Ethanol can be manufactured by the fermentation of sugar solutions using yeast. Describe what happens during fermentation and explain why ethanol produced this way is considered a more sustainable fuel than petrol.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Fermentation is an anaerobic chemical reaction carried out by yeast in which glucose (C₆H₁₂O₆) is broken down into ethanol (C₂H₅OH) and carbon dioxide (CO₂). The overall equation is C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂.
Ethanol produced in this way is considered a more sustainable fuel than petrol because it is derived from renewable biological sources such as sugar or plant material, whereas petrol comes from finite crude oil. Furthermore, the CO₂ released when ethanol is burnt was originally taken up by the plants during photosynthesis, making the fuel approximately carbon‑neutral and not adding new CO₂ to the atmosphere.
Ethanol produced in this way is considered a more sustainable fuel than petrol because it is derived from renewable biological sources such as sugar or plant material, whereas petrol comes from finite crude oil. Furthermore, the CO₂ released when ethanol is burnt was originally taken up by the plants during photosynthesis, making the fuel approximately carbon‑neutral and not adding new CO₂ to the atmosphere.
Examiner tips
- Use the exact reaction equation to show understanding of fermentation.
- Mention both the renewable source and the carbon‑neutral aspect to cover all marks.
Common mistakes
- Forgetting to state that fermentation is anaerobic.
- Confusing ethanol with other alcohols or omitting the CO₂ by‑product.
Mark scheme (4 marks)
- Fermentation is a chemical process in which glucose/sugar is broken down anaerobically (without oxygen) by yeast
- Fermentation produces ethanol (and carbon dioxide)
- Ethanol is produced from a renewable/biological source (sugar/plants) whereas petrol is derived from crude oil, which is a finite/non-renewable resource
- The carbon dioxide released when ethanol burns was previously absorbed by the plants during photosynthesis, so it is (approximately) carbon neutral / does not add new carbon dioxide to the atmosphere
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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