Question

The standard molar heats 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 ethane.

Answer 1

The required chemical equation for combustion of ethane is
$2C_2{H}_6\left(g\right)+7O_2\left(g\right)=4{CO}_2\left(g\right)+6H_{2}O\left(l\right);\Delta H^o=?$
The equation involves $2$ moles of $C_2{H}_6$; heat of combustion of ethane will be $\frac{\Delta H^o}{2}$
$\Delta H^o=\Delta H_{f\left(products\right)}^o-\Delta H_{f\left(reactants\right)}$
$[4\times \Delta H_{\left({CO}_2\right)}^o+6\Delta H_{\left(H_{2}O\right)}^o]-[2\Delta H_{\left(C_2{H}_6\right)}^o+7\Delta H_{\left(O_2\right)}^o]$
$=[4\times (-94.1)+6\times (-68.3\left)\right]-[2\times (-21.1)+7\times 0]$
$=-744.0kcal$
$=\frac{\Delta H^o}{2}=$ Heat of combustion of ethane
$=-\frac{744.0}{2}=-372.0kcal$