If \( \mathrm{C(diamond)} \rightarrow \mathrm{C(graphite)} + X \, \text{kJ mol}^{-1} \)
\( \mathrm{C(diamond)} + \mathrm{O}_2(g) \rightarrow \mathrm{CO}_2(g) + Y \, \text{kJ mol}^{-1} \)
\( \mathrm{C(graphite)} + \mathrm{O}_2(g) \rightarrow \mathrm{CO}_2(g) + Z \, \text{kJ mol}^{-1} \)
At constant temperature. Then
