The standard enthalpy change of combustion Hc of a substance is the enthalpy change when 1 mole of the substance is completely burned in oxygen under standard conditions 298 K and 1 atm pressure .For methane CH4 , the balanced combustion reaction is:CH4 g + 2 O2 g CO2 g + 2 H2O l The standard enthalpy change of combustion for methane can be determined using the standard enthalpies of formation Hf for the products and reactants:Hc = [ Hf products - Hf reactants ]The standard enthalpies of formation for the substances involved in the reaction are:Hf CH4 = -74.8 kJ/molHf O2 = 0 kJ/mol since O2 is in its standard state Hf CO2 = -393.5 kJ/molHf H2O = -285.8 kJ/molNow, we can calculate the standard enthalpy change of combustion for methane:Hc = [1 -393.5 + 2 -285.8 ] - [1 -74.8 + 2 0]Hc = -393.5 - 571.6 - -74.8 Hc = -965.1 + 74.8Hc = -890.3 kJ/molSo, the standard enthalpy change of combustion of methane gas CH4 at 298 K is approximately -890.3 kJ/mol.