Explain why hydrogen fluoride (HF) has a higher boiling temperature than hydrogen chloride (HCl) (19.4 vs. \(-85^{\circ} \mathrm{C}\) ), even though HF has a lower molecular weight.

HF and HCl are both polar covalent compounds, which means that they have a difference in electronegativity between the hydrogen and halogen atoms (fluorine in HF, chlorine in HCl). This electronegativity difference gives rise to a dipole-dipole interaction, which is an attractive force between the positive end of one polar molecule and the negative end of another polar molecule. Additionally, HF molecules exhibit hydrogen bonding, which is a special type of dipole-dipole interaction that occurs between hydrogen atoms covalently bonded to highly electronegative elements (such as fluorine, oxygen, or nitrogen) and the lone pairs of electrons on another highly electronegative atom. HCl, however, does not form hydrogen bonds because chlorine's electronegativity is not as high as fluorine's.

Hydrogen bonding in HF is a significantly stronger intermolecular force compared to the dipole-dipole interaction present in HCl. This is because fluorine is the most electronegative element, and its large electronegativity difference with hydrogen results in a highly polarized bond and consequently a strong hydrogen bond. On the other hand, while HCl exhibits dipole-dipole interactions due to its polar nature, the electronegativity difference between hydrogen and chlorine is not large enough to result in hydrogen bonding. As a result, the intermolecular forces in HCl are weaker than those in HF.

The boiling temperature of a substance is related to the strength of its intermolecular forces. Greater intermolecular forces require more energy to break the interactions between molecules, which in turn leads to a higher boiling temperature. Since HF experiences stronger intermolecular forces due to hydrogen bonding, it requires more energy to break these forces than the weaker dipole-dipole interactions in HCl, which explains the higher boiling temperature of HF (19.4°C) compared to HCl (-85°C).

Hydrogen fluoride (HF) has a higher boiling temperature than hydrogen chloride (HCl) due to the presence of stronger intermolecular forces, specifically hydrogen bonding, in HF. This stronger intermolecular force requires more energy to overcome, resulting in a higher boiling temperature for HF, even though it has a lower molecular weight compared to HCl.