Question

The standard molar heat of formation of ethane. $ CO_{2}$ and water are respectively $-21.1, -94.1$ and -$68.3\, kcal$. The standard molar heat of combustion of ethane will be:

• A. - 372 kcal
• B. 162 kcal
• C. -240 kcal
• D. 183.5 kcal

Answer 1

Answer: (A)

2 Use the following formula to find heat of combustion of
ethane.
$\Delta H^{o}=\left(\Sigma H_{f_{\text {products }}^{o}}\right)-\left(\Sigma H_{f_{\text {reactants }}}^{o}\right)$
Ethane undergoes combustion according to following reaction
$C_{2} H_{6}(g)+\frac{7}{2} O_{2}(g) \rightarrow 2 C O_{2}(g)+3 H_{2} O(g)$
Given, heat of formation of ethane $=-21.1\, kcal$
Heat of formation of $C O_{2}=-94.1\, kcal$
Heat of formation of $H_{2} O=-68.3\, kcal$
Heat of formation of $O_{2}=0$
$\therefore H^{o}=\left[2 H_{f_{\left(C O_{2}\right)}}^{o}+3 H_{f_{\left(H_{2} O\right)}}^{o}\right]-\left[H_{f_{\left(C_{2} H_{6}\right)}}^{o}+\frac{7}{2} H_{f_{\left(C O_{2}\right)}}^{o}\right]$
$=[2 \times(-94.1)+3 \times(-68.3)]-[-2.11+(7 / 2 \times 0)]$
$=(-188.2-204.9)-(-21.1)=-393.1+21.1=-372\,kcal$