Chemical Cells and Fuel Cells [chemistry only] — AQA GCSE Chemistry

⚗️ CHEMISTRY ONLY

SPEC 4.5.2.1, 4.5.2.2 • PAPER 1

Chemical Cells and Fuel Cells

How chemistry generates electricity. Two devices, two mechanisms — and the four examiner errors AQA tests every year.

Two Devices, Both Make Electricity

This lesson is about turning chemistry into electricity. Two different devices do this — they look similar on paper but work differently in practice.

Chemical cell (battery)

Two different metals dipping into an electrolyte.

Generates electricity from the chemical reaction between the metals and the electrolyte. Stops when reactants run out.

Fuel cell

Two electrodes with hydrogen fuel piped in continuously.

Generates electricity for as long as fuel is supplied. Doesn't run out — only "refuels".

"Cells contain chemicals which react to produce electricity... Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference." — AQA Specification 4.5.2.1, 4.5.2.2

Chemical Cell — How It Works

The mechanism

More reactive metal (zinc) = negative electrode — loses electrons more easily

Electrons flow through the external wire from negative to positive electrode

The electrolyte completes the circuit by allowing ions to move between the electrodes

This electron flow is the electric current — measured by the voltmeter

Voltage Depends On...

Two factors control the voltage produced by a chemical cell. Knowing both lets you predict — and explain — how voltage will change.

Factor 1 — Reactivity gap

The greater the difference in reactivity between the two metals, the greater the voltage.

e.g. Mg + Cu → big voltage; Zn + Cu → smaller voltage.

Factor 2 — Electrolyte

The type of electrolyte affects voltage. So does its concentration: a more concentrated electrolyte typically gives a higher voltage.

Why this matters for exam questions

If you're asked to investigate voltage experimentally, only ONE factor is allowed to vary at a time. If concentration is the IV (independent variable), then the two metals must stay the same for every measurement — otherwise the result is invalid. We'll see this in Practice 2.

Reading Voltage Data — The Sign Matters

AQA frequently gives voltage data and asks you to rank metals by reactivity. The trap: negative voltages in the data don't mean "small" — sign tells you which side of the reference metal you're on.

Electrode A (paired with nickel reference)Voltage / V

Magnesium (Mg)+2.12

Zinc (Zn)+0.51

Nickel (Ni)0.00

Copper (Cu)−0.59

Silver (Ag)−1.05

How to read this

Most reactive metal = highest positive voltage. Here: Mg (+2.12).

Least reactive metal = most negative voltage. Here: Ag (−1.05).

Order of reactivity (most → least): Mg, Zn, Ni, Cu, Ag.

Mark-scheme phrase: "the higher the (positive) voltage, the more reactive the metal".

Examiner Alert

Common error in P1 Jun 2024 Q06.1: students ranked metals by magnitude of voltage, ignoring the sign. So Ag (−1.05) was placed second-most reactive (because |−1.05| is large) — wrong. Negative voltage = LESS reactive than the reference. Always read the sign.

Batteries — Stacked, Drained, Recharged

Battery

A battery is two or more cells connected in series — the voltages add together to give a higher total. A 9V battery is six 1.5V cells stacked.

Non-rechargeable

Reactions cannot be reversed.

Reactants are used up over time → battery goes flat → must be discarded. Examples: standard alkaline AA batteries, zinc-carbon dry cells.

Rechargeable

Reactions can be reversed by applying an external current.

Charger drives the reaction backwards → reactants reform → battery regains voltage. Examples: lithium-ion phone batteries, car batteries.

"Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied." — AQA Specification 4.5.2.1

Alert 1 — Don't Draw An Electrolytic Cell

Examiner Alert · Most-missed mark

In P1 Jun 2022 Q04.7, students were asked to draw a chemical cell using zinc, copper, and "another substance" to light a lamp. The most common error: drawing an electrolytic cell — adding a power supply or battery to the circuit.

Source: P1 Jun 2022 Q04.7 examiner report.

