The standard enthalpy of combustion of H2, C6H12, and cyclohexane (C611,2) at 25°C are - 241, - 3800, and - 3920 kJ mol-1 respectively. Calculate the heat of hydrogenation of cyclohexane.

We have to find ∆H forC6H10+H2→C6H12GivenH2+1/2O2→H2O;                     ΔH=−241kJ .…(1)C6H10+172O2→6CO2+5H2O;       ΔH=−3800kJ  ....(2)C6H12+9O2→6CO2+6H2O;       ΔH=−3920kJ ......(3)Adding equation (1) and (2) and then subtracting equation (3)C6H10+H2→C6H12 ΔH=−121kJ.'. Heat of hydrogenation of cylohexene = 121 kJ