MCQ
The bond energies of $C -C, C = C, H -H$ and $C -H$ bonds are $350, 600, 400$ and $410\, kJ$ per mole respectively. The heat of hydrogenation of ethylene $(C_2H_4)$ is ......$ kJ\, mol^{-1}$
- ✓$-170$
- B$-260$
- C$-400$
- D$-450$
✓
Answer
Correct option: A.
$-170$
a
$\begin{array}{*{20}{c}}
{\,H\,\,\,\,\,H} \\
{|\,\,\,\,\,\,\,\,\,\,|} \\
{H - C = C - H}
\end{array}$ $+$ $H-H$ $\to $ $\begin{array}{*{20}{c}}
{\,\,H\,\,\,\,\,\,H} \\
{|\,\,\,\,\,\,\,\,\,\,|} \\
{H - C - C - H} \\
{|\,\,\,\,\,\,\,\,\,\,|} \\
{H\,\,\,\,\,H}
\end{array}$
$\begin{array}{*{20}{c}}
{\,H\,\,\,\,\,H} \\
{|\,\,\,\,\,\,\,\,\,\,|} \\
{H - C = C - H}
\end{array}$ $+$ $H-H$ $\to $ $\begin{array}{*{20}{c}}
{\,\,H\,\,\,\,\,\,H} \\
{|\,\,\,\,\,\,\,\,\,\,|} \\
{H - C - C - H} \\
{|\,\,\,\,\,\,\,\,\,\,|} \\
{H\,\,\,\,\,H}
\end{array}$
$\Delta \mathrm{H}$ reaction $=\Sigma \mathrm{B} . \mathrm{E} .$ (Rectant) $-\Sigma \mathrm{B} . \mathrm{E} .$ (product)
$=[\mathrm{B} \cdot \mathrm{E} \cdot(4 \mathrm{C}-\mathrm{H})+\mathrm{B} \cdot \mathrm{E} \cdot(\mathrm{C}=\mathrm{C})+\mathrm{B} \cdot \mathrm{E} \cdot(\mathrm{C}-\mathrm{C})]$
$-[\mathrm{B} \cdot \mathrm{E} \cdot 6(\mathrm{C}-\mathrm{H})+\mathrm{B} \cdot \mathrm{E} \cdot(\mathrm{C}-\mathrm{C})]$
$=\mathrm{B.E.}(\mathrm{C}=\mathrm{C})+\mathrm{B.E.}(\mathrm{H}-\mathrm{H})-2 \mathrm{B} \cdot \mathrm{E} \cdot(\mathrm{C}-\mathrm{H})-$
$\mathbf{B . E .}(C-C)$
$=600+400-2 \times 410-350\, \mathrm{kJ}$
$=-170\, \mathrm{kJ}$
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