What determines the physical properties of members of a homologous series?

The physical properties of members of a homologous series are determined by the size and structure of their molecules.

In a homologous series, each member differs from the next by a constant unit, often a -CH2- group. This means that as you progress along the series, the size of the molecules increases. This increase in size affects the physical properties of the compounds. For instance, the boiling and melting points of the compounds increase as the size of the molecules increases. This is because larger molecules have more electrons, leading to stronger London forces (a type of van der Waals force) between the molecules. These forces need more energy to overcome, hence the higher boiling and melting points.

The structure of the molecules also plays a role in determining the physical properties. For example, in the case of alkanes (a homologous series of hydrocarbons), the shape of the molecule can affect the strength of the intermolecular forces. Alkanes can either be straight-chain or branched. Branched alkanes have lower boiling points than straight-chain alkanes with the same number of carbon atoms. This is because branched molecules cannot pack together as closely as straight-chain molecules, reducing the strength of the intermolecular forces.

In addition, the polarity of the molecules in a homologous series can affect their physical properties. For example, alcohols are polar due to the presence of an -OH group, and this polarity leads to stronger intermolecular forces (specifically hydrogen bonding) compared to non-polar molecules of similar size. This results in higher boiling points for alcohols compared to similar-sized hydrocarbons.

In summary, the physical properties of members of a homologous series are determined by the size and structure of their molecules, with larger and more polar molecules generally having higher boiling and melting points.