The heat of formation of CO₂ is - 407 kJ/mol. The energy required for the process, 3CO₂g)  $\to$ 3C(g) + 2O₃(g) is

$\begin{array}{l}3{\mathrm{CO}}_2\left(\mathrm{g}\right)\to 3\mathrm{C}\left(\mathrm{g}\right)+2{\mathrm{O}}_3\left(\mathrm{g}\right)\\ \mathrm{\Delta }H=3\mathrm{\Delta }{H}_{\text{atomization }}\mathrm{C}\left(\mathrm{s}\right)\to \mathrm{C}\left(\mathrm{g}\right)+2\mathrm{\Delta }{H}_{\mathrm{f}}^{\circ }\left({\mathrm{O}}_3\left(\mathrm{g}\right)\right)-3\mathrm{\Delta }{H}_{\mathrm{f}}^{\circ }\\ \left({\mathrm{CO}}_2\left(\mathrm{g}\right)\right)\\ \mathrm{\Delta }H=3\mathrm{\Delta }{H}_{\text{atomization }}\mathrm{C}\left(\mathrm{s}\right)\to \mathrm{C}\left(\mathrm{g}\right)+2\mathrm{\Delta }{H}_{\mathrm{f}}^{\circ }\left({\mathrm{O}}_3\left(\mathrm{g}\right)\right)+3\times 407\end{array}$

As energy will be required for the processes

$\mathrm{C}\left(\mathrm{s}\right)\to \mathrm{C}\left(\mathrm{g}\right)\text{ and }3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{O}}_3\left(\mathrm{g}\right)$

.'. $∆\mathrm{H}$ will be more than 1221 kJ.