0654's rule that Core stoichiometry has no mole concept and no reacting-mass calculations.
| Term | Mark-scheme-precise meaning |
|---|---|
| Hydrogen-oxygen fuel cell (Core) | A cell that reacts hydrogen and oxygen together to produce electricity, with water as the only chemical product |
| Fuel cell reaction (word equation) (Core) | hydrogen + oxygen water |
| Fuel cell reaction (symbol equation) (Core) | |
| Fuel cell vs battery (Core, precision) | A fuel cell is supplied continuously with fuel and oxidant from outside and keeps working as long as the supply lasts; a battery stores its reactants sealed inside and stops when they are used up |
| Fuel cell vs combustion (Core, precision) | A fuel cell reacts hydrogen and oxygen electrochemically to give mainly electricity, with no flame; combustion ("burning") releases energy mainly as heat and light, with a flame |
Reacting-mass relationship (Extended):
Because neither nor contains carbon, water is the only possible chemical product -- there is no source of a carbon atom for , or soot to form from.
(Core) Define a hydrogen-oxygen fuel cell.
(a) (Core) State what a hydrogen-oxygen fuel cell does, and name its only chemical product. (2 marks)
(b) (Core) Write the word equation for the reaction taking place in the fuel cell. (1 mark)
