Q

Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements?

7.2     Why is $BiH_{3}$  the strongest reducing agent amongst all the hydrides of Group 15 elements ?

We see that the stability of hydrides becomes lesser as we go from $NH_3$  to $BiH_3$ .this can be seen from dissociation enthalpy of their bond. due to that, the reducing character of the hydrides will be more. Ammonia is only a very mild reducing agent while $BiH_3$ is the strongest reducing agent amongst all of the hydrides.