Bond dissociation energy of $XY,X_2$ and $Y_2$ (all diatomic molecules) are in the ratio of 1 : 1 : 0.5 and ${\text{ΔH}}_f$ of XY is $-200\text{kJ}{\text{mol}}^{-1}$. The bond dissociation energy of $X_2$ will be

B.D.E of XY, X₂ and Y₂ are in the ratio 1 : 1 : 0.5

Let B.D.E. of XY be x, X₂ be x and Y₂ be 0.5 x.

= – 2x + (x + 0.5 x) = – 0.5x

Also, $\Delta$H_f of XY is – 200 kJ mol^–1

$\therefore$$\Delta$H = 2 × $\Delta$H_f = 2 × – 200 kJ mol^–1 = – 400 kJ

So, 0.5x = + 400 kJ

$\Rightarrow$x = 800 kJ