What is the relationship between boiling points and molecular size in a homologous series?

In a homologous series, the boiling point generally increases with increasing molecular size.

In a homologous series, such as the alkanes or alcohols, each successive member differs by a -CH2- group. This means that as you move down the series, each member has one more carbon and two more hydrogen atoms than the previous one, leading to an increase in molecular size. This increase in molecular size leads to an increase in the boiling point of the substance.

The reason for this is due to the type of intermolecular forces present in these substances. In a homologous series, the primary intermolecular force is the van der Waals force, also known as London dispersion forces. These forces are temporary attractive forces that occur when electrons in the molecule form temporary dipoles. The strength of these forces increases with the size and shape of the molecule. Therefore, as the molecular size increases, so does the strength of the van der Waals forces, leading to a higher boiling point.

Furthermore, the increase in molecular size also leads to an increase in the surface area of the molecule. This increase in surface area allows for more points of contact between molecules, which also increases the strength of the van der Waals forces. This is another reason why larger molecules have higher boiling points.

However, it's important to note that this trend is not absolute. Other factors, such as the shape of the molecule and the presence of other types of intermolecular forces, can also affect the boiling point. For example, molecules with a more branched structure may have lower boiling points than their straight-chain counterparts, despite having the same molecular size. Similarly, molecules with polar groups or hydrogen bonding capabilities may have higher boiling points than expected based on their size alone.