Question

In each of the following groups of substances, pick the one that has the given property.

Highest boiling point: CS2, CH2Cl2, CCl4

Lowest freezing point: H2O, NH3, CH4

CH2Cl2, H2O

CS2, H2O

CCl4, NH3

CS2, CH4

CCl4, CH4

Answer 1

Correct answer -

CCl₄ and CH₄

The intermolecular forces (IMF) in CCl4 would be London Dispersion Forces (LDF) and the IMF in CH2Cl2 would be LDF and dipole-dipole. It appears that the CH2Cl2 would have the greater IMF and therefore the lower vapor pressure and higher boiling point, but that is not the case. This means that the IMF in CCl4 must be greater. Due to CCl4 larger size, the LDF would be greater than that in CH2Cl2. This difference would have to be enough greater to counteract the dipole-dipole of CH2Cl2, because the CCl4 has the lower vapor pressure and higher boiling point.In CS₂ only dispersion forces will be present since it is a non-polar molecule.

Boiling point (and also melting point) is a function of intermolecular forces of attraction within molecules. Among the given molecules, H2O and NH₃ can form reasonably strong intermolecular hydrogen bonds. These two molecules are expected to have higher boiling (and melting) points. Hence, Melting point order will be-

H₂O > NH₃ > CH₄