Ethanol can be produced by fermentation. Describe the process of fermentation and explain why the ethanol produced this way is considered a renewable source of fuel.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. A glucose solution is mixed with yeast.
2. The yeast acts as a biological catalyst and the reaction takes place anaerobically.
3. Glucose is converted to ethanol and carbon dioxide.
4. The ethanol can be used as fuel.
5. Because the glucose comes from plants that absorb CO₂ from the atmosphere, the CO₂ released when the ethanol is burnt was recently fixed, making the fuel renewable.
2. The yeast acts as a biological catalyst and the reaction takes place anaerobically.
3. Glucose is converted to ethanol and carbon dioxide.
4. The ethanol can be used as fuel.
5. Because the glucose comes from plants that absorb CO₂ from the atmosphere, the CO₂ released when the ethanol is burnt was recently fixed, making the fuel renewable.
Examiner tips
- Use the word "yeast" and "biological catalyst"; mention anaerobic conditions; list both products; link plant‑derived glucose to CO₂ fixation; keep answer concise to fit 5 marks.
Common mistakes
- Forgetting to state anaerobic conditions; confusing yeast as a catalyst with a reactant; not linking the CO₂ cycle to renewability; omitting that glucose is plant‑derived.
Mark scheme (5 marks)
- Sugar (glucose) solution is broken down / converted
- Yeast is used / yeast acts as a catalyst / biological catalyst
- The process occurs anaerobically / in the absence of oxygen
- Carbon dioxide is also produced / ethanol and carbon dioxide are the products
- The sugar / glucose comes from plants which absorb carbon dioxide from the atmosphere, so the carbon dioxide released on combustion was recently removed from the atmosphere / the fuel is renewable because plants can be regrown
Key terms in this question
fermentation · ethanol · renewable
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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