Question

The correct order of boiling point of NCl3, NClF2, NF3 is:

• A. NCl3 > NClF2 > NF3
• B. NCl3 < NClF2 < NF3
• C. NClF2 < NCl3 < NF3
• D. NCl3 < NF3 < NClF2

Answer 1

Answer: (A)

NCl3 > NClF2 > NF3

Answer 2

The boiling point of a substance is primarily determined by the strength of intermolecular forces. For covalent molecules like NCl$_3$, NClF$_2$, and NF$_3$, these forces include London dispersion forces and dipole-dipole forces.

1. Molecular Weight and London Dispersion Forces (LDF): The molar masses are:

   • NF$_3$: $71.01 \text{ g/mol}$
   • NClF$_2$: $87.46 \text{ g/mol}$
   • NCl$_3$: $120.36 \text{ g/mol}$ The order of increasing molar mass is: NF$_3$ < NClF$_2$ < NCl$_3$. This suggests that LDFs are weakest in NF$_3$ and strongest in NCl$_3$.

2. Molecular Geometry, Polarity, and Dipole-Dipole Forces: All three molecules have a trigonal pyramidal molecular geometry and are polar.

   • Experimental dipole moments are approximately:
     • NF$_3$: $\mu \approx 0.23$ D
     • NClF$_2$: $\mu \approx 0.57$ D
     • NCl$_3$: $\mu \approx 0.71$ D The order of increasing dipole moment is: NF$_3$ < NClF$_2$ < NCl$_3$. This suggests that dipole-dipole forces are weakest in NF$_3$ and strongest in NCl$_3$.

3. Combined Effect: Both London dispersion forces (due to molar mass) and dipole-dipole forces (due to dipole moment) increase in the order NF$_3$ < NClF$_2$ < NCl$_3$. Therefore, the boiling points are expected to follow the same trend.

   The order of boiling points is: NF$_3$ < NClF$_2$ < NCl$_3$, which can also be written as: NCl$_3$ > NClF$_2$ > NF$_3$.