Question

The bond dissociation energy of gaseous $H_2,C{l}_2$ and $HCl$ are 104, 58 and 103 $kcal.mo{l}^{-1}$ respectively. The enthalpy of formation for HCl gas will be:

• A. -44.0 kcal
• B. -22.0 kcal
• C. 22.0 kcal
• D. 44.0 kcal

Answer 1

Answer: (B)

-22.0 kcal

The balanced chemical reaction for the formation of HCl is as shown.

$\frac{1}{2}{H}_2+\frac{1}{2}C{l}_2\to HCl$

The enthalpy of formation of HCl gas is

$\Delta H_{f,HCl}=\frac{1}{2}\Delta H_{B.E,H-H}+\frac{1}{2}\Delta H_{B.E,Cl-Cl}-\Delta H_{B.E,H-Cl}$

$\Delta H_{f,HCl}=\frac{1}{2}\times 104+\frac{1}{2}\times 58-103=52+29-103=-22kcal/mol$

Hence, the enthalpy of formation of HCl gas is -22.0 kcal/mol