Question

The standard heat of combustion of $C_2{H}_2\left(g\right),$ $C$ (graphite) and $H_2\left(g\right)$ are $-310\text{kcal}$, $-94\text{kcal}$ and $-98\text{kcal}$ respectively. The heat of formation of $C_2{H}_2\left(g\right)$ is:

• A. $24\text{kcal}$
• B. $148\text{kcal}$
• C. $26\text{kcal}$
• D. $284\text{kcal}$

Answer 1

The correct option is A $24\text{kcal}$

Given,
$C+O_2\left(g\right)\to C{O}_2\left(g\right)\Delta H_c=-94\text{kcal}......\left(1\right)H_2+\frac{1}{2}{O}_2\to H_{2}O\Delta H_c=-98\text{kcal}.......\left(2\right)C_2{H}_2+\frac{5}{2}{O}_2\to 2C{O}_2+H_{2}O\Delta H_c=-310\text{kcal}......\left(3\right)$
The required chemical reaction is:
$2C+H_2\left(g\right)\to C_2{H}_2\left(g\right)$
So the heat of formation of $C_2{H}_2$ can be obtained by using the following:
$2\times \text{equation}\left(1\right)+\text{equation}\left(2\right)-\text{equation}\left(3\right)\therefore \Delta H=-\left[\right(2\times 94)+98]+310=24\text{kcal}$