Question

The standard molar heat of formation of ethane, $C{O}_2$ 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

The correct option is A $-372$ kcal

Given,
(a) $2C+3H_2\to C_2{H}_6;\Delta H=-21.1$ kcal

(b) $C+O_2\to C{O}_2;\Delta H=-94.1$ kcal

(c) $H_2+\frac{1}{2}{O}_2\to H_{2}O;\Delta H=-68.3$ kcal

Now, eqs $2\times \left(b\right)+3\times \left(c\right)-\left(a\right)$

$C_2{H}_6+\frac{3}{2}{O}_2\to 2C{O}_2+3H_{2}O$

Heat of combustion of Ethane $\Delta x=2(-94.1)+3(-68.3)-(-21.1)$
$=(-188.2)+(-204.9)-21.1$
$=-372$ kcal