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Originally Posted by curtis73
I see what you're saying, but there is no net gain. Its a wash... zero. If it takes 10 joules to make it, you get 10 joules when you combust it. Energy is energy. You can't end up with a net gain because that would be creation of energy that isn't there.
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When you break apart octane into H2 and C, it requires exactly as much energy as it would have taken to make it from H2 and C. When you break octane into H2O and CO2, however, you first lose the energy to break the octane in to H2 and C, and then you gain the energy from making H2O and CO2 from the loose H2 and C.
hf (kJ/mol):
Octane: -249950 (liquid)
Octane: -208450 (gas)
Carbon dioxide: -393520
Water: -241820 (vapor)
Water: -285830 (liquid)
Acetylene: +226730
(Don't forget to balance the equation if you want to figure this out. I'm feeling lazy. For ease of calculation, let's assume that the reactants and products go in and come out at the standard state.)