The bond dissociation energies of  ${\mathrm{X}}_2, {\mathrm{Y}}_2$ and XY are in the ratio of 1 : 0.5 : 1.  for the formation of XY is $– 200 \mathrm{kJ} {\mathrm{mol}}^{-1}$. The bond dissociation energy of XY will be

Let B.E. of  ${\mathrm{X}}_2,{\mathrm{Y}}_2$ and are   $x\mathrm{kJ}{\mathrm{mol}}^{-1}$

$0.5x\mathrm{kJ} {\mathrm{mol}}^{-1}\text{and }x\mathrm{kJ} {\mathrm{mol}}^{-1} \text{respectively}$

$\frac{1}{2}{X}_2+\frac{1}{2}{Y}_2\to XY;\Delta H=-200kJmo{l}^{-1}$

$\Delta H=\sum {\left(B.E.\right)}_{\text{reactants}}-\sum {\left(B.E.\right)}_{products}$

$\therefore -200=\left[\frac{1}{2}\times \left(x\right)+\frac{1}{2}\times \left(0.5x\right)\right]-\left[1\times \left(x\right)\right]$

$\text{B.E. of }\mathrm{XY}=x=800\mathrm{kJ}{\mathrm{mol}}^{-1}$