The strongest reducing agent among the hydrides of Group 15 elements is:

Question

Accepted Answer

The strongest reducing agent among the hydrides of Group 15 elements is $BiH_{3}$, which is Option 3. Here's a detailed explanation to justify why $BiH_{3}$ is the correct answer:

1. **Explanation for Option 3 ($BiH_{3}$):**
   - Reducing agents are substances that donate electrons in a chemical reaction. The strength of a reducing agent is often related to its ability to release hydrogen atoms or electrons.
   - In Group 15, as we move down the group from nitrogen to bismuth, the size of the central atom increases. This increase in atomic size leads to a decrease in the bond strength between the central atom and hydrogen in the hydride.
   - $BiH_{3}$, being the heaviest hydride in Group 15, has the weakest Bi-H bond compared to the other hydrides. This weak bond makes it easier for $BiH_{3}$ to release hydrogen atoms, thereby acting as a stronger reducing agent.
   - Additionally, the lower electronegativity of bismuth compared to other elements in the group contributes to its ability to donate electrons more readily, enhancing its reducing power.

2. **Explanation for why other options are incorrect:**

- **Option 1 ($NH_{3}$):**
     - Ammonia ($NH_{3}$) is a very stable molecule with strong N-H bonds. The small size of nitrogen and its high electronegativity make $NH_{3}$ a poor reducing agent compared to $BiH_{3}$.
     - The strong N-H bond in ammonia does not favor the release of hydrogen atoms, making it a weaker reducing agent.

- **Option 2 ($AsH_{3}$):**
     - Arsenic trihydride ($AsH_{3}$) has stronger As-H bonds compared to Bi-H bonds in $BiH_{3}$. This makes $AsH_{3}$ less effective as a reducing agent.
     - While $AsH_{3}$ is more reducing than $NH_{3}$, it is not as strong a reducing agent as $BiH_{3}$ due to the relatively stronger bond strength.

- **Option 4 ($PH_{3}$):**
     - Phosphine ($PH_{3}$) also has stronger P-H bonds compared to Bi-H bonds. The bond strength in $PH_{3}$ is greater than in $BiH_{3}$, making it a less effective reducing agent.
     - The electronegativity of phosphorus is higher than that of bismuth, which also contributes to its lower reducing ability compared to $BiH_{3}$.

In conclusion, $BiH_{3}$ is the strongest reducing agent among the hydrides of Group 15 elements due to its weak Bi-H bond and the lower electronegativity of bismuth, which facilitates the release of hydrogen atoms and electrons.