Question
The correct order of the thermal stability ofhydrogen halides $$\left( {H - X} \right)$$ is
A.
$$HI > HCl> HF > HBr$$
B.
$$HCll< HF < HBr < HI$$
C.
$$HF > HCl > HBr > HI$$
D.
$$HI > HBr > HCl > HF$$
Answer :
$$HF > HCl > HBr > HI$$
Solution :
The $$H - X$$ bond strength decreases from $$HF\,{\text{to}}\,HI$$. i.e. $$HF > HCl > HBr > HI.$$ Thus $$HF$$ is most stable while $$HI$$ is least stable. This is evident from their decomposition reaction: $$HF\,{\text{and}}\,HCl$$ are stable upto $$1473K,$$ $$HBr$$ decreases slightly and $$HI$$ dissociates considerably at $$713K.$$ The decreasing stability of the hydrogen halide Oxidation is also reflected in the values of dissociation energy of $$H - X$$ bond
$$\mathop {H - F}\limits_{135kcal\,mo{l^{ - 1}}} \,\,\mathop {H - Cl}\limits_{103kcal\,mo{l^{ - 1}}} \,\,\,\,\,\,\,\mathop {H - Br}\limits_{87kcal\,mo{l^{ - 1}}} \,\,\,\,\,\,\mathop {H - I}\limits_{71kcal\,mo{l^{ - 1}}} $$