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$-200kJ{\text{mol}}^{-1}$. The bond dissociation energy of $X_2$ will be:

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

Answer 1

The correct option is A $800kJ{\text{mol}}^{-1}$

$\Delta H$ for the formation of $XY$ is $-200kJ{\text{mol}}^{-1}.$

The bond dissociation energies of $X_2,Y_2$ and $XY$ are in the ratio $1:0.5:1$.

Let the bond dissociation energy of $X_2$ will be $akJ/mol$.
The bond dissociation energy of $Y_2$ will be $0.5akJ/mol$.
The bond dissociation energy of $XY$ will be $akJ/mol$.

$\frac{1}{2}{X}_2+\frac{1}{2}{Y}_2\to XY$

$\Delta H\text{(reaction)}=\Sigma \Delta H\text{(reactant~bonds)}-\Sigma \Delta H\text{(product~bonds)}$

$\Delta H\text{(reaction)}=\frac{1}{2}\Delta H(X-X)+\frac{1}{2}\Delta H(Y-Y)-\Delta H(X-Y)$

$-200=\frac{1}{2}\left(a\right)+\frac{1}{2}\left(0.5a\right)-\left(a\right)$

$-200=-0.25\left(a\right)$

$a=800$

The bond dissociation energy of $X_2$ will be $800kJmo{l}^{-1}$