Question

The enthalpies of combustion at ${25}^o$C of $H_2$, cyclohexane $\left(C_6{H}_{12}\right)$ and cyclohexene $\left(C_6{H}_{10}\right)$ are $-241,-3920$ and $-3800$ kJ/mol respectively. The heat of hydrogenation of cyclohexene is:

• A. $-121$ kJ/mol
• B. $+121$ kJ/mol
• C. $-242$ kJ/mol
• D. $+242$ kJ/mol

Answer 1

The correct option is C $-242$ kJ/mol

$H_2+\frac{1}{2}{O}_2\to H_{2}O$

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

$C_6{H}_{12}+O_2\to 6C{O}_2+6H_{2}O$

From these reactions, we have to get the following reaction-

$C_6{H}_{10}\stackrel{H_2}{⟶}{C}_6{H}_{14}$

So on reversing the third reaction and add the first and second reaction-

So $\Delta H=(-241-3800)+3920=-242KJ$

Hence, option $C$ is correct.