In each pair of compounds, pick the one with the higher...

In each pair of compounds, pick the one with the higher vapor pressure at a given temperature. Explain your reasoning.
a. $\mathrm{Br}_{2}$ or $\mathrm{I}_{2}$
b. $\mathrm{H}_{2} \mathrm{S}$ or $\mathrm{H}_{2} \mathrm{O}$
c. $\mathrm{NH}_{3}$ or $\mathrm{PH}_{3}$

Key Concepts

Vapor Pressure

Vapor pressure is the measure of a substance's tendency to evaporate, defined as the pressure exerted by its vapor when in equilibrium with its liquid or solid form. It is influenced by the strength of the intermolecular forces present; substances with weaker intermolecular attractions tend to have higher vapor pressures because their molecules more easily escape into the gas phase.

Intermolecular Forces

Intermolecular forces are the attractions between molecules, including dispersion forces, dipole-dipole interactions, and hydrogen bonding. The strength of these forces dictates many physical properties, such as boiling points and vapor pressures. Stronger attractive forces mean that more energy is required for molecules to leave the liquid phase, resulting in a lower vapor pressure.

Dispersion Forces

Dispersion forces, also known as London forces, arise from temporary fluctuations in electron density, leading to temporary dipoles in molecules. These forces become stronger with increased molecular size and greater polarizability. In comparing similar molecules, a larger or more polarizable molecule typically exhibits stronger dispersion forces, which results in a lower vapor pressure relative to its counterpart with weaker dispersion forces.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when hydrogen is directly bonded to highly electronegative elements such as oxygen, nitrogen, or fluorine. This strong attraction significantly increases the energy needed to vaporize a substance, thereby lowering its vapor pressure compared to similar molecules that do not form hydrogen bonds.

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