The hydrogen fluoride molecule, HF, is more polar than a water molecule, H2O (for example, has a greater dipole moment), yet the molar enthalpy of vaporization for liquid hydrogen fluoride is lesser than that for water. Explain.

Water forms more hydrogen bonds than HF

Explanation:

The answer to this question goes back to the idea of hydrogen bonding. Hydrogen bonding occurs when hydrogen is bonded to a highly electronegative atom such as fluorine or oxygen.

However, in HF, there are three lone pairs of electrons on fluorine atom and one hydrogen atom bonded to fluorine.

In H2O, there are two lone pairs of electrons on oxygen atom and two hydrogen atoms bonded to oxygen. This simply means that water can form four hydrogen bonds while HF only forms two hydrogen bonds.

This implies that H2O molecules possess more hydrogen bonding than HF molecules. Hence, the molar enthalpy of vaporization for liquid hydrogen fluoride is lesser than that for water.

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