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

Explain the observed trend in the boiling points of the compounds listed. Why is $\mathrm{H}_{2} \mathrm{O}$ atypical? $$\begin{array}{ll}\hline \text { Compound } & \text { Boiling Point } \\\hline \mathrm{H}_{2} \mathrm{Te} & -2{ }^{\circ} \mathrm{C} \\ \mathrm{H}_{2} \mathrm{Se} & -41.5^{\circ} \mathrm{C} \\\mathrm{H}_{2} \mathrm{~S} & -60.7{ }^{\circ} \mathrm{C} \\\mathrm{H}_{2} \mathrm{O} & +100{ }^{\circ} \mathrm{C} \\\hline\end{array}$$

Explain the observed trend in the boiling points of the compounds listed. Why is $\mathrm{H}_{2} \mathrm{O}$ atypical?
$$\begin{array}{ll}\hline \text { Compound } & \text { Boiling Point } \\\hline \mathrm{H}_{2} \mathrm{Te} & -2{ }^{\circ} \mathrm{C} \\
\mathrm{H}_{2} \mathrm{Se} & -41.5^{\circ} \mathrm{C} \\\mathrm{H}_{2} \mathrm{~S} & -60.7{ }^{\circ} \mathrm{C} \\\mathrm{H}_{2} \mathrm{O} & +100{ }^{\circ} \mathrm{C} \\\hline\end{array}$$

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Key Concepts

Intermolecular Forces

The boiling point of a substance is governed by the strength of the forces between its molecules. In the context of these compounds, dispersion (van der Waals) forces play a significant role. As molecular size and polarizability increase (with heavier atoms), the dispersion forces become stronger, which typically leads to higher boiling points.

Dispersion Forces

For the chalcogen hydrides containing heavier central atoms like tellurium, the electrons are more easily distorted (higher polarizability), resulting in stronger London dispersion forces. This explains why, despite similar molecular structures, the boiling points vary with the atomic weight of the central atom in the series.

Hydrogen Bonding

Water is atypical because unlike the other compounds in the series, it is capable of extensive hydrogen bonding. The strong dipole and the ability of water molecules to form multiple hydrogen bonds with one another lead to significantly higher intermolecular attraction, which in turn results in a much higher boiling point compared to its counterparts.

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