Question

The standard molar heat of formation of ethane , carbon dioxide and liquid water are -21.1 , -94.1 , and -68.3 kcal respectively. Calculate the standard molar heat of combustion of ethene.

Answer 1

2C(s) +3H${}_2$(g) $\to$ C${}_2$H${}_6$(g) ${}^{\Delta H}f$=-21.1 kcal

C(s) +O${}_2$(g) $\to$ CO${}_2$(g) ${}^{\Delta H}f$=-94.1 kcal

H${}_2$(s) +$\frac{1}{2}$O${}_2$(g) $\to$ H${}_2$O(l) ${}^{\Delta H}f$=-68.3 kcal

C${}_2$H${}_6$ + $\frac{1}{2}$O${}_2$ $\to$ 2CO${}_2$ + 3H${}_2$O ${}^{\Delta H}{f}^{\prime }$

${}^{\Delta H}{f}^{\prime }$ =(21.1) -(2 X 91.1) -(68.3 x 3)

=-372 kcal mor${}^{-1}$