Question

$\textbf{Choose the correct statements about the hydrides of group 15 elements.}$
A. The stability of the hydrides decreases in the order \(\text{NH}_3 > \text{PH}_3 > \text{AsH}_3 > \text{SbH}_3 > \text{BiH}_3\)
B. The reducing ability of the hydrides increases in the order \(\text{NH}_3 < \text{PH}_3 < \text{AsH}_3 < \text{SbH}_3 < \text{BiH}_3\)
C. Among the hydrides, \(\text{NH}_3\) is a strong reducing agent while \(\text{BiH}_3\) is a mild reducing agent.
D. The basicity of the hydrides increases in the order \(\text{NH}_3 < \text{PH}_3 < \text{AsH}_3 < \text{SbH}_3 < \text{BiH}_3\)
Choose the most appropriate from the option given below:

• A. B and C only
• B. C and D only
• C. A and B only
• D. A and D only

Answer 1

The Correct Option is C

Identify the correct statements (A, B, C, D) concerning the properties of Group 15 hydrides.

Concept Used:

Key trends for Group 15 hydrides (E_NH₃):

• Stability: Decreases down the group due to reduced E–H bond dissociation enthalpy.
• Reducing Character: Increases down the group as the E–H bond weakens and becomes easier to break.
• Basicity: Decreases down the group due to diminished lone pair availability on the central atom (resulting from increased size and decreased electronegativity).

Step-by-Step Solution:

Step 1: Evaluate Statement A: "The stability of the hydrides decreases in the order NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃." This is accurate. The E–H bond strength diminishes down the group owing to the expanding size of the central atom and less effective orbital overlap with hydrogen.

Step 2: Evaluate Statement B: "The reducing ability of the hydrides increases in the order NH₃ < PH₃ < AsH₃ < SbH₃ < BiH₃." This is accurate. Reducing ability correlates with the ease of E–H bond cleavage. As the bond weakens down the group, the hydride can more readily donate H^– (or H•), enhancing its reducing power.

Step 3: Evaluate Statement C: "Among the hydrides, NH₃ is a strong reducing agent while BiH₃ is a mild reducing agent." This is inaccurate. The trend is reversed. NH₃ is a very weak reducing agent (stable), whereas BiH₃ is the strongest reducing agent in the group (highly unstable and prone to decomposition).

Step 4: Evaluate Statement D: "The basicity of the hydrides increases in the order NH₃ < PH₃ < AsH₃ < SbH₃ < BiH₃." This is inaccurate. Basicity decreases down the group: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃. The lone pair on nitrogen in NH₃ is most accessible for donation due to nitrogen's small size and high electronegativity.

Therefore, the correct statements are A and B exclusively.