Pregunta 7 · Complements de química URV Examen Primer Parcial. Complements de Química

Rank the following substances in order of increasing melting point (or boiling point/stability in condensed phase): NH3, F2, HCl, CF4, He

Solución: $\mathrm{He < F_2 < CF_4 < HCl < NH_3}$

Explain the trend based on intermolecular forces

Solución: He: noble gas atom, extremely weak London dispersion forces (~4 K). F2: non-polar diatomic molecule held solely by weak dispersion forces (m.p. ≈ -219 °C). CF4: non-polar tetrahedral molecule, zero dipole moment, but higher polarizability/mass (m.p. ≈ -184 °C). HCl: polar molecule with dipole-dipole interactions (m.p. ≈ -114 °C). NH3: strong hydrogen bonding, highest melting point (m.p. ≈ -77.7 °C).