Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Why do molecules with increasing molar mass show a rising trend in boiling points? The difference can be attributed to their intermolecular forces. 

All substances have dispersion forces — the weakest intermolecular force. The strength of dispersion forces depends on how easily the atoms can be polarized.

Atoms with higher masses have more electrons and larger electron clouds with electrons dispersing more freely, leading to increased dispersion forces. The stronger the forces, the more energy is required to break the interactions between neighboring atoms, resulting in higher boiling points.

This also explains why an increase in the molar mass in the alkane series translates to a corresponding increase in boiling points and melting points.

However, molar mass alone does not determine the strength of the dispersion forces. 

Despite having the same masses, n-pentane and neopentane have different boiling points as a result of their differing shapes. 

The straight n-pentane provides a larger surface area to interact with the neighboring molecules, while the spherical neopentane has a smaller surface area and consequently reduced dispersion forces. This leads to a lower boiling point.