Ethanol is widely used as a solvent and can also be used as a fuel. Describe the complete combustion of ethanol and explain why ethanol produced by fermentation can be considered carbon neutral, whilst ethanol produced by hydration of ethene cannot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Complete combustion:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
The carbon and hydrogen atoms in ethanol are oxidised to CO₂ and H₂O.
Fermentation:
Glucose (C₆H₁₂O₆) → 2C₂H₅OH + 2CO₂
Plants absorb CO₂ from the atmosphere during photosynthesis to produce glucose. The CO₂ released when the ethanol is later combusted is therefore balanced by the CO₂ that was originally taken up by the plants, giving a net zero carbon balance – carbon neutral.
Hydration of ethene:
C₂H₄ + H₂O → C₂H₅OH
Ethene is obtained from cracking crude oil, a fossil fuel. The CO₂ released on combustion of this ethanol is not offset by any CO₂ uptake, so it is not carbon neutral.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
The carbon and hydrogen atoms in ethanol are oxidised to CO₂ and H₂O.
Fermentation:
Glucose (C₆H₁₂O₆) → 2C₂H₅OH + 2CO₂
Plants absorb CO₂ from the atmosphere during photosynthesis to produce glucose. The CO₂ released when the ethanol is later combusted is therefore balanced by the CO₂ that was originally taken up by the plants, giving a net zero carbon balance – carbon neutral.
Hydration of ethene:
C₂H₄ + H₂O → C₂H₅OH
Ethene is obtained from cracking crude oil, a fossil fuel. The CO₂ released on combustion of this ethanol is not offset by any CO₂ uptake, so it is not carbon neutral.
Examiner tips
- Write the balanced combustion equation first; include all reactants and products. Explain the oxidation of C and H. Mention photosynthesis for fermentation. Contrast with fossil‑fuel source for ethene hydration.
Common mistakes
- Omitting the balanced equation or missing the 3O₂ coefficient. Failing to link plant photosynthesis to CO₂ uptake. Confusing the source of ethene as renewable instead of fossil fuel.
Mark scheme (5 marks)
- Complete combustion of ethanol produces carbon dioxide and water (only)
- The carbon and hydrogen in ethanol are oxidised during combustion
- Fermentation uses plants/sugar/glucose as a feedstock, which absorb carbon dioxide from the atmosphere as they grow (via photosynthesis)
- So the CO₂ released when ethanol (from fermentation) burns is balanced/offset by the CO₂ absorbed during plant growth, giving an overall neutral carbon balance
- Ethanol from hydration of ethene uses ethene from cracking crude oil, which is a finite/non-renewable fossil fuel source, so the CO₂ released on combustion is not offset by any uptake
Key terms in this question
complete combustion · fermentation · carbon neutral · hydration
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
More Alcohols questions
- Alcohols form a homologous series. Describe four features of alcohols as a homol…
- Ethanol can be used as a fuel and as a solvent. Describe how ethanol is produced…
- Ethanol can be oxidised to form ethanoic acid. Compare the properties and uses o…
- Ethanol can be produced by fermentation or by hydration of ethene. Describe the …