Question

The enthalpy of combustion at $25^{\circ} C$ 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

Answer: (A)

$H _{2}+\frac{1}{2} O _{2} \longrightarrow H _{2} O$
$\Delta H=-241\, kJ$ ... (i)
$C _{6} H _{10}+\frac{17}{2} O _{2} \longrightarrow 6 CO _{2}+5 H _{2} O$
$\Delta H=-3800\, kJ$ ...(ii)
$C _{6} H _{12}+9 O _{2} \longrightarrow 6 CO _{2}+6 H _{2} O$
$\Delta H=-3920\, kJ$ ...(iii)
hydrogenation of cyclohexene-
$C _{6} H _{10}+ H _{2} \longrightarrow C _{6} H _{12}$
Eq (i) }+ Eq (ii) - Eq (iii)
$\Delta H =-241-3800-(-3920)$
$=-4041+3920$
$=-121\, kJ$