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Heat of Combustion, Enthalpy

  1. Sep 29, 2015 #1
    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.
     
  2. jcsd
  3. Sep 29, 2015 #2

    Bystander

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    Check the "phase" of water (liq/vap).
     
  4. Sep 29, 2015 #3
    Of course. Thanks. The numbers are close now.
     
  5. Sep 29, 2015 #4
    Liquid water vs water vapor accounts for only about half the difference. You also forgot to subtract the heat of formation of methane, which is -75 kJ/mole.

    Chet
     
  6. Oct 3, 2015 #5

    James Pelezo

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    You might also read about Hess's Law Equation. That is, if you haven't already done so. Heat of Rxn (or, Heat of Combustion in this case) = (Sum of Enthalpies of Formation of Products) - (Sum of Enthalpies of formation of Reactants) Google 'Enthalpy of Formation Tables' You'll get many 'energy of formation' values to apply.
     
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