Which one of the following group-15 hydride is the strongest reducing agent ?
(a) AsH3
(b) BiH3
(c)

Q: Why does reducing character depend on hydrogen release?

In chemistry, reduction can be defined as the addition of hydrogen. Therefore, a compound that can easily supply hydrogen to another substance is acting as a strong reducing agent.

Q: Why does the bond length increase down the group?

As you move down a group in the periodic table, each successive element has an additional electron shell. This increases the atomic radius, pushing the valence electrons (and thus the bonded hydrogen atom) further away from the nucleus.

Q: Is ammonia ($\text{NH}_3$) a reducing agent?

Ammonia is a very poor reducing agent. Nitrogen is a small, highly electronegative atom, making the $\text{N-H}$ bond very short and strong. It requires a lot of energy to break, so it doesn't easily release hydrogen.

Q: What is the trend for thermal stability among these hydrides?

Thermal stability decreases down the group. $\text{NH}_3$ is the most thermally stable, while $\text{BiH}_3$ is the least stable and can decompose even at room temperature.

Q: How is this different from acidic character?

Interestingly, the acidic character of these hydrides also increases down the group for the exact same reason! A weaker $\text{M-H}$ bond makes it easier to release an $\text{H}^+$ ion in solution.

Question

Which one of the following group-15 hydride is the strongest reducing agent ?

Options:

(A) $	ext{AsH}_3$

(B) $	ext{BiH}_3$

(C) $	ext{PH}_3$

(D) $	ext{SbH}_3$

Sigma Percentile · Accepted Answer

### Analyzing the Setup
When we talk about the hydrides of Group 15 elements—namely Ammonia ($	ext{NH}_3$), Phosphine ($	ext{PH}_3$), Arsine ($	ext{AsH}_3$), Stibine ($	ext{SbH}_3$), and Bismuthine ($	ext{BiH}_3$)—we are looking at a classic periodic trend. The question asks us to identify the strongest reducing agent among them.

To answer this, we first need to understand what makes a compound a good reducing agent in this context. A reducing agent is a substance that can easily donate hydrogen atoms to another substance. Therefore, the ability of a hydride to act as a reducing agent depends entirely on how easily it can break its central metal-hydrogen ($	ext{M-H}$) bond to release $	ext{H}_2$ gas.

### The Master Trend: Size and Bond Length
Let's visualize moving down Group 15 from Nitrogen to Bismuth. As we descend the group, new electron shells are added, causing the atomic radius of the central atom to increase significantly.

Because the central atom gets larger, the distance between its nucleus and the hydrogen atom increases. This means the $	ext{M-H}$ bond length increases down the group.

Now, here is the fundamental rule of chemical bonding: **a longer bond is a weaker bond**. As the bond length increases, the orbital overlap between the central atom and hydrogen becomes less effective, leading to a decrease in bond dissociation energy.

### Final Conclusion
Since Bismuth is the largest atom in this group, the $	ext{Bi-H}$ bond in Bismuthine ($	ext{BiH}_3$) is the longest and, consequently, the weakest.

Because the bond is so weak, $	ext{BiH}_3$ has the lowest thermal stability among all Group 15 hydrides. It requires very little energy to break the $	ext{Bi-H}$ bond and release hydrogen.

Mathematically, we can express this as:
$$	ext{Reducing Power} \propto \frac{1}{	ext{Thermal Stability}}$$

Thus, Bismuthine readily gives up its hydrogen, making it the **strongest reducing agent**. The correct option is **(b)**.

Animated Solution for Chemistry - s and p-Block Elements: Which one of the following group-15 hydride is the strongest reducing agent ?

Select Answer:

Visualized Solution

$Group 15 Hydrides$

$Reducing Agent$

$Bond Strength Trend$

$Thermal Stability$

$Conclusion$

$The Way Forward$

The Sigma Insight: Group 15 Elements

Analyzing the Setup When we talk about the hydrides of Group 15 elements—namely Ammonia ( $NH_{3}$ ), Phosphine ( $PH_{3}$ ), Arsine ( $AsH_{3}$ ), Stibine ( $SbH_{3}$ ), and Bismuthine ( $BiH_{3}$ )—we are looking at a classic periodic trend

The Master Trend

Final Conclusion

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