Q. The enthalpy of combustion of $H_{2}$, cyclohexane $(C_{6}H_{10})$ and cyclohexane $(C_{6}H_{12})$ are $- 241,\ - 3800$ and $- 3920$ kJ per mol respectively. Heat of hydrogenation of cyclohexane is
A) 121 kJ/mol
B) $- 121$ kJ/mol
C) $+ 242$ kJ/mol
D) $- 242\$kJ/mol
Explanation
(b)
Given: (i)$H_{2} + \frac{1}{2}O_{2} \rightarrow H_{2}O;\ \mathrm{\Delta}H = - 241\$kJ
(ii)$C_{6}H_{10} + \frac{17}{2}O_{2} \rightarrow 6CO_{2} + 5H_{2}O;\ \mathrm{\Delta}H = - 3800$ kJ
(iii)$C_{6}H_{12} + 9O_{2} \rightarrow 6CO_{2} + 6H_{2}O;\ \mathrm{\Delta}H = - 3920$ kJ for the reaction
$C_{6}H_{10} + H_{2} \rightarrow C_{6}H_{12}$
[It is infact Eq.(i)+Eq.(ii)$-$Eq.(iii)]
Thus, $\mathrm{\Delta}H = - 241 - 3800 - ( - 3920) = - 121$ kJ
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.