A) -121 kJ/mol
B) +121 kJ/mol
C) -242 kJ/mol
D) +242 kJ/mol
Correct Answer: A
Solution :
Key Idea: Use Hess's law to solve the problem. It states that total heat changes during a reaction are independent of path. \[{{H}_{2}}+\frac{1}{2}{{O}_{2}}\xrightarrow[{}]{{}}{{H}_{2}}O\] \[\Delta H=-241\,kJ\] ?. (i) \[{{C}_{6}}{{H}_{10}}+\frac{17}{2}{{O}_{2}}\xrightarrow{{}}6C{{O}_{2}}+5{{H}_{2}}O\] \[\Delta H=-3800\text{ }kJ\] ...(ii) \[{{C}_{6}}{{H}_{12}}+9{{O}_{2}}\xrightarrow{{}}6C{{O}_{2}}+6{{H}_{2}}O\] \[\Delta H=-3920\text{ }kJ\] ...(iii) \[\underset{Cyclohexene}{\mathop{{{C}_{6}}{{H}_{10}}}}\,+{{H}_{2}}\xrightarrow{{}}{{C}_{6}}{{H}_{12}}\] \[\Delta H=?\] Adding Eqs. (i) and (ii) and subtracting Eq (iii) we get, \[{{C}_{6}}{{H}_{10}}+{{H}_{2}}\xrightarrow{{}}{{C}_{6}}{{H}_{12}}\] \[H=-241-3800-(-3920)\] \[=-4041+3920~\] \[=-121kJ\]
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