Homologous Series (Edexcel GCSE Combined Science: Chemistry): Revision Note

Homologous Series

• Homologous series are families or groups of organic compounds that have similar features and chemical properties due to them having the same functional group

• All members of a homologous series have:

  • The same general formula

  • The difference in the molecular formula between one member and the next is CH₂

  • Gradation in their physical properties

  • Same functional group

  • Similar chemical properties

• Gradation in the physical properties of a homologous series can be seen in the trend in boiling points of the alkanes

Boiling Point

A graph of the boiling points of the first eight alkanes showing a gradually increasing trend

• Each alkane has a boiling point that is higher than the one before it

• As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there are more electrons in the molecules and greater surface area contact between them

• This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point