Question

Which one has the lowest boiling point?

• A. $NH_3$
• B. $PH_3$
• C. $AsCI_3$
• D. $SbH_3$

Answer 1

The Correct Option is B

To determine which of the given compounds has the lowest boiling point, we need to analyze the factors that affect boiling points, mainly molecular weight and intermolecular forces.

1. Molecular Weight: In general, as the molecular weight increases, the boiling point tends to increase because heavier molecules have stronger London dispersion forces. The molecular weights of the given compounds are approximately:
   • NH_3 (Ammonia): 17 g/mol
   • PH_3 (Phosphine): 34 g/mol
   • AsCl_3 (Arsenic trichloride): 181 g/mol
   • SbH_3 (Stibine): 124 g/mol
2. Intermolecular Forces: Besides molecular weight, intermolecular forces, such as hydrogen bonding, also influence boiling points.
   • NH_3: Exhibits hydrogen bonding due to the presence of a highly electronegative nitrogen atom bonded to hydrogen.
   • PH_3, AsCl_3, SbH_3: Primarily exhibit weaker van der Waals forces and dipole-dipole interactions.

Normally, hydrogen bonding significantly increases the boiling point, which explains why NH_3 has a relatively higher boiling point despite its low molecular weight. Meanwhile, PH_3, lacking hydrogen bonding, has weaker intermolecular forces compared to NH_3.

Now, considering the molecular weights and the nature of the intermolecular forces, PH_3 has the lowest boiling point among the given options because it has a higher molecular weight than NH_3 but lacks hydrogen bonding, making its intermolecular forces weaker overall.

Hence, the correct answer is PH_3.