Ethanol is a member of the alcohol homologous series. Explain what is meant by a homologous series and describe the chemical properties of ethanol, including its reactions with sodium and with oxygen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A homologous series is a family of compounds that share the same general formula and differ by a constant unit. For alcohols the general formula is CₙH₂ₙO and each successive member differs by one CH₂ group.
Ethanol (C₂H₅OH) is the second member of this series. It is a clear, colourless liquid that is miscible with water and has a characteristic alcoholic smell.
1. Reaction with sodium: 2 C₂H₅OH + 2 Na → 2 C₂H₅ONa + H₂↑. The alcohol donates a proton to sodium, forming sodium ethoxide and liberating hydrogen gas.
2. Combustion in oxygen: 2 C₂H₅OH + 6 O₂ → 4 CO₂ + 6 H₂O. Ethanol burns with a blue flame, producing carbon dioxide and water as the only products.
These reactions illustrate the typical behaviour of alcohols in a homologous series.
Ethanol (C₂H₅OH) is the second member of this series. It is a clear, colourless liquid that is miscible with water and has a characteristic alcoholic smell.
1. Reaction with sodium: 2 C₂H₅OH + 2 Na → 2 C₂H₅ONa + H₂↑. The alcohol donates a proton to sodium, forming sodium ethoxide and liberating hydrogen gas.
2. Combustion in oxygen: 2 C₂H₅OH + 6 O₂ → 4 CO₂ + 6 H₂O. Ethanol burns with a blue flame, producing carbon dioxide and water as the only products.
These reactions illustrate the typical behaviour of alcohols in a homologous series.
Examiner tips
- Define the series and the CH₂ increment first. Show the balanced equations with products. Mention the colourless, miscible nature of ethanol. Use the exact wording "hydrogen gas" and "complete combustion".
Common mistakes
- Writing the reaction with sodium as a simple substitution without forming sodium ethoxide. Forgetting to balance the combustion equation. Describing ethanol as a gas instead of a liquid.
Mark scheme (5 marks)
- A homologous series is a family/group of compounds with the same general formula
- Each successive member differs by CH₂ (one carbon and two hydrogen atoms)
- Ethanol reacts with sodium to produce hydrogen gas
- Ethanol burns in oxygen / undergoes combustion
- Complete combustion of ethanol produces carbon dioxide and water
Key terms in this question
homologous series · ethanol · sodium
Related
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