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. 

Intermolecular forces are also crucial when mixing liquids. Liquids of similar type and magnitude of intermolecular forces are entirely soluble in all proportions, or miscible.

Consider ethanol and water, both of which exhibit hydrogen bonding interactions. Upon mixing, ethanol molecules can form hydrogen bonds with water molecules, and the two liquids are completely miscible.

In contrast, liquids with different types and magnitudes of intermolecular forces are insoluble or immiscible. Consider the polar water molecules with dispersion and hydrogen bonding forces and the nonpolar hexane molecules with only dispersion forces.

On mixing, hexane forms the top layer and water forms the bottom layer. The weaker dispersion forces in hexane cannot compete with the stronger hydrogen bonds in water. Hence, hexane and water are immiscible