Question

The standard molar heats of formation of ethane, ${CO}_2$ and $H_{2}O\left(l\right)$ are respectively $-21.1,-94,1$ and $-68.3kcal$. The standard molar heat of combustion of ethane will be:

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

Answer 1

The correct option is A $-372kcal$

$C_2{H}_6\left(g\right)+\frac{1}{2}{O}_2\left(g\right)⟶2C{O}_2\left(g\right)+3H_{2}O\left(g\right)$
$\Delta H=2\Delta H_{f\left(C{O}_2\right)}+3\Delta H_{f\left(H_{2}O\right)}-H_{f\left(C_2{H}_6\right)}$
$=2(-94.1)+3(=68.3)-(-21.1)=-372kcal$