Talk:Combustion

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Fringe theories

I removed the following paragraph, and it has been re-added:

Combustion of an organic fuel in air is always exothermic because the double bond in O₂ is much weaker than other double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in the combustion products CO₂ and H₂O results in the release of energy.^[1] The bond energies in the fuel play only a minor role, since they are similar to those in the combustion products; e.g., the sum of the bond energies of CH₄ is nearly the same as that of CO₂. The heat of combustion is approximately −418 kJ per mole of O₂ used up in the combustion reaction, and can be estimated from the elemental composition of the fuel.^[1]

The sole reference is a self-cited paper (the author of the paper is the editor who added the paragraph).

That the claims are also incorrect isn't really relevant to this, but I'd just like to point out that no great chemical insight is required to determine that they are. Combustion, for example, is exothermic by definition (so there's no need to explain why it is in a special case), and there are organic molecules which, depending on your definition, do not combust or require no O₂ to do so.

IpseCustos (talk) 09:00, 10 June 2022 (UTC)[reply]

This short paragraph makes correct and relevant scientific statements about bond energies and the heat of combustion. It states documented, verifiable facts based on universally accepted values of bond energies. That heats of reaction, including combustion, can be estimated from bond energies is also universally accepted in chemistry. A peer-reviewed reference providing all the details is provided for verification. In short, nothing is scientifically incorrect about this paragraph.

The claim that "combustion ... is exothermic by definition" is highly questionable and needs to be referenced. Combustion of an organic fuel is an example of a chemical reaction, and the heat of reaction depends on the bond energies or on the enthalpies of formation of reactants and products. How does any heat of reaction become negative (exothermic) by definition?

Furthermore, an "analysis" just "by definition" is unable to provide good quantitative estimates of the heats of combustion, unlike the last sentence in the quoted paragraph. Why, for instance, is the heat of combustion, per gram, of carbohydrates more than twice smaller than that of fat? Because carbohydrates, containing oxygen atoms, react with less O₂ than the corresponding mass of fat. A mere definition does not provide this useful result.

The last sentence by IpseCustos also does not hold up to careful scrutiny. The quoted paragraph, being about "Combustion of an organic fuel in air", makes no statement about "organic molecules which ... do not combust". Next, if organic molecules "require no O2 to do so." then they violate the definition of combustion given in the first sentence of the Wikipedia article and possibly the premise of "Combustion of an organic fuel in air".

Editors are of course free to replace this paragraph with a better, even more widely applicable and more accurate explanation (properly sourced) of the heat of combustion and its quite strict proportionality with the amount of oxygen consumed. But just deleting this correct short paragraph with its explanation of a central quantitative aspect of the heat of combustion is not in the interest of Wikipedia readers.

Klaus Schmidt-Rohr (talk) 14:58, 10 June 2022 (UTC)[reply]

I don't think I've ever seen educational material present combustion as anything but exothermic by definition — if a reaction qualifies as combustion, it is exothermic, simply because of the meanings of the words, though whether it is vigorously exothermic is then an empirical matter. To the concerns in my edit summary, I'd add that the details of energetics may be unsuitable for that point in the article. The typical reader of an article on a broad topic like combustion, particularly its opening paragraphs, may have only limited knowledge of which chemical bonds typically occur in fuel molecules. Consequently, citing a table of bond energies is basically assigning a homework problem, and one for which the student only has some of the necessary information. For encyclopedia content that is obligated to be verifiable, that's rather poor form. Moreover, the cited table of bond energies is by itself insufficient to say that one sum of energies is nearly the same as another. What counts as "nearly"? Citing the Chang (2010) textbook at that point merely disguises the fact that the nearly claim comes from a paper about which conflict-of-interest concerns have been raised. XOR'easter (talk) 17:31, 15 June 2022 (UTC)[reply]

References

^ ^a ^b Schmidt-Rohr, K (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O₂". J. Chem. Educ. 92 (12): 2094–2099. Bibcode:2015JChEd..92.2094S. doi:10.1021/acs.jchemed.5b00333.

[Schmidt-Rohr_15-1] Schmidt-Rohr, K (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O₂". J. Chem. Educ. 92 (12): 2094–2099. Bibcode:2015JChEd..92.2094S. doi:10.1021/acs.jchemed.5b00333.

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