The enthalpies of combustion ofC(graphite) and C( diamond) are $-393.5 \mathrm{kJ} {\mathrm{mol}}^{-1}$ and $-395.3 \mathrm{kJ} {\mathrm{mol}}^{-1}$, respectively. The enthalpy change of the reaction C(graphite) __, C(diamond) is

$\text{ (a) }\mathrm{C}\text{ (graphite })+{\mathrm{O}}_2(\mathrm{g})\to {\mathrm{CO}}_2(\mathrm{g})\mathrm{\Delta }{H}_{\mathrm{a}}=-393.5 \mathrm{kJ} {\mathrm{mol}}^{-1}$

$\text{ (b) }\mathrm{C}\left(\text{ diamond }\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)\mathrm{\Delta }{H}_{\mathrm{b}}=-395.3 \mathrm{kJ} {\mathrm{mol}}^{-1}$

The given transformation C(graphite) $\to$ C(diamond) is obtained by subtracting Eq. (b) from Eq. (a). Hence

$\mathrm{\Delta }H=\mathrm{\Delta }{H}_{\mathrm{b}}-\mathrm{\Delta }{H}_{\mathrm{a}}=[-393.5-(-395.3\left)\right] \mathrm{kJ} {\mathrm{mol}}^{-1}=1.80 \mathrm{kJ} {\mathrm{mol}}^{-1}$