A student claims that CH₃OH, C₂H₅OH and C₃H₇OH belong to a homologous series. (i) State two pieces of evidence from their formulae and properties that support this claim. (ii) Predict one physical property that will differ between methanol and propanol, and explain why.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
(i) Two pieces of evidence that CH₃OH, C₂H₅OH and C₃H₇OH form a homologous series:
- Same functional group: All three compounds contain the –OH (hydroxyl) functional group, giving them similar chemical properties.
- Successive members differ by –CH₂– unit: CH₃OH → C₂H₅OH → C₃H₇OH; each successive compound differs by one –CH₂– unit (molecular mass difference = 14 u), giving the general formula CₙH₂ₙ₊₁OH.
(ii) Physical property that differs — Boiling point:
Propanol (C₃H₇OH) has a higher boiling point than methanol (CH₃OH). As molecular mass increases in a homologous series, intermolecular forces increase, so more heat energy is needed to separate the molecules. Since propanol has a greater molecular mass (60 u) than methanol (32 u), its boiling point is higher.
Source: Chapter 4, Section 4.2.4 — Homologous Series
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Explanation
- Part (i) is worth ~2 marks: examiners expect same functional group AND –CH₂– difference — give both clearly.
- Part (ii) is worth ~3 marks: name the property, state the comparative difference, and give the reason (increasing molecular mass → stronger intermolecular forces → higher boiling point). All three steps are needed for full credit.
- The textbook explicitly states: "As molecular mass increases in any homologous series, a gradation in physical properties is seen… melting and boiling points increase with increasing molecular mass." Quote or paraphrase this reasoning.
- Do not just say "boiling point differs" — always state which is higher and why.