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

Answer: (C)

$HF > HCl > HBr > HI$

Answer 2

In a group, $\Delta G^{\circ}_{f}$(HX) changes from - ve to + ve downwards. $HF \left(g\right) \Delta G =- 273 .20 \,kJ \,mol^{-1}$ $HI\left(g\right) \Delta G = + 1.72 \,kJ\, mol^{-1}$ Thus HF is thermally stable and HI not. Thus $HF > HCI > HBr > HI$