I'm calculating the energy density of certain flammable compounds in MJ/kg. To do this I compare the sum of the bond strengths of the reactants to the products and then extrapolate the energy density from the difference in the molar binding energy of each compound.

For methane my result was almost exactly 50 MJ/kg while the Wikipedia result says it's 55 MJ/kg.

For ethanol my result was 28.6 MJ/kg while Wikipedia says it's 26.4 MJ/kg

I don't expect my calculations to be spot on but I didn't expect them to be this off.

What are the possible reasons for my calculated energy density to be quite different than the supposed actual values?

  • 1
    $\begingroup$ Do you assume the product water being in liquid form or vapour form? The value of 55.5 MJ/kg for methane includes water in liquid form. The corresponding value for water in vapour form is 50.0 MJ/kg. $\endgroup$ – Loong Sep 15 '16 at 20:45
  • $\begingroup$ So the hydrogen bonds in the liquid water contribute to the number found on Wikipedia? If so then do you have any idea what happened with my ethanol calculation? Thanks for the feedback. $\endgroup$ – Derek Farkas Sep 15 '16 at 20:49
  • $\begingroup$ As far as I know, the formula involving bond enthalpies is approximate and valid only when all reactants and products are in gaseous form. This is because the formula neglects the intermolecular forces of attraction. SO the results you got I would say are approximately correct considering you obtained them from bond enthalpy calculations. $\endgroup$ – Amritansh Singhal Sep 16 '16 at 5:48

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