What is a homologous series? Find the difference in molecular mass between the two consecutive members of a homologous series. State how in a homologous series of carbon compounds the following properties vary with increase in molecular mass: (i) Melting and boiling points (ii) Chemical properties
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Model Answer
A homologous series is a series of carbon compounds that have the same functional group, the same general formula, and similar chemical properties, differing from each other by a –CH₂– unit.
Difference in molecular mass: Each successive member differs by a –CH₂– unit, so the difference in molecular mass = 12 + 2(1) = 14 u.
(i) Melting and boiling points: They increase with increasing molecular mass in a homologous series, showing a gradation in physical properties.
(ii) Chemical properties: They remain similar throughout the series, as chemical properties are determined solely by the functional group, not the length of the carbon chain.
Source: Chapter 4, Section 4.2.4 – Homologous Series
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Explanation
- Examiners expect the definition to mention: same functional group, differ by –CH₂–, similar chemical properties.
- The 14 u calculation (C = 12, 2×H = 2) must be shown or stated.
- Key contrast to remember: physical properties (MP/BP) increase; chemical properties remain the same — this distinction is a common exam point.