Question

Why is $Bi{H}_3$ the strongest reducing agent amongst all the hydrides of Group $15$ elements?

Answer 1

Reducing nature of hydrides
Down the group, the atomic size of the element increases and the bond length of the corresponding $E—H$ bond also increases. This results in the decrease in bond dissociation enthalpy.

So, amongst trihydrides of the members of nitrogen family, the bond dissociation enthalpy of $Bi—H$ bond is the least.

$N{H}_3$ is only a very mild reducing agent while $Bi{H}_3$ is the strongest reducing agent amongst all of the hydrides.