Why Combustion Is Exothermic
pubs.acs.org
pubs.acs.org
"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"
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.
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.
So when you go the other direction, when the strong bond is formed, that energy is released.
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.
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:)
This cracks me up. Despite a chemistry degree, I have never really thought to consider whether or not endothermic combustion exists, even though endothermic reactions exist. Nitrogen "combustion" with oxygen is endothermic. But really, the definition of combustion implies a high temperature, relatively rapid and self-sustaining reaction, e.g. rust and glucose metabolism aren't combustion.
If you make your pellets small enough, it is. The combustion of steel brushes is quite interesting to watch.
If you make your pellets too small, then is stops being an ordinary combustion and becomes an explosion.
https://en.m.wikipedia.org/wiki/Dust_explosion
Another interesting phenomenon is spontaneous combustion where exothermic reactions get hot enough to start the combustion process in a medium.
https://en.m.wikipedia.org/wiki/Spontaneous_combustion#:~:te....
I was a fire engineer in a past life, forgot the science but remember the cool phenomenons.
If a reaction were to be endothermic would we have called it combustion in the first place ?
Implied in the name there is this notion that the reagents are looking for an excuse to combust, a release of bottled potential, this in turn implies this has to be an energy producing reaction rather than one where the reagents need to be coerced into reacting by providing external energy
being endothermic or exothermic and activation energy are orthogonal.
as a matter of fact, burning wood with air is exothermic, but you don't see thees spontaneously burning until some activation energy is provided.
heck, it's not even a given that all exothermic reaction are self sustaining, there's plenty that will go on producing small quantity of heat but will required external energy to sustain the reaction; sometimes it's even the same reaction but in a different environment with a different chemistry or average temperature.
You can also drive reactions by removing the end products in lieu of providing external energy.
That also means that the energy we use to fuel our lives mostly comes directly from recent photosynthesis, and not actually from historical sunlight embodied in the fossil fuels we're burning.
Oxygen rusts metals.
Humans need to breathe it to survive. It's very reactive and fuels many of the domains of life.
But since it's so reactive it also wears you down and ages you. And causes cancer.
The narcissist view: The issues surrounding who should keep breathing is pretty serious. But the other benefits could be worth finding a solution...
Obviously not something I subscribe to. But those people with lack of consideration might.
AFAIK our oxygen was not made recently. It accumulated over billions of years.
https://en.wikipedia.org/wiki/Great_Oxidation_Event
This here tries to project oxygen levels:
> It is found that anthropogenic fossil fuel combustion is the largest contributor to the current O2 deficit, which consumed 2.0 Gt/a in 1900 and has increased to 38.2 Gt/a by 2015. Under the Representative Concentration Pathways (RCPs) RCP8.5 scenario, approximately 100Gt (gigatonnes) of O2 would be removed from the atmosphere per year until 2100, and the O2 concentration will decrease from its current level of 20.946% to 20.825%.
https://www.sciencedirect.com/science/article/pii/S209592731...
According to wikipedia [1], the atmosphere has 34e18 mol of oxygen, and photosynthesis produces 8800e12 mol/yr of oxygen, meaning the atmosphere turns over in a little under four thousand years. There's a separate estimate of atmospheric oxygen's "residence time" in the text, of 4500 years.
So although oxygen levels have been high for billions of years, there is a sense in which the oxygen in the atmosphere right now has only been there for thousands.
If it's "how long is it likely that the oldest free oxygen molecule I'm inhaling right now has been in the atmosphere", that's about 4k years.
If it's "how long has the atmosphere had a double-digit partial pressure of O2", very different question with a different answer.
Only if the reservoir is a fifo queue. :) Otherwise there's a distribution, with a tail. But oldest in atmosphere still looks (envelope scribbling...) order Myr? Rather than Gyr. Similar for breath?
Err it says the exact opposite:
> 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 CO2 and H2O results in the release of energy
Weaker bond = less energy needed to break it.
In your example, 9 moles of ethanol produces the same heat as 2 moles of isooctane, but the combustion products are substantially higher:
Ethanol: 9C2H5OH+27O2→18CO2+27H2O
Isooctane: 2C8H18+27O2→16CO2+18H2O
One important detail around use of hydrocarbons for fuel to power things like cars is that you need the energy to be delivered in a timely manner -- typically within milliseconds of the spark. Whereas something like a power plant could be happy to have just a hot pile of iron metal powder giving off heat for weeks at a time rather than in one massive flash explosion. Using something like iron powder that is in a pure Fe unoxidized state would require some process to get the iron into that state. Likely requiring energy -- so probably the cycle for iron powder power plants would be to use solar/hydro/wind power to generate the power liberate the oxygen from the FeO2 molecule, then transport the iron to the place where it would be used that has poor access to solar/hydro/wind, then use that there.
