JEE Main 2021 · 22 July · evening · Single correct · +4 / −1
Which one of the following group-15 hydride is the strongest reducing agent?
- (a) AsH₃
- (b) BiH₃
- (c) PH₃
- (d) SbH₃
Answer and solution
Answer (b) BiH₃
Solution
Chemistry ma’am · 3 steps
Down group 15 the E–H bond gets weaker, so the hydride at the bottom, BiH3, gives up hydrogen most easily.
- What reducing means. A hydride acts as a reducing agent by giving up its hydrogen, so the E–H bond has to break. The weaker that bond, the more easily it breaks and the stronger the reducing agent.
- Bond strength trend. Going down from P to Bi, the central atom gets bigger. Its large orbitals overlap poorly with the small 1s orbital of hydrogen, so the E–H bond becomes longer and weaker.
Hydride Size of central atom E–H bond strength Reducing power PH₃ smallest of these strongest weakest AsH₃ larger weaker stronger SbH₃ larger still weaker still stronger still BiH₃ largest weakest strongest - So. The order is PH₃ < AsH₃ < SbH₃ < BiH₃, and BiH₃, option (b), is the strongest reducing agent.
The idea to keep. For hydrides down a group, a bigger central atom means a weaker E–H bond. That means lower thermal stability and stronger reducing power. Both trends come from the same cause.
Answer from the NTA final answer key. Solution by PrepWiser.