(a) Which compound, A or B, has the higher boiling point? (b) Which compound, C or D, has the higher melting point?
Analysis
Determine the types of intermolecular forces in each compound. The compound with the stronger forces has the higher boiling point or melting point.
a. NH_{3} (A) has an N—H bond, so it exhibits intermolecular hydrogen bonding. CH_{4} (B) has only London forces since it has only nonpolar C—H bonds. NH_{3}has stronger forces and the higher boiling point.
b. Methanol (C) has an O—H bond, so it can intermolecularly hydrogen bond. Chloromethane (D) has a polar C—Cl bond, so it has dipole–dipole interactions, but it cannot hydrogen bond. C has stronger forces, so C has the higher melting point.