Ethanol is an alcohol that can be produced by the fermentation of glucose. Describe the fermentation process and explain why ethanol is classified as an alcohol.

WJEC A-Level Chemistry (Wales) — 4.3 Alcohols and phenols · Describe and Explain · 5 marks · View as Markdown

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

Ethanol is widely used as a fuel and as a solvent in industry. It can be manufactured by fermentation of glucose using yeast.

Model answer (5 marks)

Yeast is used as a catalyst in the fermentation of glucose. Enzymes in the yeast break down glucose into ethanol and carbon dioxide. The reaction occurs in the absence of oxygen, i.e. under anaerobic conditions. Ethanol is classified as an alcohol because it contains a hydroxyl –OH functional group that is bonded to a carbon atom in a hydrocarbon chain.

Examiner tips

  • Use the word "yeast" and mention enzymes as catalysts
  • State the products (ethanol + CO₂) and that it is anaerobic
  • Explain the –OH group bonded to carbon to justify the alcohol classification

Common mistakes

  • Forgetting to mention anaerobic conditions
  • Describing ethanol as a ketone or aldehyde instead of noting the –OH group
  • Using vague terms like "oxidation" instead of "enzymatic breakdown"

Mark scheme (5 marks)

  1. Yeast is used / enzymes in yeast act as a catalyst
  2. Glucose is broken down / converted to ethanol and carbon dioxide
  3. The process occurs in the absence of oxygen / anaerobic conditions
  4. Ethanol contains the hydroxyl / –OH functional group
  5. The –OH group is bonded to a carbon atom / hydrocarbon chain (making it an alcohol)

Key terms in this question

fermentation · ethanol · alcohol

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