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. #ERROR!
• C. - 242 kJ/mol
• D. #ERROR!

Answer 1

Answer: (A)

- 121 kJ/mol

Answer 2

$H _{2}+\frac{1}{2} O _{2} \longrightarrow H _{2} O$ $\Delta H=-241\, kJ$ ... (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) $\Delta H =-241-3800-(-3920)$ $=-4041+3920$ $=-121\, kJ$