Question

The strongest reducing agent among the following is:

• A. SbH$_3$
• B. NH$_3$
• C. BiH$_3$
• D. PH$_3$

Hint:

In a group, reducing character increases down the group due to an increase in atomic size and a decrease in bond strength.

Answer 1

The Correct Option is C

Step 1: Understanding Reducing Agent Strength 
A reducing agent donates electrons and gets oxidized. - The strength of a reducing agent increases as we move down the group in the periodic table. - This happens because the atomic size increases, and the bond strength between the central atom and hydrogen weakens, making electron donation easier. 
Step 2: Trend in Group 15 Hydrides 
- Group 15 hydrides: \( NH_3, PH_3, AsH_3, SbH_3, BiH_3 \). - The reducing character increases as: \[ NH_3<PH_3<AsH_3<SbH_3<BiH_3 \] - Thus, BiH$_3$ is the strongest reducing agent among the given choices. 
 

Answer 2

Step 1: Understand what a reducing agent is
A reducing agent is a substance that donates electrons in a redox reaction and gets oxidized itself.
The stronger the reducing agent, the more easily it loses electrons or gets oxidized.

Step 2: Analyze the given class of compounds
The compounds under consideration are metal hydrides of Group 15 elements (NH₃, PH₃, AsH₃, SbH₃, BiH₃).
These are known as pnictogen hydrides, and their reducing ability generally increases down the group.

Step 3: Trend in reducing power down the group
As we move down Group 15 from nitrogen to bismuth:
- The size of the central atom increases
- The E–H bond strength decreases
- The bond becomes more easily broken, releasing H₂ gas more readily
This leads to an increase in reducing power.

Step 4: Compare reducing strengths
- NH₃: Very stable, poor reducing agent
- PH₃: Slightly better, still weak
- AsH₃ < SbH₃ < BiH₃: Increasing reducing ability
- BiH₃ has the weakest E–H bond, making it the best electron donor and strongest reducing agent

Step 5: Final conclusion
Among the Group 15 hydrides, BiH₃ is the strongest reducing agent due to its large atomic size and weak Bi–H bond.

Final Answer: BiH₃