✗ Electrolytic cell (wrong)

Two electrodes in solution + power supply in circuit

Used to cause a reaction (e.g. extracting aluminium)

Belongs to topic 4.4 (electrolysis), not 4.5.2

✓ Chemical cell (correct)

Two different metals + electrolyte + just a wire (and lamp/voltmeter)

NO power supply — the cell IS the power source

Generates current from the spontaneous reaction between metals and electrolyte

Mark-scheme requirements (3 marks)

MP1: Solution in a container

MP2: Zinc and copper electrodes, both inserted into the solution

MP3: Complete circuit that functions as an electrochemical cell, with the electrolyte clearly labelled (e.g. "dilute sulfuric acid")

Ignored by examiner: voltmeter/ammeter regardless of location; polarity labels on electrodes (you don't need to mark + / −); molten electrolytes also accepted.

Fuel Cells — Different Beast

A fuel cell is fundamentally different from a chemical cell: instead of consuming a fixed amount of metal, it consumes a continuous stream of fuel — typically hydrogen. As long as fuel keeps coming, electricity keeps flowing.

Definition · Spec verbatim

A fuel cell is supplied by an external source of fuel (e.g. hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.

Hydrogen fuel cell — overall reaction

2H₂ + O₂ → 2H₂O Hydrogen and oxygen react to produce water — the only product. The energy from this reaction is converted directly into electrical energy, not heat.

Fuel Cell Mechanism

Step by step

At the anode (−): hydrogen molecules (H₂) are oxidised — they lose electrons.

Electrons flow through the external circuit to the cathode — this is the electric current.

H⁺ ions travel through the membrane (electrolyte) to the cathode side.

At the cathode (+): oxygen is reduced — it gains electrons and combines with H⁺ ions to form water.

Half Equations HT

Higher Tier students need to write the half equations for both electrodes in a hydrogen fuel cell. The trick: balance both atoms AND charges.

⚡ Higher Tier Only · Spec 4.5.2.2

At the anode (oxidation — loss of electrons):

2H₂ → 4H⁺ + 4e⁻

At the cathode (reduction — gain of electrons):

O₂ + 4H⁺ + 4e⁻ → 2H₂O

Adding the half equations gives the overall reaction:

2H₂ + O₂ → 2H₂O

How to balance — quick rules

Balance atoms first — make sure same number of H, O on both sides.

Then balance charge using electrons. Add e⁻ to whichever side needs negative charge to balance.

For the anode: 2H₂ has 4 H atoms → produces 4H⁺ → balance by adding 4e⁻ on the right.

For the cathode: O₂ becomes 2H₂O → needs 4H⁺ + 4e⁻ to balance both atoms and charge.

Alert 2 — Hydrogen MOLECULES, Not Ions

Examiner Alert

A widespread student misconception is to write "hydrogen ions react at the anode" or "H⁺ is oxidised". This is fundamentally wrong. H₂ molecules are oxidised at the anode (they lose electrons). H⁺ ions are produced at the anode — they're the result of oxidation, not the reactant.

Source: P1 Jun 2024 Q06.3 mark scheme rewards "hydrogen is oxidised" or "hydrogen loses electrons" — both unambiguously describe H₂ molecules, not ions.

✗ Wrong (no marks)

"Hydrogen ions react at the anode"

"H⁺ is oxidised"

"Hydrogen ions lose electrons"

✓ Correct

"Hydrogen molecules are oxidised" — accepted

"H₂ loses electrons" — accepted

"Hydrogen is oxidised electrochemically" — direct mark-scheme phrase

Direction of conversion

Easiest way to remember: H₂ becomes H⁺. Reactants → products. So H₂ is the reactant (lost electrons), H⁺ is the product (now has lost an electron and carries positive charge).

Alert 3 — Electrochemical, NOT Combustion

Examiner Alert · The biggest fuel-cell trap

A widespread student misconception is to describe fuel cells using language that belongs to combustion engines: "hydrogen is combusted" or "steam drives a turbine". Both are wrong. A fuel cell is NOT a combustion engine.

Source: the mark scheme for P1 Jun 2024 Q06.3 is "hydrogen is oxidised (electrochemically) to produce water". The bracketed word "(electrochemically)" is in the MS specifically to rule out combustion-style answers.

✗ Wrong (no marks)

"Hydrogen is combusted to produce energy"

"Hydrogen burns and produces steam"

"Steam from the reaction drives a turbine"

"Heat is converted into electricity"

✓ Correct

"Hydrogen is oxidised electrochemically"

"Direct electrochemical reaction"

"The only product is water"

"Electrons flow through the external circuit"

Why "combustion" is wrong

Combustion of hydrogen produces heat (and light) — that heat would then need a turbine, generator, etc. to make electricity. A fuel cell skips ALL of that. The chemical energy of H₂ + O₂ is converted directly to electrical energy at the electrodes. No flames. No steam. No turbine. No combustion.

