AQA A-Level Chemistry Paper 1, 2023: Question 8

✅ Correct Answer

State symbols are not strictly required unless requested, but equations must be fully balanced for atoms and charges.

💡 Key Knowledge

• The positive electrode is the cathode in an electrochemical cell, where reduction (gain of electrons) takes place.
• In acidic fuel cells, oxygen gas is reduced by reacting with hydrogen ions (H⁺) to produce water.
• Oxygen's oxidation state decreases from 0 in O₂ to -2 in H₂O.

🧠 Exam Technique

The question explicitly tells you the reactants and products: "oxygen reacts with hydrogen ions to form water". Start with O₂ + H⁺ → H₂O , balance oxygen atoms first by doubling H₂O, then balance H with 4H⁺, and finally add 4e⁻ to balance charges.

❌ Common Errors

• Writing the alkaline half-equation ( O₂ + 2H₂O + 4e⁻ → 4OH⁻ ) instead of the acidic one, ignoring the prompt that states H⁺ ions are involved.
• Incorrect charge balancing, such as writing 2e⁻ instead of 4e⁻ .

✅ Correct Answer

💡 Key Knowledge

• The negative electrode is the anode, where oxidation (loss of electrons) occurs.
• Methanol is oxidised to carbon dioxide. The carbon changes oxidation state from -2 in CH₃OH to +4 in CO₂ (a loss of 6 electrons).
• Water provides the extra oxygen atom needed to convert the alcohol group into carbon dioxide.

🧠 Exam Technique

1. Write reactants and products given in the prompt: CH₃OH + H₂O → CO₂ + H⁺ .
2. Check C and O atoms: 1 C and 2 O on both sides (already balanced).
3. Count hydrogens: Left has 4 + 2 = 6 H. Therefore, produce 6H⁺ on the right.
4. Balance charge: Left has 0 charge, right has +6, so add 6e⁻ to the right.

❌ Common Errors

• Forgetting to include H₂O as a reactant, making it impossible to balance the oxygen atoms.
• Writing electrons on the wrong side (reduction instead of oxidation).
• Miscounting hydrogen atoms in methanol ( CH₃OH has 4 H atoms, not 3).

📐 Step-by-Step Calculation

1. Identify given data:
   Overall cell EMF = +1.20 V
   E°(negative electrode, CO₂ / CH₃OH) = +0.03 V
2. State the electrochemical relationship:
   EMF = E°(positive electrode) − E°(negative electrode)
3. Rearrange to solve for E°(positive electrode):
   E°(O₂ / H₂O) = EMF + E°(CO₂ / CH₃OH)
4. Substitute numerical values:
   E°(O₂ / H₂O) = 1.20 + 0.03 = +1.23 V

✅ Correct Answer

Always include a positive sign or double check that the value aligns with standard electrode potential tables (the standard potential for oxygen reduction in acid is known to be +1.23 V).

❌ Calculation Traps

• Subtraction error: Subtracting incorrectly: 1.20 − 0.03 = 1.17 V . Remember that the positive electrode must have the more positive potential!
• Sign confusion: Assigning a negative sign ( -1.23 V ) — spontaneous cells must result in a positive overall EMF.

🧠 Sanity Check

Top students recognise standard data from memory: the O₂ / H₂O reduction potential in acidic media is a textbook constant ( +1.23 V ). Knowing this helps immediately verify your arithmetic.

✅ Correct Answer

Any one of the following:

• Reactants are supplied continuously.
• Fuel is continuously supplied.
• Continuous supply of chemicals/reactants.

💡 Key Knowledge

• In conventional secondary (rechargeable) batteries, reactants are stored inside a closed system and are depleted until an external electrical current reverses the chemical reaction.
• In a fuel cell, the system is open: fuel and oxidant flow in continuously from an external reservoir, generating current as long as reactants are fed in.

❌ Common Errors

• Vague statements like "it produces electricity all the time" or "the reaction is spontaneous" without referencing the continuous supply of fuel/reactants.
• Saying "it runs on fuel, not electricity" without explaining that fuel is added continuously.

🧠 Exam Technique

The word "continuously" or "constant supply" is the essential examiner trigger word for AQA fuel cell questions.

✅ Correct Answer

Any one valid point:

• Methanol is a liquid, so it is easier to store / transport (than gaseous hydrogen).
• More energy can be produced from 1 cm³ of methanol (liquid) than from 1 cm³ of hydrogen (gas) / higher energy density per unit volume.
• Existing petrol station infrastructure can be used.

💡 Key Knowledge

• Hydrogen is a low-density gas requiring either high-pressure tanks (hundreds of atmospheres) or cryogenic liquification (-253 °C).
• Methanol ( CH₃OH ) is a liquid at ambient temperature and pressure, dramatically reducing storage volume and engineering complexity in vehicles.

❌ Disallowed Answers (Mark Scheme Restrictions)

• DO NOT write "Methanol produces no emissions / no greenhouse gases" — Methanol combustion/oxidation produces CO₂!
• IGNORE generic references to "safety" or "cost" without precise explanation (hydrogen is explosive, but methanol is toxic and flammable too).

🧠 Top-Grade Tip

Always relate physical state to practical utility: state that methanol is a liquid, which makes storage and transport straightforward compared to high-pressure gas cylinders.