Question

The standard molar heat of formation of ethane, $C{O}_2$ and water (l) are $-21.2,-94.1$ and $-68.3kcal$ respectively. The standard molar heat of combustion of ethane will be :

• A. $-372kcal$
• B. $235.4kcal$
• C. $141.3kcal$
• D. $183.5kcal$

Answer 1

The correct option is A $-372kcal$

$△H_{reaction}=\sum △H$ of products $-\sum △H$ of reactants
$△H_f{C}_2{H}_6=-21.2kcal$,
$△H_{f}C{O}_2=-94.1kcal$,
$△H_f{H}_{2}O=-68.3kcal$
$C_2{H}_6+\frac{7}{2}{O}_2\to 2C{O}_2+3H_{2}O$
$△H_{combustion}=[2△H_{f}C{O}_2+3△H_f{H}_{2}O]-[△H_f{C}_2{H}_6+\frac{7}{2}△H_f{O}_2]$
$=[2\times (-94.1)+3(-68.3\left)\right]-[-21.2+\frac{7}{2}\times (0\left)\right]$
$=(-1882-204.9)+21.1$
$=-393.1+21.1$
$=-371kcal$