Question

The strongest reducing agent amont the following is:

• A. \( \text{NH}_3 \)
• B. \( \text{SbH}_3 \)
• C. \( \text{BiH}_3 \)
• D. \( \text{PH}_3 \)

Answer 1

The Correct Option is C

To determine the strongest reducing agent among the given hydrides—\( \text{NH}_3 \), \( \text{SbH}_3 \), \( \text{BiH}_3 \), and \( \text{PH}_3 \)—we need to consider their chemical properties, particularly their reducing abilities. 

The reducing ability of a hydride depends on the ease with which it can donate electrons to other substances. This ability is influenced by the bond strength between hydrogen and the central atom in the hydride, with weaker bonds leading to stronger reducing agents.

1. These compounds are hydrides of Group 15 elements, and reducing behavior typically increases down the group. This trend is due to:
   • Decreasing electronegativity of the central atom as we move down the group, which makes the bond between the central atom and hydrogen weaker.
   • Increasing atomic size, which also weakens the bond, making hydrogen easier to release.
2. Analyzing the given hydrides:
   • \( \text{NH}_3 \) has relatively strong N-H bonds due to nitrogen's high electronegativity. Hence, it is a weaker reducing agent.
   • \( \text{PH}_3 \) has weaker P-H bonds compared to N-H, increasing its reducing ability.
   • \( \text{SbH}_3 \) and \( \text{BiH}_3 \) have even weaker bonds due to larger atomic sizes of Sb and Bi, further increasing reducing capabilities.
3. Since \( \text{BiH}_3 \) is at the bottom of the group, it has the weakest bond with hydrogen and is, therefore, the strongest reducing agent among the options due to its ability to easily donate electrons.

Thus, the correct answer is \(\text{BiH}_3\), which is the strongest reducing agent among the given options.

Answer 2

The reducing strength of hydrides of Group 15 elements increases as we move down the group. This trend can be explained based on the following factors: - As we move down the group, the size of the central atom increases, which weakens the E − H bond (where E is the central atom.) The bond dissociation energy decreases. This makes it easier for the hydride to donate hydrogen (as H$^-$), thereby increasing its reducing power. Among the given options:
NH$_3$ (ammonia) has the strongest N--H bond and, therefore, is the weakest reducing agent.
PH$_3$, SbH$_3$, and BiH$_3$ follow the trend of increasing reducing strength due to weaker bonds as we move down the group.
BiH$_3$ (bismuthine) has the weakest Bi--H bond, making it the strongest reducing agent among the given hydrides.
Conclusion: The strongest reducing agent is BiH$_3$.