Question

Which one is correct regarding thermal stability of hydrides of group $15$ ?

• A. $NH_{3}>PH_{3}>AsH_{3}$
• B. $PH_{3}>NH_{3}>AsH_{3}$
• C. $NH_{3}>AsH_{3}>PH_{3}$
• D. $AsH_{3}>PH_{3}>NH_{3}$

Answer 1

Answer: (A)

Thermal stability is the property of a compound to resist break down of the bond under heating effect.

Bond energy is the amount of energy required to break the bond in the molecule.

Therefore, higher the bond energy, higher is the thermal stability.

Thermal stability is directly proportional to the bond energy and inversely proportional to the bond length.

Bond length increases down the group due to increasing the atomic radius by increasing the number of inner shell.

As the bond length increases bond energy and thermal stability decreases down the group.



[ $\because $ Bond energy $\text{N} - \text{H} > \text{P} - \text{H} > \text{A} \text{S} - \text{H}$ ]