Dear PF Forum, I have read this link in Wiki. But I don't understand what it means. Can someone help me? https://en.wikipedia.org/wiki/Heat_of_combustion A: Heat of combustion of CH4 is 50.09 MJ/kg https://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table) B: Enthalpy of water: -285.88 KJ/mol C: Enthalpy of CO2: -393.509 KJ/mol Then, I'll try to do a simple calculation. A: CH4 heat of combustion is 50.09 MJ/kg. What does it mean? Does it mean that if we burn 1 kg of CH4 completely CH4 + 2O2 -> CO2 + 2H2O It will produce 50.09 mega joules? B: Enthalpy of water is 285.88 kj/mol What does it mean? If we combine 2 moles of Hydrogen and 1 moles of Oxygen, it will explode and gives 285.88 kilo joules? C: Then I did some calculation... The heat of combustion of 1 moles CH4 + 2 moles O2 -> 1 moles CO2 + 2 moles H2O 16 gr CH4 + 32 gr O2 -> 44 gr CO2 + 36 gr H2O will gives 50.09 MJ/kg * 16 gr = 801.44 KJ The enthalpy of 1 moles H2O: -285.88 KJ 1 moles CO2: -393.519 KJ Combining those two: 2 moles H2O: -571.76 KJ/mol 1 moles CO2: -393.519 KJ/mol = 965.269 KJ 801.44 KJ ≠ 965.269 KJ Where did I go wrong? Or my understanding of the concept of heat combustion and enthalpy is wrong. Thanks for any answer.