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3.2.3Additional lessons

Homologous Series — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 60 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Features of a Series

3.2.3 Homologous Series
Same functional group
  • Every member has the same functional group, which controls the characteristic chemical reactions of the series.
General formula
  • All members fit one general formula, such as CₙH₂ₙ₊₂ for alkanes and CₙH₂ₙ for alkenes.
CH₂ difference
  • Neighbouring members differ by one CH₂ unit, which adds 14 to the relative molecular mass.
Trend in properties
  • Physical properties such as boiling point change gradually along the series as the chain lengthens.
Similar chemistry
  • Members react in similar ways because the functional group is the reactive site, although rates may vary.
SeriesGeneral formulaFunctional group
AlkaneCₙH₂ₙ₊₂C–C single bonds
AlkeneCₙH₂ₙC=C
AlcoholCₙH₂ₙ₊₁OH–OH
Carboxylic acidCₙH₂ₙ₊₁COOH–COOH

Part 2 of 5Why Boiling Point Rises

1
Longer chains have more electrons, so the instantaneous dipoles that cause London dispersion forces are larger.
2
A longer molecule has a greater surface area of contact with its neighbours.
3
Stronger intermolecular forces need more energy to overcome, so boiling point rises with chain length.
4
Alcohols have higher boiling points than alkanes of similar mass because hydrogen bonds act in addition to dispersion forces.
5
Solubility in water falls with chain length, as the non-polar chain grows larger relative to the polar group.

Part 3 of 5Common Series

Common Series
Halogenoalkanes
  • Contain a carbon-halogen bond and are named with a chloro, bromo or iodo prefix.
Aldehydes and ketones
  • Both contain C=O, but aldehydes have it at the end of the chain and ketones within the chain.
Esters
  • Contain –COO– and are formed from an alcohol and a carboxylic acid.
Amines and amides
  • Amines contain –NH₂ on a carbon chain, while amides contain –CONH₂.

Part 4 of 5Worked Example

Question Deduce the molecular formula and relative molecular mass of the fifth member of the alkene series.

  1. The fifth alkene has n = 5 carbon atoms.
  2. Formula CₙH₂ₙ gives C₅H₁₀.
  3. Mᵣ = 5 × 12.01 + 10 × 1.008 = 60.05 + 10.08.
  4. Mᵣ = 70.13.

Answer C₅H₁₀ (pentene), Mᵣ = 70.13

Part 5 of 5Exam Checklist & Tips

3.2.3 – Key Points for the Exam

  • Homologous series A family of compounds with the same functional group and general formula, differing by CH₂.
  • General formula An algebraic formula that gives the molecular formula of any member of a series.
  • Functional group The atom or group that determines the characteristic reactions.
  • Dispersion forces Weak forces between temporary dipoles in molecules.

Common Mistakes

  • Saying members of a series have identical physical properties.
  • Describing the difference between members as a CH₃ group instead of CH₂.
  • Forgetting that the general formula for cycloalkanes differs from that of alkanes.
  • Claiming that boiling point rises because covalent bonds break on boiling.

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