If C (diamond) →C\rightarrow \mathrm{C}→C (graphite) +X kJ mol−1+ X \,\text{kJ}\,\text{mol}^{-1}+XkJmol−1 C(\mathrm{C}(C( diamond )+O2(g)→CO2(g)+Y kJ mol−1)+ \mathrm{O}_2( \mathrm{g}) \rightarrow \mathrm{CO}_2( \mathrm{g})+ Y \,\text{kJ}\,\text{mol}^{-1})+O2(g)→CO2(g)+YkJmol−1 C (graphite) +O2(g)→CO2(g)+Z kJ mol−1+ \mathrm{O}_2( \mathrm{g}) \rightarrow \mathrm{CO}_2( \mathrm{g})+ Z \,\text{kJ}\,\text{mol}^{-1}+O2(g)→CO2(g)+ZkJmol−1 at constant temperature. Then
X=−Y+ZX=-Y+ZX=−Y+Z
X=Y−ZX=Y-ZX=Y−Z
X=Y+ZX=Y+ZX=Y+Z
−X=Y+Z-X=Y+Z−X=Y+Z