A company that makes synthetic diamonds needs to know the standard enthalpy change for the conversion of graphite into diamond. The direct conversion requires extremely high pressure and temperature, so its enthalpy change cannot be measured directly. Both graphite and diamond burn completely in oxygen. The standard enthalpy change of combustion of graphite is −393.5 kJ mol⁻¹ and that of diamond is −395.4 kJ mol⁻¹.
Suggest how the standard enthalpy change of the conversion could be determined using Hess's law and combustion data only. In your answer, describe the energy cycle, write the equations involved, state how the enthalpy change of the conversion is related to the two combustion enthalpy changes, and explain what the values show about the relative stability of graphite and diamond.