a
\(\mathrm{H}_{2}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl}\)

\(\Delta H_{\text {reaction }}=\Sigma(\mathrm{B.E})_{\text {reactant }}-\Sigma(\mathrm{B.E})_{\text {product }}\)

\(=\left[(\mathrm{B.E})_{\mathrm{H}-\mathrm{H}}+(\mathrm{B.E})_{\mathrm{Cl}-\mathrm{Cl}}\right]-\left[2 \mathrm{B.E}_{(\mathrm{H}-\mathrm{Cl})}\right]\)

\(=434+242-(431) \times 2\)

\(\Delta H_{\text {reaction }}=-186 \mathrm{kJ}\)

Heat of formation is the amount of heat absorbed or evolved when one mole of substance is directly obtained from its constituent element.

Hence, enthalpy of formation of \(\mathrm{HCl}=-186 / 2\; \mathrm{kJ}\)

\(=-93 \;\mathrm{kJ} \mathrm{mol}^{-1}\)