Question

The enthalpy of combustion for the $H_2$, cyclohexene and cyclohexane are $-241,-3800$ and $-3920kJmo{l}^{-1}$, respectively. Heat of hydrogenation of cyclohexene is:

• A. $+121kJmo{l}^{-1}$
• B. $-121kJmo{l}^{-1}$
• C. $+242kJmo{l}^{-1}$
• D. $-242kJmo{l}^{-1}$

Answer 1

The correct option is B $-121kJmo{l}^{-1}$

Different combustion reactions are shown below.
$C_6{H}_{12}+9O_2\to 6C{O}_2+6H_{2}O$

$\Delta H_1=-3920$ kJ/mol ......(1)

$C_6{H}_{10}+8.5O_2\to 6C{O}_2+5H_{2}O$

$\Delta H_2=-3800$ kJ/mol ......(2)

$H_2+O_2\to H_{2}O$ $\Delta H_3=-241$ kJ/mol ......(3)

The balanced chemical equation for the hydrogenation of cyclohexene is

Cyclohexene $+H_2\to$ Cyclohexane .....(4)

Add reaction (3) to reaction (2). Then substract reaction (1) to obtain reaction (4).

$\Delta H_4=\Delta H_2-\Delta H_1+\Delta H_3$
$\Delta H_4=-3800-(-3920)+-241$

$\Delta H_4=-121$ kJ/mol

Hence, the heat of hydrogenation of cyclohexene is $\Delta H=-121$ kJ/mol