A) \[890.4\,kJ\,mo{{l}^{-1}}\]
B) \[-298.8\,kJ\,mo{{l}^{-1}}\]
C) \[-74.7\,kJ\,mo{{l}^{-1}}\]
D) \[-107.7\,kJ\,mo{{l}^{-1}}\]
Correct Answer: C
Solution :
\[C{{H}_{4}}+2{{O}_{2}}\to C{{O}_{2}}+2{{H}_{2}}O,\,\Delta H=-\,890.4\,kJ\] ?.(i) \[C+{{O}_{2}}\to C{{O}_{2}},\,\,\,\,\Delta H=-\,395.5\,kJ\] ?..(ii) \[C+{{O}_{2}}\to C{{O}_{2}},\,\,\,\,\Delta H=-\,395.5\,kJ\] .?.(ii) \[{{H}_{2}}+\frac{1}{2}{{O}_{2}}\to {{H}_{2}}O,\,\,\Delta H=-\,285.8\,kJ\] ?..(iii) from (i), (ii), (iii). \[\Delta {{H}_{comb\text{.}}}(C{{H}_{4}})\] \[=\Delta {{H}_{f}}(C{{O}_{2}})+2\Delta {{H}_{f}}({{H}_{2}}O)-\Delta {{H}_{f}}(C{{H}_{4}})-2\Delta {{H}_{f}}({{O}_{2}})\] \[=-890.4=-395.5+2\,(-285.5)-\Delta {{H}_{f}}(C{{H}_{4}})-2\times 0\] \[\Delta {{H}_{f}}(C{{H}_{4}})=-76.7\,kJ\,mo{{l}^{-1}}\].
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