Explain the observed trend in the boiling points of these...

Explain the observed trend in the boiling points of these compounds. $$ \begin{array}{ll}{\mathrm{H}_{2} \mathrm{Te}} & {-2^{\circ} \mathrm{C}} \\ {\mathrm{H}_{2} \mathrm{Se}} & {-41.5^{\circ} \mathrm{C}} \\ {\mathrm{H}_{2} \mathrm{S}} & {-60.7^{\circ} \mathrm{C}} \\ {\mathrm{H}_{2} \mathrm{O}} & {100^{\circ} \mathrm{C}}\end{array}$$

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions between molecules that determine how much energy is required to overcome these forces during a phase change such as boiling. In the context of small molecules, these forces include hydrogen bonding, dipole–dipole interactions, and London dispersion forces. Understanding these forces is essential to explain why similar compounds can have different boiling points; stronger intermolecular forces result in higher boiling points because more energy is needed to separate the molecules.

London Dispersion Forces

London dispersion forces are weak attractions that arise from temporary fluctuations in the electron distribution within molecules, leading to instantaneous dipoles. The magnitude of these forces tends to increase with the size and molar mass of the molecule. Thus, for a series of related hydrides, as the central atom becomes larger and has more electrons, the associated increase in polarizability results in stronger dispersion forces, which consequently raises the boiling point.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole–dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms such as oxygen, nitrogen, or fluorine. This interaction can cause an anomalously high boiling point compared to molecules of similar size by greatly increasing the energy needed to separate molecules. In a series of hydrides, when one of the compounds is capable of hydrogen bonding, its boiling point is markedly affected by this strong intermolecular attraction.

Polarizability

Polarizability is a measure of how easily the electron cloud around a molecule can be distorted into an instantaneous dipole. Larger atoms with more electrons are typically more polarizable, leading to stronger London dispersion forces. In a homologous series, the increased polarizability of heavier atoms contributes to a rising boiling point, as the induced dipoles create additional attractive forces between molecules.

Transcript

Please give Ace some feedback