AQA A-Level Chemistry Paper 2, June 2025: Question 4

✅ Correct Answer

Any one of the following valid hazard statements:

• N₂O is of unknown toxicity so should be treated as toxic / can cause respiratory problems.
• Gases or vapours released (N₂O, cyclohexanol, methanol, or concentrated HNO₃) are toxic, corrosive, harmful, or respiratory irritants.

❌ Common Errors

• Too vague: Stating simply that chemicals "are dangerous" or "smell bad" scores 0 marks.
• Wrong hazard: Calling concentrated nitric acid merely "flammable" instead of corrosive/oxidising.

✅ Correct Answer

Oxidising agent (or oxidant).

💡 Key Knowledge

Cyclohexanol (a secondary alcohol, C₆H₁₁OH) is converted into a dicarboxylic acid, HOOC(CH₂)₄COOH.

• This requires breaking a C–C bond and increasing the oxidation state of the carbon atoms from 0 to +3.
• Concentrated HNO₃ is reduced (N goes from +5 in HNO₃ to +1 in N₂O), confirming it acts as an oxidising agent.

❌ Common Errors

• Writing "catalyst" or "dehydrating agent" — concentrated HNO₃ is a stoichiometric reactant (consumed in a 2:1 ratio), not a catalyst.
• Writing "acid" or "proton donor" — while it is an acid, its functional chemical role in this conversion is oxidation.

🧠 Exam Technique & Keywords

• Always specify "minimum" volume: Too much hot solvent prevents crystals from precipitating upon cooling, drastically lowering yield.
• Distinguish the two filtration steps clearly:
  • Filtration 1 (Hot): Removes insoluble impurities. Must be gravity filtration so premature crystallisation does not block the funnel.
  • Filtration 2 (Cold): Collects the pure solid crystals, leaving soluble impurities in the cold filtrate. Done under reduced pressure for speed and dryness.

❌ Common Student Pitfalls

• Omitting the word "minimum" or "hot" when describing dissolving (loses M1).
• Confusing the filtration types (e.g. attempting Büchner filtration while hot, which causes rapid cooling and crystallization in the stem).
• Forgetting to state drying at the end (loses M5).

✅ Correct Answer

• Water at room temperature would dissolve some of the product / crystals (causing it to be lost in the filtrate).
• OR: Ice-cold water minimises / prevents the dissolution of the product.

💡 Key Principle

Hexanedioic acid has two polar –COOH groups, meaning it has moderate solubility in water. Solubility increases with temperature. Washing with room-temperature water washes pure crystals away into the suction flask, reducing the final mass obtained.

✅ Mistake 1 & Problem Caused

Mistake: Water is used as the heating liquid / bath.

Problem: Water boils at 100 °C, so it cannot reach the melting point of 152 °C (the sample will never melt).

Alternative: Mineral oil should be used instead of water.

✅ Mistake 2 & Problem Caused

Mistake: The sample is positioned significantly higher than the bulb of the thermometer (not level with it).

Problem: The thermometer will measure the temperature of a different region of liquid; the temperature recorded will be different / lower than the true temperature of the sample.

Alternative Mistake: No stirrer used / boiling tube used instead of Thiele tube → Problem: Uneven heat distribution / temperature variation.

✅ Justification: MORE Sustainable (Any 1)

• Glucose is a renewable resource / renewable feedstock (whereas cyclohexanol is derived from crude oil).
• Biological catalyst / E. coli bacteria is a renewable resource operating under mild biological conditions.
• Platinum (Pt) catalyst can be reused / recycled.
• Higher atom economy in Stage 2 (addition of H₂ has 100% atom economy).

✅ Justification: LESS Sustainable (Any 1)

• Synthesis requires a two-stage process (the original is single-stage).
• Hydrogen gas (H₂) is non-renewable (currently sourced from fossil fuels / steam reforming of methane).
• High pressure is required in Stage 2 (350 kPa), which consumes significant energy and requires specialised pressure vessels.
• Uses a Platinum (Pt) catalyst, a scarce transition metal that must be extracted/mined from the Earth.