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If the intro paragraphs are correct, this is a great illustration of why so many subjects in math, stats and science are so much more difficult and unapproachab
by icedistilled 6y ago
If the intro paragraphs are correct, this is a great illustration of why so many subjects in math, stats and science are so much more difficult and unapproachable than they should be. Often there are no clear explanations of why fundamental principles make sense or actually work in a way normal people can understand. In this case it seems to have gone beyond that, in that there wasn't even an unclear explanation apparently.
"Surprisingly, a simple, valid explanation of the
exothermicity of combustion reactions has apparently not been
provided, [1−5] not even in textbooks on combustion [6−9] or in the
be provided in conceptual general-chemistry terms and results in a simple, predictive formula for heats of combustion.
The heats of reaction of a few specific combustion reactions have been explained in terms of bond energies, [5,7] but this has not revealed why all combustions of organic molecules are exothermic. Most bond-energy analyses have remained opaque since double bonds were treated on par with single bonds. We count double bonds as two bonds since the total number of electron-pair bonds is the same in reactants and products; only when the number of bonds remains unchanged is a general analysis per bond meaningful. On this basis, it becomes apparent that combustions are exothermic because of the unusually small bond-dissociation energy of O2 (per pair of
technical literature on heats of combustion"
- wcp 6y agoI have a Ph.D. in chemistry and I have taught general chemistry at a large research university and at a small liberal arts college. For what it's worth I teach this explanation and know of other colleagues that do, and the explanation is not novel to this article. That said, it's probably less well known and less widely taught than it should be. It's counterintuitive to many people that the formation of strong bonds results in the production of heat!
- gizmo686 6y ago> It's counterintuitive to many people that the formation of strong bonds results in the production of heat! Not an expert in anything physics, but this strike me as a result that, while counterintuitive, should be obvious after a moment of thinking about it: as reaction tend to favor low energy states, a strong bond bond would mean low energy; and therefore the energy has to go somewhere.
- CamperBob2 6y agoOr, put another way: strong bonds require the input of more energy to break them apart, so it's not too surprising if they emit energy in the form of heat at the time of formation.
- galaxyLogic 6y agoIt's counter intuitive because when we burn something say a wooden house it turns into ashes. Where are the stronger bonds then?
- thunderbong 6y agoThe strong bonds are in CO2 and H2O i.e. carbon dioxide and water vapour, both of which dissipate into the air.
- galaxyLogic 6y agoRight. It is true but counter-intuitive. For us humans strong bonds in a gas don't look like strong bonds.
- kurthr 6y agoIf it's a single step well, and the width of the well is fairly constant, then the steeper the slope is (stronger bonds are like stiffer springs) the deeper the well will be. You can extract the most energy with the deepest well. It seems like gravity potential energy mental model works moderately well for chemical potentials (or more accurate enthalpy) at relatively low temperatures (vs bond energy) for statistically large numbers of molecules.
- Gravityloss 6y agoIt takes a lot of energy to break a strong bond - ie that's the definition of a strong bond. So when you go the other direction, when the strong bond is formed, that energy is released.
- deleted 6y ago[deleted]
- Retric 6y agoYes, in the same way an object on mercury takes more energy to reach orbit than the moon, an object falling to mercury releases more energy than it would falling to the moon. What’s counter intuitive is what makes something a strong bond. Chemical bonds are more complex than simple gravity fields, further most chemistry involves both forming and breaking bonds. As such talking about specific bonds as strong or weak isn’t really an explanation so much as a corollary.
- Gravityloss 6y agoWith reversible reactions the energy to break it or make it should be equal and opposite.
- mncharity 6y ago> it's probably less well known and less widely taught than it should be This seems an important mechanism for science education content and instruction remaining wretched. Consider "a 5-year old asks 'the Sun is a ball?! What color is the ball?!". Certainly some instructors teach it correctly. But the top 10-ish most used introductory astronomy textbooks have it wrong. And thus so do most first-tier astronomy graduate students. And this state has been stable for decades. > the explanation is not novel to this article Science education research is distinct from the underlying science research. If those colleagues didn't write it up and publish it, perhaps because they didn't see chemistry education research as their field... oh well. The paper's existence makes it ever so slightly more likely some future content author gets it right, or is ever so slightly more embarrassed at having it wrong, and thus to revise it. Which over decades can sometimes move the needle. And being part of a research literature permits incremental collaborative correction, refinement, reference, and citation. > It's counterintuitive to many people that the formation of strong bonds results in the production of heat! A similar case. Some instructors do mention that attraction in bonds is almost all classical electrostatic attraction. Which makes this an intuitive extension of gravitational and electrostatic potential. But most instructors and textbooks don't. And so people struggle. Maybe a science education research paper or three might help. Or an interactive electron density model web app? (Something on my infinite todo list, using precomputed densities from GPAW:)
- esperent 6y agoGranted it's been a few years since I studied chemistry, but isn't the word "combustion" defined as "exothermic reaction"? At least that's what wiki states. If a reaction doesn't create heat, we don't call it combustion.