Question

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

• A. $121\, kJ\, mol^{-1}$
• B. $-121\, kJ\, mol^{-1}$
• C. $+242\, kJ\, mol^{-1}$
• D. $ - 242\, kJ\, mol^{-1}$

Answer 1

Answer: (B)

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

$C _{6} H _{10}+\frac{17}{2} O _{2} \longrightarrow 6 CO _{2}+5 H _{2} O$

$\Delta \,H=-3800 \,kJ\,\,\,\,\dots(ii)$

$C _{6} H _{12}+9 O _{2} \longrightarrow 6 CO _{2}+6 H _{2} O$

$\Delta \,H=-3920\, kJ \,\,\,\,\dots(iii)$

For the reaction, $C _{6} H _{10}+ H _{2} \longrightarrow C _{6} H _{12}$

Adding Eqs. (i) and (ii) then subtracting Eq. (iii), we get

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