question_answer

                The enthalpy of combustion of \[{{H}_{2}}\], cyclohexene \[({{C}_{6}}{{H}_{10}})\] and cyclohexene \[({{C}_{6}}{{H}_{12}})\] are -241, -3800 and -3920 kJ per mol respectively. Heat of hydrogenation of cyclohexene is:                                                

A)                  -121 kJ per mol

B)                  +121 kJ per mol

C)                  +242 kJ per mol

D)                  -242 kJ per mol

Correct Answer: A

Solution :

                                \[\Delta H=\] [\[\Delta H\]of combustion of cyclohexane                 - (\[\Delta H\] of combustion of cyclohexene                 +\[\Delta H\]of combustion of H2)]                 \[=-[-3920-(3800-24)]\,kJ\]                 \[=-[3920+4041]\,kJ\]                 = -[121] kJ                 = -121 kJ