Which is a stronger reducing agent, SbH3 or BiH3, and why?

BiH3 is a stronger reducing agent because a good reducing agent is a substance that can donate a hydrogen atom easily. As we move down a particular group, the size of the atom increases, so the bond length between the atom and hydrogen increases, and thus the bond strength decreases. Hence, it can lose hydrogen easily. So, the reducing nature of the hydrides of the VA group is given as BiH3 > SbH3 > AsH3 > PH3 > NH3.