Does energy come from fuel or oxygen? (surprise?)

Question

Question: Is it oxygen or fuel that releases energy during burning?

The answer from this article:

Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2 (Journal of Chemical Education, 2015)

Our analysis provides simple answers to two related, important questions: “What makes fire hot?” and “How much heat is produced?” in the combustion of a certain fuel. One can now explain that the double bond in O2 is unusually weak, and therefore the formation of the stronger bonds in CO2 and H2O results in the release of heat or an increase in thermal motion.

Generally, our analysis indicates that it is not the organic fuel but rather O2 that is “energy-rich”.

It is usually said that fuel contains energy and that oxygen only enables the release of energy in the sense like enzymes enable reactions. Does anyone want to comment this? Because, from this article it seems that it is just the opposite: fuel enables the release of energy from oxygen.

(This question was initially nutrition-related: Does energy come from nutrients or oxygen?)

Answer 1

[OP:] It is usually said that fuel contains energy and that oxygen only enables the release of energy in the sense like enzymes enable reactions.

An enzyme is a catalyst, so it does not change the enthalpy of a reaction. That part is correct. "Oxygen only enables the release of energy" is incorrect. Oxygen is one of the reactants, and the oxygen atoms also appear as products.

[Dr. Schmidt-Rohr in JChemEd paper] Generally, our analysis indicates that it is not the organic fuel but rather O2 that is “energy-rich”.

This might be true in the context the combustion reaction with the set of organic molecules he chose to include. The organic fuels exclusively had C-H, C-C, C=C, O-H, C-O and C=O bonds, if I understand correctly from the abstract and the figure. He is saying that the bond energy of most bonds in reactants and products are similar (e.g. C-H vs O-H, C=C vs C=O, 2 C-C vs C=O), and that the O=O "double bond" is an outlier.

I think it is worthwhile to state the amazing fact that we have a biradical (see https://chemistry.stackexchange.com/a/15061) in our atmosphere and are breathing in a very reactive substance. The only reason that our red blood cells aren't constantly undergoing mutations from oxidative damage is that they lack DNA once they are released into the bloodstream.

If I were to study the combustion reactions with elemental metals, I might come to a different conclusion than the paper. Try comparing $\ce{Na + O2}$ with $\ce{Hg + O2}$.

Final remark (quoting myself):

When a raw egg rolls off a countertop and falls on the floor, does it crash because the countertop is so high or because the floor is so low?

Answer 2

Energy usually comes from reactions not one component of the reactions

Describing oxygen as "energy rich" doesn't make sense outside of the context of reactions oxygen can participate in. And, in that context, it doesn't make sense to partition the "energy* between the things that are reacting.

Chemical reactions can release energy. This happens because the bonds in the results of the reaction are in a lower energy state than those in the starting materials. Oxygen often participates in exothermic energy-releasing reactions because the resulting bonds (eg the bonds between carbon and oxygen in carbon dioxide) are stronger (or in a lower energy state) than the sum of all the bonds in the starting materials.

It is the strength of the bonds in the product that is significant here and not the "energy stored" in the starting materials (eg a hydrocarbon and oxygen gas). It isn't that dioxygen molecules "store" energy, it is that many oxygen compounds in the products are in low energy states with strong bonds. The release of energy makes sense only in the context of a chemical reaction and not in terms of one of the starting materials. Many compounds with strong bonds to oxygen (even silicon dioxide (silica) can still release lots of energy when reacted with the right compound (which is why they don't let schools do experiments with chlorine trifluoride which sets sand on fire).

And the idea that oxygen is the "store" of energy will confuse us when talking about compounds which, by themselves, are effective stores of energy. Many explosives can be persuaded to undergo strongly exothermic reactions without having to do something as gross as reacting with another compound but just by decomposing. Describing these as "stores" of energy makes sense. Getting compounds like nitroglycerine or nitrogen triiodide to release that energy sometimes takes little more than a bad glance or the touch of a feather. But try persuading pure oxygen to do anything similar and you won't get very far.

In short, describing one component of a reaction as a "store" of energy distracts form the key issue in chemistry which is thinking about reactions and confuses reactions involving multiple components with decompositions that release energy all by themselves without having to engage with other molecules.