$\underset{(\text{(graphite)}}{\mathrm{C}}+\underset{\left(g\right)}{{\mathrm{O}}_2}\to \underset{\left(g\right)}{{\mathrm{CO}}_2},\mathrm{\Delta H}=-393.5\mathrm{KJ}$

$\mathrm{\Delta H}$ of above reaction cannot be

$\underset{\left(\text{graphite}\right)}{\mathrm{C}}+\underset{\left(g\right)}{O_2}\to \underset{\left(g\right)}{{\mathrm{CO}}_2}$

In above reaction one mole ${\mathrm{CO}}_2$ formed It is of combustion reaction above reaction takes place at constant P, T i.e $\mathrm{\Delta }H$ heat of reaction