AQA A-Level Chemistry Paper 3, June 2025: Question 24

📌 What This Question Tests

This question assesses your recall and understanding of organic synthesis from renewable resources (AQA Organic Chemistry: Alcohols):

Conditions for industrial fermentation: The biological requirement for anaerobic conditions (absence of oxygen) to produce ethanol rather than ethanoic acid or water and carbon dioxide.
Separation methods: Selecting the correct physical technique to separate a mixture of miscible liquids (ethanol and water) based on differing boiling points.

Question 24 • 1 Mark (AO1)

D — It is anaerobic and the ethanol is separated by fractional distillation.

Mark Scheme: Award 1 mark for option D.

Treat this two-part multiple-choice question systematically by splitting it into two independent choices:

Aerobic vs Anaerobic: Yeast ferments sugars anaerobically. Eliminate A and B immediately.
Filtration vs Fractional Distillation: Ethanol and water are miscible liquids with different boiling points. Filtration only separates insoluble solids from liquids. Eliminate C.

This leaves D as the only valid answer in under 15 seconds.

Option	Reaction Type	Separation Method	Status	Reason
A	Aerobic	Filtration	Incorrect	Both statements are false. Oxygen prevents fermentation; filtration cannot separate miscible liquids.
B	Aerobic	Fractional distillation	Incorrect	Distillation is correct, but fermentation requires anaerobic conditions.
C	Anaerobic	Filtration	Incorrect	Anaerobic is correct, but filtration only removes yeast cells, not the dissolved ethanol.
D	Anaerobic	Fractional distillation	Correct	Anaerobic ensures ethanol is produced; fractional distillation separates ethanol (b.p. 78 °C) from water (b.p. 100 °C).

Equation:
C₆H₁₂O₆ (aq) → 2C₂H₅OH (aq) + 2CO₂ (g)
Catalyst / Organism: Yeast (supplying the enzymes zymase).
Temperature: Approximately 30 °C to 40 °C (optimum for enzymes; too cold = reaction too slow, too hot = enzymes denature).
Anaerobic Condition: Oxygen must be excluded. If air is present:
- Yeast respires aerobically, producing CO₂ and H₂O instead of ethanol.
- Any ethanol formed would oxidise biologically to ethanoic acid (vinegar).
Distillation: Fermentation stops around 14–15% v/v ethanol because higher alcohol concentrations denature yeast enzymes. Fractional distillation exploits the difference in boiling points between ethanol (78.3 °C) and water (100 °C) to achieve a higher concentration.

Confusing Filtration with Separation of Ethanol: While a reaction mixture may be filtered to remove yeast sediment, filtration does not isolate ethanol from the aqueous solution because both ethanol and water pass straight through filter paper.
Confusing Fermentation with Hydration: Industrial ethanol is also produced via the direct hydration of ethene ( C₂H₄ + H₂O ⇌ C₂H₅OH ) using a concentrated H₃PO₄ catalyst at 300 °C and 60–70 atm. Do not mix up the conditions of these two pathways.
Forgetting Why Oxygen is Excluded: In extended-response questions, examiners frequently ask why fermentation must be anaerobic. Always state both that respiration switches away from ethanol production and that ethanol oxidises to ethanoic acid.

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