The heat of formation of ${\mathrm{CO}}_2$ is $-407 \mathrm{kJ}/\mathrm{mol}.$ The energy required for the process $3{\mathrm{CO}}_2\left(\mathrm{g}\right)\stackrel{ }{\to }3\mathrm{C}\left(\mathrm{g}\right)+2{\mathrm{O}}_3\left(\mathrm{g}\right) \mathrm{is}$ 

$$\begin{gathered} 3{\mathrm{CO}}_2\left(\mathrm{g}\right)\stackrel{ }{\to }3\mathrm{C}\left(\mathrm{g}\right)+2{\mathrm{O}}_3\left(\mathrm{g}\right), ∆\mathrm{H}=?---\left(\mathrm{a}\right) \\ \mathrm{Given} {\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {{\mathrm{CO}}_2}_{\left(\mathrm{g}\right)} , ∆{\mathrm{H}}_{\mathrm{f}}=-407\mathrm{kJ}/\mathrm{mol} ---\left(1\right) \end{gathered}$$

Eq.(a) is obtained as follows

$$\begin{gathered} \frac{1}{\left(1\right)}\times 3\Rightarrow {3\mathrm{CO}}_2\left(\mathrm{g}\right)\to 3\mathrm{C}\left(\mathrm{g}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right) , ∆{\mathrm{H}}_1=3\left(407\right)\mathrm{KJ}/\mathrm{mol} \\ \underline{ 3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{O}}_3\left(\mathrm{g}\right) , ∆{\mathrm{H}}_2=\mathrm{x} } \\ 3{\mathrm{CO}}_2\left(\mathrm{g}\right) \to 3\mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{O}}_3\left(\mathrm{g}\right) , ∆\mathrm{H} = 1221+\mathrm{x} \end{gathered}$$

As energy will be required for the processes

$$\begin{gathered} \mathrm{C}\left(\mathrm{s}\right)\stackrel{ }{\to }\mathrm{C}\left(\mathrm{g}\right) \mathrm{and} 3{\mathrm{O}}_3\left(\mathrm{g}\right) \stackrel{ }{\to }2{\mathrm{O}}_3\left(\mathrm{g}\right) \\ \therefore ∆\mathrm{H} \mathrm{will} \mathrm{be} \mathrm{more} \mathrm{than} 1221 \mathrm{kJ}. \end{gathered}$$