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)

(i) $H _{2}+1 / 2 O _{2} \longrightarrow H _{2} O , \Delta H=-241 \,kJ$

(ii) $C _{6} H _{12}+9 O _{2} \longrightarrow 6 CO _{2}+6 H _{2} O , \Delta H=-3920 \,kJ$

(iii) $C _{6} H _{10}+\frac{17}{2} O _{2} \longrightarrow 6 CO _{2}+5 H _{2} O , \Delta H=-3800 \,kJ$

Required reaction is

$C _{6} H _{10}+ H _{2} \longrightarrow C _{6} H _{12}, \Delta H=?$

Eq. (i) + eq. (iii) - eq. (ii)

$\Delta H=-241-3800-(-3920)=-4041+3920=-121 \,kJ$