Propan-1-ol undergoes combustion when it is burned in air. Describe what is observed when propan-1-ol burns in a plentiful supply of air, and explain why propan-1-ol is classified as a member of the alcohol homologous series rather than as an alkane, even though both contain only carbon and hydrogen atoms in addition to any oxygen.

Eduqas A-Level Chemistry — 4.3 Alcohols and phenols · Describe · 5 marks · View as Markdown

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

Propan-1-ol has the molecular formula C₃H₇OH and is a flammable liquid at room temperature.

Model answer (5 marks)

Propan‑1‑ol burns with a relatively clean, blue flame, indicating complete combustion.

The combustion reaction produces mainly carbon dioxide and water:
C₃H₇OH + 4 O₂ → 3 CO₂ + 4 H₂O.

Propan‑1‑ol is an alcohol because it contains a hydroxyl (–OH) functional group. All members of the alcohol homologous series share this –OH group and the general formula CₙH₂ₙ₊₂O. Alkanes, by contrast, contain only C–H and C–C bonds and have the general formula CₙH₂ₙ₊₂. Since propan‑1‑ol possesses an –OH group, it belongs to the alcohol series rather than the alkane series.

Examiner tips

  • Mention the blue flame and complete combustion first; this shows you understand the observation. Explain the products with the balanced equation to gain the product marks. State the –OH group and the general formula of the alcohol series to link structure to classification. Contrast with alkanes by noting the absence of –OH in their general formula.

Common mistakes

  • Writing a yellow or soot‑producing flame, which suggests incomplete combustion. Forgetting to balance the combustion equation or omitting the water product. Claiming propan‑1‑ol is an alkane because it contains only C, H and O, ignoring the functional‑group criterion.

Mark scheme (5 marks)

  1. Propan-1-ol burns with a (relatively) clean/blue flame
  2. Carbon dioxide and water are produced as combustion products
  3. Propan-1-ol contains an –OH (hydroxyl) functional group / alcohol functional group
  4. Members of a homologous series share the same functional group / general formula
  5. Alkanes do not contain an –OH group / only contain C–H and C–C bonds, so propan-1-ol cannot be an alkane

Key terms in this question

homologous series · combustion · alcohol

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