Enthalpy change of formation?

Question

Which from the following is the enthalpy change of formation of CO2

C(graphite) + O2 --> CO2 (Delta H = -393.5kJ/mol)

or

C (diamond) + 02 --> C02 (Delta H = -395.4 kJ/mol)

Is it anyone of them or do I have to obtain and average?

Answer 1

The difinition of DHf says that the element must be in its normal state at the temperature specified, which is graphite. Look up DHf of diamond, and it has a value which is non-zero! In fact, it is the difference between your two values.