Question

The bond dissociation energies of $X_2,Y_2$ and $XY$ are in the ratio of $1:0.5:1$. $\Delta H$ for the formation of $XY$ is $-200{\text{kJ mol}}^{-1}$. The bond dissociation energy of $X_2$ will be:

• A. $200{\text{kJ mol}}^{–1}$
• B. $100{\text{kJ mol}}^{–1}$
• C. $800{\text{kJ mol}}^{–1}$
• D. $400{\text{kJ mol}}^{–1}$

Answer 1

The correct option is C $800{\text{kJ mol}}^{–1}$

Let, bond dissociation energies of $X_2,Y_2$ and $XY$ be $x{\text{kJ mol}}^{-1},0.5x{\text{kJ mol}}^{-1},x{\text{kJ mol}}^{-1}$ respectively.
$\frac{1}{2}{X}_2+\frac{1}{2}{Y}_2\to XY;\Delta H=-200{\text{kJ mol}}^{-1}$
$\Delta H=\sum (B.E.{)}_{reactants}-(B.E.{)}_{products}$
$\therefore -200=\left[\frac{1}{2}\right(x)+\frac{1}{2}(0.5x\left)\right]-1\times x$
Bond dissociation energies of $X_2=800{\text{kJ mol}}^{–1}$.