Consider the homologous series of alkanes and the homologous series of alcohols. (i) Boiling points increase steadily as you move from methane to butane, and from methanol to butanol. Explain why this gradation in boiling points occurs, and why the chemical properties within each series remain similar despite this physical change. (ii) A student claims that ethane and ethanol belong to the same homologous series because they differ by only one atom. Identify the error in the student's reasoning and explain what conditions must be satisfied for two compounds to belong to the same homologous series.
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
(i) As we move from methane to butane (or methanol to butanol), each successive compound has one extra –CH₂– unit, increasing molecular mass. Greater molecular mass means stronger intermolecular forces, so more energy (higher boiling point) is needed to separate molecules. Chemical properties remain similar because they are determined solely by the functional group (e.g., –OH in alcohols), which stays the same throughout the series.
(ii) The student's error is that ethane (C₂H₆) and ethanol (C₂H₅OH) do not differ merely by one atom — they belong to different homologous series (alkanes and alcohols respectively) with different functional groups. For two compounds to belong to the same homologous series, they must: (a) have the same functional group, (b) differ by a –CH₂– unit, and (c) follow the same general formula.
Source: Carbon and its Compounds, Section 4.2.4 Homologous Series
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Explanation
- Examiners look for: molecular mass → intermolecular forces → boiling point (for part i), and functional group → same chemical properties.
- For part (ii), clearly name the error (different functional groups / different series) and then state the three conditions of a homologous series. Don't just say "they differ by one atom is wrong" — explain why and state the correct conditions.
- Key phrase to use: "chemical properties are determined solely by the functional group."