Quote:
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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.
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I see the other flaw... in chemical reactions you simply meausure the net delta H. The equation doesn't show C8H18 breaking up into 4C2 and 9H2, then recombining with O2. The activation energy (spark) begins combustion, which does not break it down into H and C, it begins the combination of oxygen with the HC molecule. You don't split it up in to componenents and recombine, its just one reaction. The (unbalanced and simplified) equation looks like this:
C
8H
18 + O
2 + E
A = H
2O + CO
2 + heat
what you are suggesting is that the equation look like this:
C
8H
18 + heat = 8C + 9H
2 and THEN
C + H
2 + O
2 + E
A = H
20 + CO
2 + heat
... and that's just not the case.
Its all one reaction. You don't get a net gain, you simply unlock the heat stored in the bond of octane and the atoms settle into their end compounds. Its not a double reaction. Your enthalpy numbers only apply if that is the reaction that is being created. If you separate acetylene into H and C and then combine the H with O, your enthalpy will be that of a hydrogen combustion reaction. Period. You can't count the separation of acetylene because its not included in the reaction.
Combustion is combustion. Its a single equation, so the only enthalpy numbers you can use are those for THAT reaction, not any other reactions that could or might occur. Burning acetylene means you get the enthalpy involve with burning acetylene, not the enthalpy that you get from separating acetylene then burning its components.