Question

The correct order of the thermal stability of hydrogen halides $(H-X)$ is

• A. $HI>HCl<HF>HBr$
• B. $HCl<HF>HBr<HI$
• C. $HF>HCl>HBr>HI$
• D. $HI<HBr>HCl<HF$

Answer 1

The correct option is C $HF>HCl>HBr>HI$

The $H-X$ bond strength decreases from $HF$ 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: $HF$ and $HCl$ are stable upto $1473K$, $HBr$ decreases slightly and $HI$ dissociates considerably at $714K$. The decreasing stablility of the hydrogen halide is also reflected in the values of dissociation energy of the $H-X$ bond.

$H-F$	$H-Cl$	$H-Br$	$H-I$
$574{\text{kJmol}}^{-1}$	$432{\text{kJmol}}^{-1}$	$363{\text{kJmol}}^{-1}$	$295{\text{kJmol}}^{-1}$