Question

Among the hydrides \( NH_3, PH_3, \) and \( BiH_3 \), the hydride with highest boiling point is \( X \) and the hydride with lowest boiling point is \( Y \). What are \( X \) and \( Y \) respectively?

• A. \( PH_3, NH_3 \)
• B. \( NH_3, PH_3 \)
• C. \( BiH_3, PH_3 \)
• D. \( NH_3, BiH_3 \)
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Hint:

- Ammonia (\( NH_3 \)) has an unusually high boiling point due to hydrogen bonding.
- The boiling point of hydrides increases down the group due to stronger van der Waals forces.

Answer 1

The Correct Option is C

Step 1: Understanding the Boiling Points of Group 15 Hydrides
The boiling points of hydrides of Group 15 elements are influenced by: - Hydrogen bonding (in case of \( NH_3 \)). - Van der Waals forces, which increase with molecular mass.
The boiling points of the hydrides are: \[ NH_3 = -33^\circ C, \quad PH_3 = -88^\circ C, \quad AsH_3 = -62^\circ C, \quad SbH_3 = -17^\circ C, \quad BiH_3 = +17^\circ C \] Step 2: Identifying \( X \) and \( Y \)
- The hydride with the highest boiling point is \( BiH_3 \) (Bismuth Hydride, 17°C) due to the large molecular mass and strong van der Waals interactions.
- The hydride with the lowest boiling point is \( PH_3 \) (Phosphine, -88°C) because it has weaker intermolecular forces and lacks hydrogen bonding.
Step 3: Identifying the Correct Option
- \( X = BiH_3 \), \( Y = PH_3 \) matches Option (3). ✅
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