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

Evaluate the given statements (A, B, C, D) regarding the properties of Group 15 hydrides to identify the correct ones.

Concept Used:

Key trends for Group 15 hydrides (E_NH₃):

• Stability: Decreases down the group due to decrease in E–H bond dissociation enthalpy.
• Reducing Character: Increases down the group as the E–H bond becomes weaker and easier to break.
• Basicity: Decreases down the group due to decreased availability of the lone pair on the central atom (increase in size, decrease in electronegativity).

Step-by-Step Solution:

Step 1: Analyze Statement A: "The stability of the hydrides decreases in the order NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃." This is correct. The E–H bond strength decreases down the group due to the increasing size of the central atom and poorer orbital overlap with hydrogen.

Step 2: Analyze Statement B: "The reducing ability of the hydrides increases in the order NH₃ < PH₃ < AsH₃ < SbH₃ < BiH₃." This is correct. Reducing ability depends on the ease of E–H bond cleavage. As the bond becomes weaker down the group, the hydride can more readily donate H^– (or H•), making it a stronger reducing agent.

Step 3: Analyze Statement C: "Among the hydrides, NH₃ is a strong reducing agent while BiH₃ is a mild reducing agent." This is incorrect. The trend is the opposite. NH₃ is a very weak reducing agent (stable), while BiH₃ is the strongest reducing agent in the group (very unstable and decomposes easily).

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

Thus, the correct statements are A and B only.