question_answer

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

A)  \[-121\text{ }kJ\] per mol

B)  \[+121\text{ }kJ\] per mol

C)  \[+242\text{ }kJ\] per mol

D)  \[-242\text{ }kJ\] per mol

Correct Answer: A

Solution :

  \[\Delta H=?\] \[\Delta H=[\Delta H\]of combustion of cyclohexan\[-(\Delta H\] of combustion of cyclohexene\[+\Delta H\]of combustion of\[{{H}_{2}}\])] \[=-[-3920-(3800-24)]\text{ }kJ\] \[=-[3920+4041]\] \[=-[121]kJ\] \[=-121kJ\]