AQA A-Level Chemistry Paper 3, 2019: Question 32

1 mark · Easy difficulty · Multiple Choice

Identify a likely reason why the direct hydration of ethene has largely replaced the fermentation method for industrial ethanol production.

Practise this question

Question

Question 32 asks: 'In the UK industrial ethanol is now produced by the direct hydration of ethene. This process has largely replaced the fermentation method. Which is a likely reason for this change of method?' with four options: A: 'The direct hydration route produces purer ethanol.', B: 'The direct hydration route employs milder conditions.', C: 'The direct hydration route does NOT use a catalyst.', and D: 'The direct hydration route produces ethanol by a slower reaction.' Each option has a selection bubble next to it.
Question text

32 In the UK industrial ethanol is now produced by the direct hydration of ethene. This

process has largely replaced the fermentation method.

Which is a likely reason for this change of method?

[1 mark]

A The direct hydration route produces purer ethanol.

B The direct hydration route employs milder conditions.

C The direct hydration route does NOT use a catalyst.

D The direct hydration route produces ethanol by a slower reaction.

Mark scheme

Show the mark scheme Mark scheme table shows question 32 corresponds to answer A, with 1 mark awarded.

32 A 1

How to answer it

AQA A-Level Chemistry • Alcohols & Industrial Processes

Industrial Production of Ethanol: Hydration vs Fermentation

What this question tests

This question evaluates your knowledge of the two industrial routes to produce ethanol: catalytic hydration of ethene versus fermentation of glucose. You must be able to compare reaction conditions, catalysts, rates, atom economy, and the purity of the ethanol produced by each method.

Question 32 Analysis

Multiple Choice [1 Mark]

✅ Correct Option: A

"The direct hydration route produces purer ethanol."

  • Hydration of ethene yields a high-purity product (around 95–100% ethanol after standard condensation and minor purification).
  • Fermentation produces an aqueous mixture containing only about 15% ethanol (yeast dies at higher concentrations) along with organic impurities, requiring fractional distillation to concentrate.

💡 Comparison of Industrial Routes

Feature Fermentation Hydration of Ethene
Raw Material Glucose (renewable, plant-based) Crude oil / ethene (non-renewable)
Conditions Mild: 30–40 °C, atmospheric pressure Harsh: ~300 °C, 60–70 atm
Catalyst Yeast enzymes (zymase) Concentrated H₃PO₄ (phosphoric acid)
Process Type Batch (slow, high labour cost) Continuous (fast, low labour cost)
Purity Impure (~15% aq. solution) Very pure (essentially pure ethanol)

❌ Why Options B, C, and D are Incorrect

  • B is false: Direct hydration uses harsh conditions (300 °C, 60–70 atm), whereas fermentation uses mild conditions (35 °C, 1 atm).
  • C is false: Direct hydration definitely uses a catalyst — concentrated phosphoric acid (H₃PO₄) absorbed onto a solid silica support.
  • D is false: Direct hydration is a continuous, fast chemical process, whereas fermentation is a slow batch process that takes days.

🧠 Exam Technique & Examiner Commentary

Elimination Strategy:

  • Identify the chemistry first: write down the hydration equation: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH .
  • Notice it has 100% atom economy and produces no side-reaction liquid by-products, making product separation straightforward and yielding high purity.
  • Check off known industrial conditions: high pressure and temperature immediately rule out option B; knowing H₃PO₄ is used rules out option C; knowing it is a rapid continuous reaction rules out D.
Mark Scheme Insight: Only A gains the 1 mark. Students frequently confuse hydration conditions with fermentation conditions or forget that direct hydration requires a phosphoric acid catalyst.

Topics

Organic Chemistry · 3.3.5 Alcohols

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.