Why is `BiH_(3)` the strongest reducing agent amongst all the hydrides of group `15` elements ?

Explanation

Down the group, the atomic size of an element increases and the bond length of the corresponding `E-H` bond also increases. This adversely affects the bond dissociation enthalpy. This means that amongst the trihydrides of the members of nitrogen family, the bond dissociation enthalpy of `Bi-H `bond is the least. Therefore, `BiH_(3)` is the strongest reducing agent among the hydrides of group `15` elements.