The normal boiling point of a substance depends on both the...

The normal boiling point of a substance depends on both the mass of the molecule and the attractive forces between molecules. Arrange the compounds in each set in order of increasing boiling point and explain your answer: (a) $\mathrm{HCl}, \mathrm{HBr}, \mathrm{HI}$ (b) $\mathrm{O}_{2}, \mathrm{HCl}, \mathrm{H}_{2} \mathrm{O}_{2}$

The normal boiling point of a substance depends on both the mass of the molecule and the attractive forces between molecules. Arrange the compounds in each set in order of increasing boiling point and explain your answer:
(a) $\mathrm{HCl}, \mathrm{HBr}, \mathrm{HI}$
(b) $\mathrm{O}_{2}, \mathrm{HCl}, \mathrm{H}_{2} \mathrm{O}_{2}$

Key Concepts

Normal Boiling Point

The normal boiling point is defined as the temperature at which a liquid’s vapor pressure equals atmospheric pressure. It is a key property that reflects how much energy is required for molecules to overcome intermolecular attractions and transition from the liquid phase to the gas phase.

Molecular Mass

Molecular mass is an important factor in determining boiling points because larger molecules generally have more electrons and a greater polarizability. This increased polarizability leads to stronger London dispersion forces, which in turn require higher temperatures to overcome during boiling.

Intermolecular Forces

Intermolecular forces include various types of attractions such as London dispersion forces, dipole-dipole interactions, and hydrogen bonding. The strength and type of these forces greatly affect the boiling point: stronger intermolecular attractions typically result in higher boiling points due to the increased energy required to separate the molecules.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of intermolecular force that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. This bonding significantly increases the boiling point, as it creates a strong attraction between molecules that must be overcome for the substance to boil.

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