A chemical company produces ethanol for use in hand sanitisers. The company must decide whether to produce ethanol by fermenting sugar cane or by reacting ethene with steam. Evaluate the two methods of producing ethanol from the perspective of a chemical company.

OCR A-Level Chemistry B: Salters (H433) — CI The chemical industry · Evaluate · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Ethanol can be produced by two routes: fermentation of glucose (from sugar cane) using yeast, or by the reaction of ethene with steam using a catalyst. Fermentation operates at around 37 °C and produces a dilute solution of ethanol that must be purified. The ethene–steam reaction operates at high temperature and pressure and produces ethanol continuously at high purity.

Model answer (5 marks)

1. Fermentation uses a renewable raw material (sugar cane/glucose) whereas the ethene–steam route uses a finite, non‑renewable raw material (crude oil/ethene from cracking).
2. The ethene–steam process is continuous, giving a higher rate of production and greater yield per unit time compared with the batch process of fermentation.
3. The ethene–steam process requires high temperatures and pressures, increasing energy costs, whereas fermentation operates at low temperature (≈37 °C), so energy costs are lower for fermentation.
4. The ethene–steam route produces ethanol of high purity directly, whereas fermentation produces a dilute solution that requires additional purification (e.g. distillation), adding cost and time.
5. Overall, for a large chemical company the ethene–steam route may be preferred for high‑volume, high‑purity output, but fermentation is more sustainable long‑term as fossil fuels become scarcer and renewable feedstock is valued.

Examiner tips

  • Use the exact terminology from the mark scheme (renewable, non‑renewable, continuous, batch, high purity, energy costs).
  • Show a clear comparison and finish with a justified overall judgement.
  • Keep each point brief but complete – 1–2 sentences per mark.

Common mistakes

  • Confusing renewable with non‑renewable or vice versa; not mentioning the continuous vs batch nature; failing to note the energy cost difference; omitting the purity/purification point; giving a vague or unsupported overall judgement.

Mark scheme (5 marks)

  1. Fermentation uses a renewable raw material (sugar cane / glucose) whereas the ethene–steam route uses a finite/non-renewable raw material (crude oil / ethene from cracking)
  2. The ethene–steam process is continuous, giving a higher rate of production / greater yield per unit time compared with the batch process of fermentation
  3. The ethene–steam process requires high temperatures and pressures, increasing energy costs, whereas fermentation operates at low temperature (around 37 °C), so energy costs are lower for fermentation
  4. The ethene–steam route produces ethanol of high purity directly, whereas fermentation produces a dilute solution requiring additional purification (e.g. distillation), adding cost/time
  5. Overall judgement with reasoning — e.g. for a large chemical company the ethene–steam route may be preferred for high-volume, high-purity output, but fermentation is more sustainable long-term as fossil fuels become scarcer / or any valid justified conclusion

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