Anyway, articles like this really do a great job of making people think about things..
Edit: slightly changed wording
And this, in turn, is a specific example of a general principle: breaking any chemical bond always requires energy, while forming any bond always releases energy.
(e.g. it's thought that many asteroids are like Saudi Arabia but with the relative proportions of sand and hydrocarbons reversed)
Edit: I assume people are missing my reasoning. Yes, water requires energy to split, but assuming you are doing some other process that ends up with a resulting split anyway, you could use the hydrocarbons as a fuel source at that stage. Think recycling, not energy production.
That's partially illustrated by nuclear hydrogen engines - there is no chemistry, and the gas coming from the nozzle is actually colder than what conventional chemical rocket engine produces. Hydrogen doesn't participates in any transformations, it's purely a working fluid, yet Isp is about twice as good as with LOX-LH2 engines.
So at least adding water to burning process might sometimes improve the result.
To do it from water for instance you could use electricity to split up the H2O molecules; you could directly use the resulting O2 or you could use the H2 to reduce the iron ore back to H2O and thus recycle the H2. Maybe you get electricity from solar panels or a nuclear reactor.
You could burn this with cosmic hydrocarbons but you have only accomplished energy storage as opposed to an energy source. You are then competing with a sun that shines all the time and every other energy storage technology as well as nuclear fission, fusion, and decay.
The oxygen rich environment is the gift we get from plants here on Earth, ultimately they have stored a lot of "energy" in the atmosphere which we can tap.
Future wildcatters will see asteroid hydrocarbons as the material to make human bodies and cattle, everything from wood to plastics to pharmaceuticals -- every bit as much of a gold rush, but not a new energy source.
If you could make plastics from asteroid materials you could blow very large films and coat them with layers of metals and semiconductors and thus function as solar sails if not energy collectors -- these could sail to their destinations on their own power, say to be installed at the Earth-Sun L1 point to fight global warming.
Anyhow it took people a long time to get oxygen right. Cave men "mastered" fire but people couldn't make steel commercially until people understood that 80% of the atmosphere is inert stuff that goes along for the ride.
I forget exactly the proportions, but... the entire point of iron-processing is to turn rust back into raw Iron (FE) by removing the O from it.
http://www.fchea.org/in-transition/2019/11/25/hydrogen-in-th...
Let's say you have oxygen from somewhere. You use the oxygen to burn hydrocarbons, to make energy and water and CO2.
To turn that into a closed cycle, you need to regenerate the water and CO2 into oxygen, to burn more hydrocarbons. The only way to do that is to add energy - on earth, we use solar energy via photosynthesis, but in space, you could use solar or nuclear energy via some chemical process.
But if you have the energy to regenerate the oxygen, why not just use it? Why make oxygen and then burn the hydrocarbons?
And why bother burning more hydrocarbons when a by-product of the regeneration process is carbon, a perfect fuel!
Close to the sun volatile substances such as hydrocarbons and water got cooked off so the Earth is still a dry place rich in aluminum and silicon compared to objects outside the frost line (Jupiter) which tend to have water, carbon dioxide, and hydrocarbons as major components.
Neat
I keep seeing comments about how certain science topics are initially presented in an overly complicated fashion. But there are two forces at play here: correctness versus accessibility. The theory presented in this paper "predicts most heats of combustion with an error of only a few percent". This is good enough for most practical applications, especially if your goal is to introduce students to this topic.
But this is not the most "correct" description of reality that we currently have. A better model to decrease the error would incorporate information about the 3D structures/conformations of the molecules and some funky terms related to the quantum mechanics.
Anybody publishing literature about/teaching something like organic chemistry is stuck between this balancing act of correctness versus accessibility. This difficulty is compounded by the fact that you basically have to unlearn some of the stuff you picked up during the path to "accessibility" in order to properly move into the "correctness" phase. I genuinely sympathize with anybody dealing with the balancing act.
After studying organic chemistry for an extended period of time, it dawned on me that the well-thought-out explanations in my textbooks were just post-hoc rationalizations the field uses to avoid delving into the true quantum mechanical nature of the reactions. I'm happy I sacrificed some correctness for the huge amount of accessibility I got. But I'm also happy to unlearn some of the stuff on my path to correctness.
The heat is a byproduct of the cascade reaction that allows combustion to occur, which structurally in my mind is very similar to fission processes where neutron bombardment produces excess neutrons to bombard more atoms.