Thermodynamics and Thermochemistry 2 Question 28
28. The bond dissociation energies of gaseous $\mathrm{H}{2}, \mathrm{Cl}{2}$ and $\mathrm{HCl}$ are 104,58 and $103 \mathrm{kcal} / \mathrm{mol}$ respectively. Calculate the enthalpy of formation of $\mathrm{HCl}$ gas.
$(1985,2 \mathrm{M})$
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Solution:
- $\frac{1}{2} \mathrm{H}{2}(g)+\frac{1}{2} \mathrm{Cl}{2}(g) \longrightarrow \mathrm{HCl}(g)$; $\quad \Delta H_{f}^{\circ}$
$\Delta H_{f}^{\circ}=\Sigma \mathrm{BE}$ (reactants) $-\Sigma \mathrm{BE}$ (products)
$$ =\frac{1}{2}(104+58)-103=-22 \mathrm{kcal} / \mathrm{mol} $$
