Which member of each of the following pairs of compounds has...

Which member of each of the following pairs of compounds has the higher boiling point?
(a) $\mathrm{O}_{2}$ or $\mathrm{N}_{2}$
(b) $\mathrm{SO}_{2}$ or $\mathrm{CO}_{2}$
(c) HF or HI
(d) $\mathrm{SiH}_{4}$ or $\mathrm{GeH}_{4}$

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions that occur between molecules. They include dispersion forces, dipole-dipole interactions, and hydrogen bonding. The strength of these forces largely determines the boiling point of a substance since more energy is required to separate molecules with strong attractions.

London Dispersion Forces

London dispersion forces are a type of van der Waals force that exists between all molecules, regardless of polarity. They are more significant in larger and more polarizable molecules, which tend to have higher boiling points due to increased electron cloud distortion, leading to stronger temporary dipoles.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative elements such as fluorine, oxygen, or nitrogen. This leads to significantly stronger intermolecular attractions, which in turn increases the boiling point of the substance.

Molecular Weight and Polarizability

Molecular weight and polarizability are key factors influencing the strength of London dispersion forces. Generally, heavier molecules with larger electron clouds are more polarizable, leading to stronger dispersion forces and higher boiling points, even if the molecules are non-polar.

Molecular Polarity

Molecular polarity refers to the distribution of electrical charge over the atoms in a molecule. Polarity arises from differences in electronegativity between atoms and the geometrical arrangement of these atoms. Polar molecules have dipole moments that lead to dipole-dipole interactions, which generally result in higher boiling points compared to non-polar molecules of similar size.

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