Within each group, assign each of the boiling points to the...

Within each group, assign each of the boiling points to the respective substances on the basis of intermolecular forces. (a) $\mathrm{N}_{2}, \mathrm{HCN}, \mathrm{C}_{2} \mathrm{H}_{6}:-196^{\circ} \mathrm{C},-89^{\circ} \mathrm{C}, 26^{\circ} \mathrm{C} ;$ (b) $\mathrm{H}_{2}, \mathrm{HCl}, \mathrm{Cl}_{2}:-35^{\circ} \mathrm{C},-259^{\circ} \mathrm{C},-85^{\circ} \mathrm{C} .$

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions between molecules that dictate many physical properties of substances, including boiling points and melting points. Their types and relative strengths determine how much energy is needed for molecules to separate during phase changes.

London Dispersion Forces

London dispersion forces are weak, temporary attractions that occur in all molecules due to fluctuating electron distributions. They become more significant in larger, more polarizable molecules and affect the physical properties of nonpolar substances.

Dipole-Dipole Interactions

Dipole-dipole interactions take place between molecules that have permanent dipoles. The partial positive charge on one molecule attracts the partial negative charge on another, leading to moderately strong attractions that influence properties like boiling points.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative atoms such as oxygen, nitrogen, or fluorine. This interaction significantly increases the boiling and melting points of substances by creating strong attractions between molecules.

Boiling Point Correlation with Intermolecular Forces

The boiling point of a substance is an indirect measure of the strength of its intermolecular forces. Stronger forces require more energy to overcome, resulting in higher boiling points, while weaker forces are associated with lower boiling points.

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