The strongest reducing agent among the following is :

Solution

Chemistry ma’am · 3 steps

Go down group 15 and the E–H bond keeps getting weaker, so the hydride gives up hydrogen more easily.

1. What reducing means. A hydride EH₃ acts as a reducing agent by giving away its hydrogen, or the electrons in the E–H bond. The weaker the E–H bond, the more easily this happens, and the stronger the reducing agent.
2. Trend down the group. From N to Bi the central atom gets bigger. Its orbitals overlap less well with the small 1s orbital of hydrogen, so the E–H bond gets longer and weaker.

   Hydride	E–H bond enthalpy (kJ mol⁻¹)	Reducing power
   NH₃	389	weakest
   PH₃	322	stronger
   SbH₃	255	stronger still
   BiH₃	lowest of all	strongest

3. So. The order is NH₃ < PH₃ < SbH₃ < BiH₃. BiH₃ has the weakest E–H bond, so it is the strongest reducing agent, option (c).

The idea to keep. For hydrides down a group, reducing power follows bond weakness: a bigger central atom gives a weaker E–H bond, which breaks more easily. So the bottom hydride is always the strongest reducer.

Answer from the NTA final answer key. Solution by PrepWiser.