A research team studies the same polar molecule in both the gas phase (400 K, 0.10 atm) and the solid phase (150 K). The molecule has a permanent dipole moment of 2.2 D and a molar mass of 72 g mol⁻¹.
Respond to the tasks below concerning the relative importance of dipole–dipole and London dispersion forces for this substance.
(a) Describe qualitatively how rotational motion of the molecules at 400 K influences the time-averaged dipole–dipole interaction energy in the gas phase.
(b) Explain why, upon freezing at 150 K, the contribution of dipole–dipole interactions to the crystal lattice energy becomes much larger relative to that in the gas phase.
(c) Even in the solid state, London dispersion forces remain significant. Give one molecular property and one structural feature of the solid that help account for the continued importance of dispersion forces.