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 $-200kJmo{l}^{-1}$. The bond dissociation energy of $X_2$ will be:

• A. $800kJmo{l}^{-1}$
• B. $200kJmo{l}^{-1}$
• C. $400kJmo{l}^{-1}$
• D. $100kJmo{l}^{-1}$

Answer 1

The correct option is A $800kJmo{l}^{-1}$

$\Delta H$ for the formation of $XY$ is $-200kJmo{l}^{-1}$.

The bond dissociation energies of $X_2,Y_2$ and $XY$ are in the ratio of $1:0.5:1$.
Let the bond dissociation energy of $X_2$ will be $a$ kJ/mol.
The bond dissociation energy of $Y_2$ will be $0.5a$ kJ/mol.
The bond dissociation energy of $XY$ will be $a$ kJ/mol.

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

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

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

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

$-\text{200}=-0.25\text{(a )}$

$\text{a}=\text{800}$

The bond dissociation energy of $X_2$ will be $800kJ/mol$