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?