Fuel Cells vs Rechargeable Batteries

The spec requires you to evaluate hydrogen fuel cells in comparison with rechargeable batteries. That means giving both sides — pros AND cons.

Fuel cells advantagesRechargeable batteries advantages

Pollution at point of useOnly water — no CO₂, no air pollutionNone at point of use

Refuel/recharge timeFast — just refill the H₂ tankSlow — recharging takes minutes to hours

Energy per kgHigher energy densityLower than fuel cells

LifetimeLasts as long as fuel is suppliedCapacity degrades after many recharge cycles

Disadvantages of fuel cells

Hydrogen storage: H₂ is highly flammable; must be stored at very high pressure or very low temperature — this is dangerous and energy-intensive.

Hydrogen production: most hydrogen today is made from fossil fuels (steam reforming of methane), so the carbon footprint isn't zero — just shifted upstream.

Infrastructure: very few hydrogen filling stations exist; you can't refuel a fuel-cell car most places.

Cost: fuel cells are currently expensive to manufacture.

Common Errors — Cells & Fuel Cells

What students doCorrect version

Drew an electrolytic cell (with power supply) when asked for a chemical cellTwo metals + electrolyte + wire/voltmeter — NO power supply

Used same metal for both electrodesTwo different metals — voltage requires reactivity difference

Ranked metals by magnitude of voltage (ignoring sign)Most reactive = most positive voltage; least reactive = most negative

"Hydrogen is combusted in the fuel cell""Hydrogen is oxidised electrochemically "

"Hydrogen ions react at the anode""Hydrogen molecules lose electrons at the anode"

"Steam is the product of a fuel cell"" Water is the only product" (at point of use)

For investigation: varied multiple variablesVary only one factor; keep two metals and electrolyte type the same

Practice — Draw A Chemical Cell

Zinc and copper can be used with another substance to produce electricity. The symbol ⊗ represents a lamp.

Complete the diagram to show how zinc, copper and another substance can be used to light a lamp. Label:

zinc

copper

the other substance used.3 MARKS

Source: P1 Jun 2022 Q04.7 (verbatim)

MP1

Solution in a container — a beaker (or similar vessel) holding the solution.

MP2

Zinc electrode AND copper electrode, both inserted into the solution — clearly labelled "zinc" and "copper".

MP3

Complete circuit functioning as an electrochemical cell, including a labelled electrolyte — both electrodes connected by a wire (with the lamp ⊗ in the circuit), and the solution labelled with what it is (e.g. "dilute sulfuric acid", "copper sulfate solution", "sodium chloride solution"). A named molten electrolyte is also accepted.

📝

Ignored by the marker: "ignore voltmeter / ammeter regardless of location" — adding extra meters doesn't lose marks, AND they don't earn extra marks either. "Ignore polarities on electrodes" — you don't need to mark which is + or −. "Ignore labels" generally (other than what's required by MP3).

⚠️

Common error to avoid: do NOT add a power supply or battery to your circuit — that turns it into an electrolytic cell. The chemical cell IS the power source.

Practice — Rank Metals From Voltage Data

A student investigated the voltage produced by different pairs of metal electrodes. Each metal was paired with a nickel electrode in 1.0 mol/dm³ sodium chloride solution. Results:

Cu: −0.59 V • Mg: +2.12 V • Ni: 0.00 V • Ag: −1.05 V • Zn: +0.51 V

Write the symbols of the five metals in order of reactivity. Justify your answer.3 MARKS

Source: P1 Jun 2024 Q06.1 (verbatim)

MP1+2

Order (most → least reactive): Mg, Zn, Ni, Cu, Ag (2 marks for full correct order; 1 mark for partial).

MP3

Justification: "The higher the (positive) voltage, the more reactive the metal". Equivalent: "the most reactive metal has the highest positive voltage" OR "the least reactive metal has the most negative voltage" OR "the greater the reactivity difference, the greater the magnitude of the voltage".

⚠️

Watch the sign: Ag has voltage −1.05 V. The magnitude (1.05) is large, but the SIGN is negative — so Ag is the LEAST reactive, not the most. Students who ranked by absolute magnitude lost marks here.

