JEE Main 2024 · 1 February · morning · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion (A ) and the other is labelled as Reason (R) .
Assertion (A) : PH₃ has lower boiling point than NH₃.
Reason (R) : In liquid state NH₃ molecules are associated through vander Waal's forces, but PH₃ molecules are associated through hydrogen bonding.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) Both (A) and (R) are correct and (R) is the correct explanation of (A)
- (b) (A) is not correct but (R) is correct
- (c) (A) is correct but (R) is not correct
- (d) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
Answer and solution
Answer (c) (A) is correct but (R) is not correct
Solution
Chemistry ma’am · 4 steps
The assertion is right, but read the reason carefully: it has swapped the two forces between the molecules.
- Check the assertion. NH₃ boils at about -33 °C and PH₃ at about -88 °C. So PH₃ does have the lower boiling point, and (A) is correct.
- Who can H-bond. Hydrogen bonding needs H attached to a small, highly electronegative atom such as N, O or F. Nitrogen fits, so NH₃ molecules are associated through hydrogen bonds. Phosphorus is larger and much less electronegative, so PH₃ molecules are held only by van der Waals forces.
- Check the reason. (R) gives these the other way round: van der Waals forces for NH₃ and hydrogen bonding for PH₃. So (R) is not correct. The true reason for (A) is the extra energy needed to break the hydrogen bonds in liquid NH₃.
- So. (A) is correct but (R) is not, which is option (c).
The idea to keep. Hydrogen bonding needs H on N, O or F. Among the hydrides of a group, the second-period one (here NH₃) boils unusually high for exactly this reason.
Answer from the NTA final answer key. Solution by PrepWiser.