Question

The standard enthalpy of combustion at ${25}^{\circ }C$ of hydrogen, cyclohexene $\left(C_6{H}_{10}\right)$ and cyclohexane $\left(C_6{H}_{12}\right)$ are $-241,-3800$ and $-3920kJ/mole$ respectively. Calculate the heat of hydrogenation of cyclohexene

• A. $-111kJ/mole$
• B. $-121kJ/mole$
• C. $-118kJ/mole$
• D. $-128kJ/mole$

Answer 1

The correct option is B $-121kJ/mole$

The required reaction is
$\underset{Cyclohexene}{C_6{H}_{10}}+H_2\to \underset{Cyclohexane}{C_6{H}_{12}},△H_1=?.....\left(1\right)$
Given:
$H_2+\frac{1}{2}{O}_2\to H_{2}O$;
$△H_2=-241kJ/mole........\left(2\right)$
$C_6{H}_{10}+\frac{17}{2}{O}_2\to 6C{O}_2+5H_{2}O$,
$△H_3=-3800kJ/mole.......\left(3\right)$
$C_6{H}_{12}+9O_2\to 6C{O}_2+6H_{2}O$,
$△H_4=-3920kJ/mole.......\left(4\right)$
The required reaction $\left(1\right)$ can be obtained by adding equations $\left(2\right)$ and $\left(3\right)$, and subtracting $\left(4\right)$ from the sum of $\left(2\right)$ and $\left(3\right)$
$C_6{H}_{10}+H_2\to C_6{H}_{12}$
$△H_1=(△H_2+△H_3)-△H_4$
$=[-241+(-3800\left)\right]-(-3920)$
$=(-241-3800)-(-3920)$
$=-4041+3920=-121kJ/mole$.

Hence, option B is correct