Question

The heat of combustion of $C_{\left(graphite\right)}$ is $-393.5kJ$ ${mol}^{-1}$. The heat of formation of ${CO}_2$ from graphite is ________ $kJ{mol}^{-1}$.

• A. $393.5$
• B. $-393.5$
• C. $-787.0$
• D. $787.0$

Answer 1

The correct option is B $-393.5$

Heat of combustion reaction of graphite means the reaction involving combustion of 1 mole of graphite i.e, reaction of 1 mole of graphite with enough oxygen.
$C+O_2\to C{O}_2$
Also heat of formation of $C{O}_2$ means the formation of 1 mole of $C{O}_2$ from its constituent elements i.e,
$C+O_2\to C{O}_2$
Since both the reactions are same. So the value of heat of formation of $C{O}_2$$\text{= heat of combustion of graphite = -393.5 KJ}$ $mo{l}^{-1}$.
Hence, answer is option B.