📝

Partial credit: 1 mark for getting either Mg–Zn–Ni (top half) OR Ni–Cu–Ag (bottom half) correctly ordered, even if the full order is wrong.

Practice — Plan An Experiment

The voltage produced by a chemical cell depends on the concentration of the electrolyte solution.

Plan an experiment to investigate how the voltage produced by a chemical cell varies with the concentration of the electrolyte solution.6 MARKS

The following substances are available:

the metal electrodes from earlier (Cu, Mg, Ni, Ag, Zn)

1.0 mol/dm³ sodium chloride solution

pure water

Source: P1 Jun 2024 Q06.2 (verbatim) — Level marked

📝

This is a Level-marked question (no individual MPs). Marks come from logical sequencing and complete steps:

Level 3 (5–6 marks): Method leads to a valid outcome. Key steps identified and logically sequenced.

Level 2 (3–4 marks): Method may not lead to valid outcome. Most steps identified but not fully sequenced.

Level 1 (1–2 marks): Some relevant steps but links not clear.

METHOD

A model Level 3 answer:

Place a zinc electrode and a copper electrode (or any two different metals) in 100 cm³ of 1.0 mol/dm³ NaCl solution in a beaker.

Connect the electrodes to a voltmeter using wires.

Measure and record the voltage.

Prepare diluted NaCl solutions using a measuring cylinder (or pipette/burette): for 0.8 mol/dm³, mix 80 cm³ of 1.0 mol/dm³ NaCl with 20 cm³ pure water. Repeat to make 0.6, 0.4, 0.2 mol/dm³ solutions.

Repeat the voltage measurement at each concentration.

Plot a graph of voltage against concentration.

CTRL

Critical control variables (do NOT skip these): use the same two metals each time; use the same volume of electrolyte solution each time; same temperature.

⚠️

Most-common Level 1 trap: changing the metals AND the concentration → multiple variables, invalid outcome. Lock the two metals first; only concentration is the IV.

Practice — How A Fuel Cell Works

Describe how a hydrogen fuel cell produces a potential difference.2 MARKS

Source: P1 Jun 2024 Q06.3 (verbatim)

MP1

"Hydrogen is oxidised (electrochemically)" — the word "electrochemically" is in brackets meaning optional but it makes the answer cleaner. Also accepted: "hydrogen loses electrons".

MP2

"To produce water" — one of the key marks. The water is the product; mentioning it is essential.

📝

Mark scheme says: "ignore references to electrodes". You don't need to mention anode/cathode for the 2 marks — but if you do mention them, they should be correctly assigned (anode = oxidation; cathode = reduction).

⚠️

Avoid these wrong answers: "hydrogen is combusted" / "hydrogen ions react" / "steam drives a turbine" — all explicit zero marks.

Checkpoint — Lesson 5.04

You should now be able to:

Chemical cells

Describe how a chemical cell produces electricity (more reactive metal = negative; electrons flow externally; ions move through electrolyte)

Draw a correct chemical cell diagram — two different metals + electrolyte + external wire (NO power supply)

State that voltage depends on reactivity difference between metals AND the electrolyte

Rank metals by reactivity from voltage data, attending to the sign (positive = more reactive)

Distinguish rechargeable from non-rechargeable cells

Fuel cells

State the overall reaction: 2H₂ + O₂ → 2H₂O

Describe the mechanism: H₂ molecules oxidised at anode; O₂ reduced at cathode; only product is water

Avoid the combustion / hydrogen-ions / steam-turbine traps

Evaluate fuel cells vs rechargeable batteries — both pros and cons

(HT only) Write the half equations: 2H₂ → 4H⁺ + 4e⁻ and O₂ + 4H⁺ + 4e⁻ → 2H₂O

Examiner-proof technique

"Hydrogen is oxidised (electrochemically) to produce water" — verbatim mark-scheme phrase

"The higher the positive voltage, the more reactive the metal" — verbatim ranking justification

For investigations: vary ONE factor only; keep the two metals constant

Module 5 complete — onwards to Module 6

Lesson 6.01 — Calculating Rates of Reactions [RP5 part 1]. We move from energy changes to rate of reaction. Mean rate vs instantaneous rate, drawing tangents to curves, and the single most-tested technique error in Paper 2 (drawing a vertical line instead of a tangent).