The heat of formation Hf of a compound is the change in enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states at 25C and 1 atm pressure.For methane CH4 , the heat of formation Hf is -74.8 kJ/mol. This means that 74.8 kJ of energy is released when 1 mole of methane is formed from its constituent elements carbon and hydrogen under standard conditions.