Answer the question on the basis of the passage given below:
There are some deposits of nitrates and phosphates in the earth's crust. Nitrates are more soluble in water. Nitrates are difficult to reduce under laboratory conditions, but microbes do it easily. Ammonia forms a large number of complexes with transition metals. Hybridization easily explains the ease of sigma donation of \(NH_3\) and \(PH_3\).
Phosphine is an inflammable gas, it is prepared from white phosphorus.
\(PH_3\) is a better reducing agent than \(NH_3\) because:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Bond dissociation enthalpy of P–H is low
The stability of the group 15 hydrides decreases from \( \ce{NH3} \) to \( \ce{BiH3} \), as seen from their bond dissociation enthalpies. The P–H bond is weaker than the N–H bond (larger P atom, poorer overlap), so \( \ce{PH3} \) gives up its hydrogen more easily and is the stronger reducing agent. Ammonia is only a mild reducing agent.
Why the other options are incorrect
Option (1) PH3 can give its lone pair of electron easily — Incorrect. Donating a lone pair is Lewis base behaviour; \( \ce{PH3} \) is in fact a weaker base than \( \ce{NH3} \), so this does not explain its reducing power.
Option (3) Bond dissociation enthalpy of P–H is high — Incorrect. A high P–H bond enthalpy would make \( \ce{PH3} \) more stable and a poorer reducing agent; the bond is actually weaker than N–H.
Option (4) PH3 is tetrahedral in shape — Incorrect. \( \ce{PH3} \) is pyramidal, not tetrahedral, and shape has no bearing on reducing character here.
Note 1: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Reactivity towards Hydrogen – …
Note 2: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 02-06 S3 31-05 S1 | – |