chemistry-intermolecular force

a)
In an H2O molecule, the 2 H atoms can at most form 2 hydrogen bonds,while the O atom can at most form 2 hydrogen bonds as it has 2 lone pairs ofelectrons. Therefore, each H2Omolecule can averagely form 2 intermolecular hydrogen bonds.

In an HF molecule, the H atom can at most form 1 hydrogen bond, while the Fatom can at most form 3 hydrogen bond as it has 3 lone pair of electrons. Therefore, each HF molecule can averagely atmost form 1 intermolecular hydrogen bond.

Although the hydrogen bond between HF molecules is stronger than that between H2Omolecules, H2O has a higher boiling point than HF because there area greater number of hydrogen bonds in H2O.


b)
In a NH3 molecule, the 3 H atoms can at most form 3 hydrogen bonds,while the N atom can at most form 1 hydrogen bond as it has only 1 lone pair ofelectrons. Each NH3 moleculecan thus averagely form at most 1 intermolecular hydrogen bond. From a), each HF molecule can also form atmost 1 intermolecular hydrogen bond. Therefore, when number of moles of NH3 is equal to that ofHF, the numbers of hydrogen bonds would be approximately equal to each other.

HF has a higher boiling point than NH3 because the hydrogen bondbetween HF molecules are stronger than that between NH3­molecules. The difference in the strengthof hydrogen bonds can be explained by the fact that F has a higherelectronegative than N.