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

Answer 1

Answer: (C)

$\Delta H$ 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 $kJ/mol$.
The bond dissociation energy of $Y_2$ will be $0.5akJ/mol$.
The bond dissociation energy of $XY$ will be $kJ/mol$.
$\frac{1}{2}{X}_2+\frac{1}{2}{Y}_2\to XY$
$\Delta H$ (reaction) $=\Sigma \Delta H$ (reactant bonds ) $-\Sigma \Delta H$ ( product bonds)
$\Delta H($ reaction $)=\frac{1}{2}\Delta H(X-X)+\frac{1}{2}\Delta H(Y-Y)-\Delta H(X-Y)$
$-200=\frac{1}{2}(a)+\frac{1}{2}(0.5a)-(a)$
$-200=-0.25\text{ (a) }$
$a=800$
The bond dissociation energy of $X_2$ will be $800kJ/mol$