Question

which is stronger reducing agent, SbH3 or BiH3, and why

Answer 1

BiH3 is stronger reducing agent because of its large size and low bond dissociation enthalpy.
Hope this will be helpful :)

Answer 2

Explanation:

Substances which loose electrons in a chemical reaction are known as reducing agents.

The larger is the size of an atom, the more easily it can loose electrons because the attraction between nucleus and outermost electrons decreases.

Both Sb and Bi belongs to the same group. Bi has larger size as compared to Sb. Therefore, it bismuth can easily loose its electrons and thus, it becomes a stronger reducing agent as compared to antimony.

Thus, we can conclude that $BiH_{3}$ is strong reducing agent as compared to $SbH_{3}$.