Answer
Please see the work below.
Work Step by Step
Hess’s law states that the overall enthalpy is the same whether the reaction takes place in one step or in several steps.
This law is helpful in the calculation of enthalpy of allotropic transformations.
We are able to calculate the enthalpy of the transformation of $C_{graphite} → C_{diamond}$
If we are given
Equation 1: $C(graphite) + O2(g) →CO2(g)$ $ H^{\circ}rxn = - 393.5 kJ/mol$
Equation 2: $C(diamond) + O2(g) →CO2(g)$ $ H^{\circ}rxn = - 395.4 kJ/mol$
By adding equation 1 with the reversed equation of 2 we get the enthalpy of the transformation of $C_{graphite} → C_{diamond}$.
$\Delta H_{r} = - 393.5 – (- 395.4) kJ/mol = + 1.9 kJ/mol $
