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> In hydrocarbons the majority of energy comes from the oxidation of hydrogen. That's not quite true. To add some numbers, the average formula for long-chain
by cyberax 2y ago
> In hydrocarbons the majority of energy comes from the oxidation of hydrogen.
That's not quite true.
To add some numbers, the average formula for long-chain hydrocarbons is roughly CH2 (one carbon atom for two hydrogen atoms). The enthalpy of formation of water is -286kJ/mole, and for carbon dioxide it's -394kJ/mole.
Conveniently enough, one mole of long-chain hydrocarbons produces one mole of water and one mole of carbon dioxide.
It's better for pure methane, as with its formula CH4 it produces 2 moles of water for each mole of CO2. So you get 572kJ of energy from hydrogens versus the same 394kJ from carbon.
- Tade0 2y agoFormation is just one part of the equation. On the other side there's an additional C-C bond per segment of the hydrocarbon chain longer than methane, which you need to break during combustion.
- cyberax 2y agoIt's already taken into the account. The CO2 enthalpy of formation has C-C bond breaking "baked in", by convention the "standard" form of an element has zero enthalpy of formation. For carbon, it's graphite with its C-C bonds. I did neglect the C-H bond enthalpy, but it's close enough to the C-C bond energy to matter too much.