The boiling point of NH3, PH3,AsH3 and SbH3 are respectively -33.4 oC,-87.5 oC, -62.4 oC, -18.4oC. Explain the variation of their boiling points in terms of the types of intermolecular forces.
The correct answer and explanation is :
The boiling points of ammonia (NH₃), phosphine (PH₃), arsine (AsH₃), and stibine (SbH₃) are -33.4 °C, -87.5 °C, -62.4 °C, and -18.4 °C, respectively. This variation in boiling points is primarily attributed to the types and strengths of intermolecular forces present in each compound.
Intermolecular forces are the attractive forces between molecules, influencing physical properties such as boiling points. The main types include hydrogen bonding, dipole-dipole interactions, and London dispersion forces.
Ammonia (NH₃):
Ammonia exhibits a relatively high boiling point of -33.4 °C among the group. This is due to the presence of hydrogen bonding, a strong intermolecular force occurring when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. In NH₃, nitrogen’s high electronegativity creates a significant dipole, allowing hydrogen bonds to form between NH₃ molecules. These bonds require more energy to break, resulting in a higher boiling point.
Phosphine (PH₃), Arsine (AsH₃), and Stibine (SbH₃):
For PH₃, AsH₃, and SbH₃, hydrogen bonding is negligible due to the lower electronegativities of phosphorus, arsenic, and antimony compared to nitrogen. Consequently, these molecules primarily rely on London dispersion forces, which are weaker intermolecular forces arising from temporary fluctuations in electron distribution that induce temporary dipoles. The strength of London dispersion forces increases with molecular size and mass.
As we move down Group 15 from PH₃ to SbH₃, the molecular size and mass increase, enhancing the polarizability of the molecules and strengthening the London dispersion forces. This leads to higher boiling points:
- PH₃ (-87.5 °C): Smallest size and mass, weakest dispersion forces, lowest boiling point.
- AsH₃ (-62.4 °C): Larger size and mass than PH₃, stronger dispersion forces, higher boiling point.
- SbH₃ (-18.4 °C): Largest size and mass, strongest dispersion forces, highest boiling point among PH₃, AsH₃, and SbH₃.
Summary:
The boiling point trend among NH₃, PH₃, AsH₃, and SbH₃ is influenced by the types and strengths of intermolecular forces. NH₃’s higher boiling point is due to strong hydrogen bonding. In contrast, PH₃, AsH₃, and SbH₃ lack significant hydrogen bonding and rely on London dispersion forces, which increase with molecular size and mass, leading to progressively higher boiling points from PH₃ to SbH₃.
This analysis underscores the impact of intermolecular forces on the physical properties of molecular compounds.