Energetics 問題

Firstly, you need to know the term entropy S
Entropy is the tendency of a substance to acheive randomness.

For diamond , each carbon atom is tetrahedrally bonded to four other carbon atoms(sp3 hybridized). All the four bonds are covalently bonded. It forms a fine and regular structure.

For graphite, the carbon atoms are more randomly distrubuted. Eg. sliding effect because of the delocalized p-orbitals.

Then according to entropy , the higher the randomness(entropy) , the more stable the substance it is.

By the way, the enthalpy of formation of diamond is more endothermic than the enthalpy of formation of graphite. THe more endothermic for the formation, the less stable it is.