Question

If the bond dissociation energies of XY, X2 and Y2 (all diatomic molecules) are in the ratio of 1 : 1 : 0.5 and $\Delta$Hf for the formation of XY is - 200 KJ mol–1. The bond dissociation energy of X2 will be

• A. 100 KJ mol–1
• B. 200 KJ mol–1
• C. 300 KJ mol–1
• D. 800 KJ mol–1

Answer 1

Answer: (D)

800 KJ mol–1

Answer 2

Let the bond dissociation energy of XY, X2 and Y2 be x,x and x,$\frac{x}{2}$KJ/mol respectively,

$\frac{1}{2}X_{2} + \frac{1}{2}Y_{2} \rightarrow$XY; DH¦ = – 200 KJ mol–1.

$\Delta$H reaction = [(sum of bond dissociation energy of all reactants) – (sum of bond dissociation energy of all product)]

= $\left\lbrack \frac{1}{2}\Delta H_{X2} + \frac{1}{2}\Delta H_{Y2} - \Delta H_{XY} \right\rbrack$= $\frac{x}{2} + \frac{0.5x}{2}$– x = – 200

$\therefore$ x = $\frac{200}{0.25}$ = 800 KJ mol–1.