Question

$H_{2}O$ has a higher boiling point than HF because:
A) $H_{2}O$ is more polar than HF.
B) $H_{2}O$ can form more hydrogen bonds.
C) $H_{2}O$ has a higher molecular weight.
D) $H_{2}O$ has more atoms.
E) $H_{2}O$ does not have a higher boiling point than HF.

Answer 1

Hint: Hydrogen-bonding is the strongest intermolecular force in both HF and $H_{2}O$. Interaction strength depends on the interaction angle, interaction distance and extensivity of the network. Each water molecule has two hydrogen atoms. Whereas, each HF molecule has only one hydrogen atom.

Complete answer:
Fluorine is more electronegative than Oxygen, so technically, HF should engage in more hydrogen bonding than water. But the situation is different. Each HF molecule possesses three lone pairs of fluorine and one hydrogen. HF molecules on average make two hydrogen bonds. Each $H_{2}O$ molecule possesses two lone pairs of oxygen. $H_{2}O$ molecules are thus able to form an average of four hydrogen bonds. Hence, water molecules take part in more hydrogen bonding than Hydrofluoric acid. Water molecules form a bulky molecule and it is very difficult to break its bonds. To break all its bonds, a large amount of the energy is required. Thus, $H2O$ has a higher boiling point than HF.

Thus, the correct answer is B. i.e., $H_{2}O$ can form more hydrogen bonds.

Additional Information: Water can make a more balanced hydrogen-bonding making the bulk system more uniformly interacting and thus stronger as a whole. Water forms a more extensive hydrogen bonding network rendering the bulk system less easily vaporizable.

Note: In determination of Boiling point we have to consider the intermolecular forces rather than intramolecular forces. $H_{2}O$ has a higher boiling point than HF because it contains twice as many hydrogen bonds, despite these being individually weaker.