Why does HF have a lower boiling point and lower heat of...

Why does HF have a lower boiling point and lower heat of vaporization than $\mathrm{H}_{2} \mathrm{O}$, even though their molecular weights are nearly the same and the hydrogen bonds between molecules of $\mathrm{HF}$ are stronger?

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Key Concepts

Hydrogen Bonding

Hydrogen bonding is a type of intermolecular force where a hydrogen atom bonded to a highly electronegative atom interacts with a lone pair on another electronegative atom. Its strength and occurrence significantly affect boiling points and heats of vaporization. Although individual hydrogen bonds in HF can be quite strong, the overall physical properties depend on the network of these bonds throughout the substance.

Hydrogen Bond Network Dimensionality

The geometry of a molecule determines how many hydrogen bonds it can form and in what spatial arrangement. Water has a bent geometry that allows each molecule to form up to four hydrogen bonds, creating an extensive three-dimensional network. In contrast, HF tends to form linear chains, which limit the number of hydrogen bonding interactions per molecule, decreasing the total cohesive energy in the liquid phase.

Intermolecular Forces and Boiling Point

The boiling point of a substance is influenced by the strength and extent of its intermolecular forces. Although individual hydrogen bonds in HF might be strong, water's ability to form a robust, multidirectional network results in higher cohesive energy. This strong network in water requires more energy to break apart, leading to higher boiling point and heat of vaporization compared to HF.

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