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 NH₃ (-33.4°C), PH₃ (-87.5°C), AsH₃ (-62.4°C), and SbH₃ (-18.4°C) vary due to the different types of intermolecular forces acting on them. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces.
Explanation of Variation in Boiling Points:
- Ammonia (NH₃) – Strong Hydrogen Bonding
NH₃ has a relatively high boiling point (-33.4°C) because of the presence of strong hydrogen bonding. Nitrogen is highly electronegative, creating a large dipole moment and allowing strong hydrogen bonds to form between NH₃ molecules. This significantly increases the energy required to break these intermolecular forces, leading to a higher boiling point. - Phosphine (PH₃) – Weak Dipole-Dipole Interactions
PH₃ has a much lower boiling point (-87.5°C) because phosphorus is less electronegative than nitrogen, making hydrogen bonding almost negligible. Instead, PH₃ molecules experience weaker dipole-dipole interactions and London dispersion forces, resulting in a lower boiling point. - Arsine (AsH₃) and Stibine (SbH₃) – Increasing London Dispersion Forces
- AsH₃ (-62.4°C) and SbH₃ (-18.4°C) follow an increasing trend in boiling points. This is because as we move down the group, the molecular size and molar mass increase, leading to stronger London dispersion forces (temporary dipole-induced dipole interactions).
- SbH₃, being the largest molecule, has the strongest London dispersion forces, giving it the highest boiling point (-18.4°C) among PH₃, AsH₃, and SbH₃.
Conclusion:
- NH₃ has the highest boiling point due to hydrogen bonding.
- PH₃, AsH₃, and SbH₃ primarily rely on London dispersion forces, which increase with molecular mass.
- This results in SbH₃ having the highest boiling point among the three, while PH₃ has the lowest due to weak intermolecular forces.
Here is the scientific diagram illustrating the intermolecular forces affecting NH₃, PH₃, AsH₃, and SbH₃. The image highlights hydrogen bonding in NH₃, weaker dipole-dipole interactions in PH₃, AsH₃, and SbH₃, and increasing London dispersion forces with molecular size. Additionally, it includes a boiling point trend graph.