Q75 / 160 · JIPMER 2006 · Chemistry
The enthalpy of combustion at $25^{\circ} \mathrm{C}$ of $\mathrm{H}_2$, cyclohexane $\left(\mathrm{C}_6 \mathrm{H}_{12}\right)$ and cyclohexene $\left(\mathrm{C}_6 \mathrm{H}_{10}\right)$ are $-241,-3920$ and $-3800 \mathrm{~kJ} / \mathrm{mol}$ respectively. The heat of hydrogenation of cyclohexene is :
- Correct answerA$-121 \mathrm{~kJ} / \mathrm{mol}$
- B$+121 \mathrm{~kJ} / \mathrm{mol}$
- C$-242 \mathrm{~kJ} / \mathrm{mol}$
- D$+242 \mathrm{~kJ} / \mathrm{mol}$
Solution
$\begin{aligned} & \mathrm{H}_2+\frac{1}{2} \mathrm{O}_2 \longrightarrow \mathrm{H}_2 \mathrm{O} \\ & \Delta H=-241 \mathrm{~kJ} . .\end{aligned}$
$\begin{aligned} \mathrm{C}_6 \mathrm{H}_{10}+\frac{17}{2} \mathrm{O}_2 \longrightarrow 6 \mathrm{CO}_2+5 \mathrm{H}_2 \mathrm{O} \\ \Delta H=-3800 \mathrm{~kJ} \ldots \text { (ii) }\end{aligned}$
$\begin{aligned} \mathrm{C}_6 \mathrm{H}_{12}+9 \mathrm{O}_2 \longrightarrow & 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \\ & \Delta H=-3920 \mathrm{~kJ} \ldots \text { (iii) }\end{aligned}$
hydrogenation of cyclohexene-
$\begin{aligned} & \mathrm{C}_6 \mathrm{H}_{10}+\mathrm{H}_2 \longrightarrow \mathrm{C}_6 \mathrm{H}_{12} \\ & \mathrm{Eq} \text { (i) } \begin{aligned} \mathrm{Eq} & \text { (ii) }-\mathrm{Eq} \text { (iii) } \\ \Delta H & =-241-3800-(-3920) \\ & =-4041+3920 \\ & =-121 \mathrm{~kJ}\end{aligned}\end{aligned}$
Hence, the correct answer is option: 'A'.