The characteristics of a homologous series, a series of compounds…

The idea

A homologous series is a family of compounds that share one functional group, so its members share chemical properties. Four characteristics are worth learning. They fit one general formula, such as CₙH₂ₙ₊₂ for alkanes or CₙH₂ₙ₊₁OH for alcohols. Successive members differ by CH₂, so relative molecular masses rise in steps of 14. They react in the same way because the functional group is the same, so propan-1-ol behaves like butan-1-ol. And their physical properties change gradually along the series: boiling point rises as the chain lengthens, because a longer chain has more electrons and a greater surface contact, giving stronger London forces that need more energy to overcome.

What earns the marks

• The same functional group in every member of the series.
• The same general formula, such as CₙH₂ₙ₊₂ for alkanes.
• Successive members differing by CH₂, or by 14 in relative molecular mass.
• A gradual trend in a physical property explained by increasing London forces.

Common mistakes and how to put them right

• Saying members differ by CH₃ rather than by CH₂.

• Claiming all members have identical physical properties instead of a gradual trend.

• Explaining the boiling point trend by bigger molecules without naming London forces.

• Giving an example series whose members do not actually share a single functional group.

Exam tips

• Give the general formula in terms of n and check it against a member you already know.
• Name the intermolecular force explicitly when explaining a boiling point trend.
• Keep chemical similarity and physical gradation as two separate points.