Question

The correct order of the thermal stability of hydrogen halide $(H-X)$ 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 $HF>HCl>HBr>HI$

In a group, $\Delta G_f^o\left(HX\right)$ changes from negative to positive downwards.

$HF\left(g\right)\Delta G=-273.20kJ/mol$

$HI\left(g\right)\Delta G=+1.72kJ/mol$

Thus, $HF$ is thermally stable and $HI$ not.

The thermal stability of these halides decreases down the group, due to decrease in bond (H-X) dissociation enthalpy.

Thus, $HF>HCl>HBr>HI$