Chloromethane (CH₃Cl) and propane (C₃H₈) have similar molar masses, 50.5 g mol⁻¹ and 44.1 g mol⁻¹ respectively. At 1 atm, their normal boiling points are 249 K (CH₃Cl) and 231 K (C₃H₈). The gas-phase dipole moment of chloromethane is 1.87 D, whereas that of propane is effectively 0 D.
Answer the following questions about the intermolecular forces operating in these two liquids.
(a) Identify the principal types of intermolecular forces that contribute to the cohesive energy of liquid chloromethane and of liquid propane.
(b) Explain, in terms of the molecular properties provided, why the boiling point of chloromethane is higher than that of propane even though their molar masses are similar.
(c) A sample of each liquid is cooled from 300 K to 200 K at constant pressure. Predict which liquid will exhibit the larger percentage increase in viscosity over this temperature interval, and justify your answer using intermolecular-force arguments.