Question

The correct order of the thermal stability of hydrogen halide(H-X) is:
(a) $\text {HI}\succ \text {HCl}\succ \text {HF}\succ \text {HBr}$
(b)$\text {HCl}\succ \text {HF}\succ \text {HBr}\succ \text {HI}$
(c) $\text {HF}\succ \text {HCl}\succ \text {HBr}\succ \text {HI}$
(d) $\text {HI}\succ \text {HBr}\succ \text {HCl}\succ \text {HF}$

Answer 1

Hint: Thermal stability decreases with atomic size and increases with the bond strength of the atom and smaller the size of the atom, greater is the bond strength and vice-versa. Identify the order.

Complete step by step answer:
By the thermal stability, we simply mean the stability of the molecule or any compound when it is subjected to the very high temperatures. If the molecule has higher thermal stability, then the molecule would not be able to decompose off easily or get broken down at higher temperature.
Thermal stability depends on the size of the atom and the bond strength. As the size of the atom decreases, the bond dissociation energy increases and hence, the thermal stability increases and vice-versa i.e. it is inversely proportional to the size of the atom.
Similarly, greater the bond strength, greater is the thermal stability of the molecule i.e. as the bond strength increases, thermal stability also increases and vice-versa.
So, in case of the hydrogen halides, as we move down the group from F to I , the size of the atom increases and the outermost electrons of the atom become loosely bound to the it and can be easily removed from it and hence, its bond dissociation decreases as we know down the group and hence the bond strength also decreases, thus , the thermal stability decreases and vice-versa.
Hence, the order of thermal stability of hydrogen halide is;
$\text {HF}\succ \text {HCl}\succ \text {HBr}\succ \text {HI}$
So, the correct answer is “Option C”.

Note: Keep in mind that as we go down the group from top to bottom, the atomic radius increases and hence, the size of the atom increases because as we move down the group, besides the addition of the electrons, the number of the shells also increases.