Fuel cell — water is the only product
A hydrogen–oxygen fuel cell makes electricity from hydrogen reacting with oxygen, without combustion. Hydrogen is fed to the negative electrode and oxidised (loses electrons); oxygen to the positive electrode and reduced (gains electrons). Electrons flow round the external circuit as a continuous current. Overall: . The only product is water — no carbon dioxide or soot — and, unlike a battery, it does not run down.
Advantages over petrol and diesel engines
Compared with a petrol or diesel engine, a fuel cell emits only water at the point of use, so no carbon dioxide is produced and it does not add to global warming. It gives off no toxic carbon monoxide, nitrogen oxides or soot, so air quality is better. It is also more efficient, wasting less energy as heat, and runs near-silently. In the exam, name a specific pollutant avoided plus its consequence, not just that it is cleaner.
Disadvantages — hydrogen storage and supply
The main drawbacks concern the hydrogen fuel. Hydrogen is a gas at room temperature, so it is hard and expensive to store — needing high-pressure tanks or cryogenic cooling, unlike liquid petrol. It is highly flammable and forms explosive mixtures with air, a safety hazard. Most hydrogen is made by steam reforming of natural gas, which releases carbon dioxide. Refuelling stations are scarce and fuel-cell systems are costly to build.
Drawn from real examiner reports.
Naming the electrode, not the product
Do not answer "anode" or "cathode" when a question asks what is produced or what happens at an electrode — that scores zero. Match the command word: "what is formed" wants the substance (water); "what happens" wants the process (oxygen is reduced); only "name the electrode" wants anode or cathode.
S22 P31 Q7(c)(ii); W22 P32 Q7(c)(ii) — candidates named the electrode instead of the product formed at it.
Vague environmental answers score zero
Answers like "better for the environment" or "cleaner" earn nothing. Name the pollutant avoided and its effect: "no carbon dioxide, so it does not add to global warming", or "hydrogen is flammable, so storage is a safety hazard". Avoid saying it makes no pollution at all — if the hydrogen came from steam reforming, carbon dioxide was released in production.
W22 P31 Q5(d)(ii); W22 P32 Q5(b)(ii) — vague answers such as "affects health" recorded as scoring zero.
Oxygen is reduced, hydrogen oxidised
Do not assume oxygen is oxidised — the reverse is true. Hydrogen is oxidised at the negative electrode (loses electrons); oxygen is reduced at the positive electrode (gains electrons). Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons. Oxidation means loss of electrons, not just combining with oxygen.
W22 P31 Q4(b)(ii); W22 P32 Q4(b)(ii) — candidates gave the opposite definition of oxidation/reduction.
A fuel cell is not a battery
A battery holds a fixed amount of reactants inside and runs down as they are used up, so it must be recharged or replaced. A fuel cell is fed continuously with hydrogen and oxygen from outside, so it does not run down and is never recharged — it generates current for as long as fuel is supplied. Writing that a fuel cell "must be recharged" is wrong.
Zero emissions only at point of use
The fuel cell itself emits only water, but that does not make the whole system carbon-free. Most hydrogen is made by steam reforming of natural gas, which releases carbon dioxide. So "zero emissions" is only true if the hydrogen is made using renewable electricity, e.g. electrolysis powered by wind or solar.
Hydrogen to negative, oxygen to positive
Getting the gases the wrong way round wrecks the half-equations. Hydrogen is always supplied to the negative electrode, where it is oxidised; oxygen is supplied to the positive electrode, where it is reduced to form water. If you swap them, both the electrode charges and the oxidation and reduction labels come out reversed, and the whole answer scores zero.
Match the command word
Check whether the question wants a substance, a process or a label. "What is produced" wants the substance (water); "what happens" wants the process (oxygen is reduced); "name the electrode" wants anode or cathode. The wrong type scores zero even when the chemistry is right.
Two marks: pollutant plus consequence
For a 2-mark advantage, give the pollutant avoided AND its consequence, e.g. "no carbon dioxide, so no contribution to global warming". Half an answer earns one mark. Good pairs: only product is water; no carbon monoxide or soot (cleaner air); more efficient (less heat wasted).
Balancing electrode half-equations
Put electrons on the correct side: oxidation makes electrons on the right (); reduction uses them on the left. Balance both atoms and charge, and match the electron count (four in each half) so the two combine correctly.
Fuel cell: An electrochemical device that converts the chemical energy of a fuel (hydrogen) directly into electrical energy, without combustion. The fuel cell works continuously as long as hydrogen and oxygen are supplied.
Overall reaction in a hydrogen–oxygen fuel cell:
Hydrogen is oxidised; oxygen is reduced. The only product at the point of use is water ().
Oxidation (in electron terms): loss of electrons. Reduction (in electron terms): gain of electrons. OIL RIG is a mnemonic (Oxidation Is Loss, Reduction Is Gain) — not the definition itself.
State the overall equation for the reaction in a hydrogen–oxygen fuel cell.
A hydrogen–oxygen fuel cell generates electricity from the reaction between hydrogen and oxygen.
(a) State what type of energy the fuel cell converts chemical energy into.
(b) State the name and formula of the only product formed in the fuel cell.
(c) Write a balanced chemical equation for the overall reaction in the fuel cell.
