The standard enthalpy of formation Hf of a compound is the change in enthalpy when 1 mole of the compound is formed from its constituent elements in their standard states at a specified temperature and pressure, which is usually 25C and 1 atm.For methane CH4 , the formation reaction is:C s + 2H2 g -> CH4 g The given enthalpy change for this reaction is -74.8 kJ/mol. Since this reaction represents the formation of 1 mole of methane from its constituent elements in their standard states, the standard enthalpy of formation of methane Hf is equal to the enthalpy change of this reaction.Therefore, the standard enthalpy of formation of methane CH4 at 25C and 1 atm is -74.8 kJ/mol.