A Levels Chemistry (9701)•9701/11/M/J/20
Explanation
Bonding types dictate melting point trends: molecular < ionic Steps:
- Identify bonding: MgCl₂ is ionic (strong lattice), CCl₄ and PCl₃ are molecular covalent (weak forces).
- Compare molecular compounds: PCl₃ (polar, MW 137) has weaker dispersion forces than CCl₄ (nonpolar, MW 154, tetrahedral symmetry), so PCl₃ mp (-93°C) < CCl₄ mp (-23°C).
- Ionic MgCl₂ has highest mp (714°C) due to electrostatic attractions.
- Order increasing mp: lowest molecular (PCl₃), higher molecular (CCl₄), ionic (MgCl₂).
Why B is correct:
- Matches trend of increasing forces: polar molecular < nonpolar molecular < ionic, per lattice energy and intermolecular force definitions.
Why the others are wrong:
- A: Reverses ionic as lowest, ignoring strong ionic bonds.
- C: Places ionic lowest and nonpolar highest, defying bonding strength order.
- D: Ionic not highest, violating ionic lattice dominance.
Final answer: B
Topic: Chemical bonding
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