enthalpy change

Thestandard enthalpy change of formation of a substance is the enthalpy changewhen one mole of the substance is formed from its element in standard state.

Diamond is an allotrope of carbon, and the standard state of carbon is graphite. Therefore, the standard enthalpy change offormation of diamond involves the following equation :
C(graphite) → C(diamond)

Theenthalpy of diamond and that of graphite are different, as they have differentstructures. Therefore, the reactioninvolves non-zero enthalpy change.