Question

The correct order of heat of formation of halogen acids is

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

Answer 1

Answer: (B)

The order of bond dissociation energy of hydrogen halide (or halogen acid) is as
$\begin{matrix}Hydrogen\, halide\, Bond \,dissociation &HF&>&HCl&>&HBr&>&HI\end{matrix}$
$\begin{matrix}energy \, KJ\,mol^{-1}&566&431&366&299\end{matrix}$
Bond dissociation energy $\propto$ heat of formation
As bond dissociation energy decreases the heat of formation of halogen acids is also decreases. Hence, the order of heat of formation of halogen acids is
$HF >\, HCl > \, HBr >\, HI$