Elements of group- 15, 16, and 17 form compounds with hydrogen with general formula \(MH_3\), \(H_2M\), and \(HX\) (called hydrogen halides). The stability of these hydrides, along with \(M-H\) bond strength and basic nature decreases down the group. Boiling point of these hydrides mainly depends on the strength of their intermolecular forces.
Which of the following is the strongest reducing agent?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → BiH3
Down Group 15 the E–H bond dissociation enthalpy falls; a weaker E–H bond gives up hydrogen more easily, so the reducing character increases from \( \ce{NH3} \) to \( \ce{BiH3} \).
Why the other options are incorrect
Option (1) NH3 — Incorrect. The N–H bond is the strongest (389 kJ mol−1), so \( \ce{NH3} \) is the weakest reducing agent of the group.
Option (2) PH3 — Incorrect. P–H (322 kJ mol−1) is still much stronger than Bi–H.
Option (3) AsH3 — Incorrect. A stronger reducing agent than \( \ce{PH3} \) but weaker than \( \ce{SbH3} \) and \( \ce{BiH3} \).
Note: 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
