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

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

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?)

• Surprised that it makes its way to an ACS journal. What is pointed out it is obvious and not a surprise at all. It might be still necessary to bring it to the attention of a class, but I would say in general and rather elementary education. And surely (now?)adays teachers might spread misconceptions. That a reactant behaves as a catalyst might be one of them, as seen in your question when you mentioned oxygen as comburent treated as an enzyme. – Alchimista Jul 18 '19 at 11:29
• Not to be pedantic. I think is worth pointing to this fact, and might be indeed necessary. But already the title is at least curious, as for exothermic reactions that are not combustions exist. Or their mentioning that fire is always hot. I just comment because not a real answer comes to my mind. Except that on elemental basis yes, most of the energy comes from oxygen changes - tge rest being necessary to order fuels for the energy that they release/allow to be released. – Alchimista Jul 18 '19 at 12:23
• Alchimista, this is an educational article in the Journal of Chemical Education. Why are you surprised :-)? It is an educational article. – M. Farooq Jul 18 '19 at 13:28
• Because as it is is not highly educational starting from the title. @M. Farooq because as I said it might be important to point out facts which are probably obscured by bad practices and /or colloquial wording. But those facts shouldn't be presented as a sort of findings nor a title like this makes sense. Plus a lot of considerations - more substantial or just technical and semantic - that are rooted in thermohenistry. The entire idea of where the energy comes from is exaggerated as for we are dealing with reaction hentalpy not with the formation hentalpy of reactants. – Alchimista Jul 19 '19 at 9:43
• It seems that the statement "combustions are always exothermic" is simply based on the defintion of combustion, rather than something fundamental about the chemistry. For instance, the formation of nitric oxide, $\ce{1/2N2(g) + 1/2O2(g) →NO(g)}$, is endothermic, with $\Delta _f H^{\circ}[NO]=91.3 kJ/mol$. However, combustion is typically defined as an exothermic reaction. [See, e.g., en.wikipedia.org/wiki/Combustion] Thus while redox reactions exist in which a substance combines with oxygen or some other oxidant endothermically, these aren't combustion reactions only by definition. – theorist Apr 26 at 1:49

[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?

• Excellent answer (+1) and I loved the egg thing! I have always believed that it is the chemical reaction, in all its (often very complex and/or obscure) glory that matters. So, for example, powdered PTFE is the oxidizer of powdered Mg 'fuel' in MTV countermeasure flares (en.wikipedia.org/wiki/Flare_(countermeasure) ). Of course, PTFE is just the generic version of Dupont's trademarked Teflon and, aside from run-ins with NaK and such like, I have never considered my Teflon/PTFE beakers and such to be particularly high energy! There are also 'cold flame' combustions that are hand safe. – Ed V Jul 18 '19 at 16:14
• and does the egg crash because its shell is weak or because the floor is hard? – cbeleites unhappy with SX Jul 19 '19 at 21:20

Energy usually comes from reactions not one component of the reactions

The problem with the way the description of oxygen as "energy rich" is that it 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 that make up the results of the reaction are in a lower energy state than the starting materials. The reason why oxygen often participates in exothermic, energy releasing reactions, is 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 are significant here 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 to oxygen in those products. The release of energy makes sense only in the context of a chemical reaction not in terms of a single starting material. 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 is combustion reactions is confusing for another reason. It will confuse us when talking about compounds which, but 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. 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 isn't going to get you 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 the release energy all by themselves without having to engage with other molecules.