8 ms·

Why Combustion Is Exothermic

136 points by jasonhansel 6y ago

PaulHoule 6y ago
Considering space resources people have a hard time recognizing that hydrocarbons are a structural material but not an energy source w/o molecular O2. (e.g. it's thought that many asteroids are like Saudi Arabia but with the relative proportions of sand and hydrocarbons reversed)
- bluejellybean 6y ago
  I don't understand your point, if you are claiming that O2 is not abundant in space, I think the idea is that we also find an ice asteroid. Where there is water, there is oxygen. 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.
  - Metacelsus 6y ago
    Yeah but you have to expend energy to split water into hydrogen and oxygen. So burning hydrocarbons using water doesn't make much sense.
    - avmich 6y ago
      Imagine you're making a rocket engine, and both fuel and combustion products have high molecular weight. A rough example is burning CO (carbon monoxide) in O2, with reasoning being "both of them can be obtained from martian athmosphere". So the fuel isn't great, and specific impulse - Isp - is low, but if you add a relatively low-molecular weight component to fuel - like water - you can hope to have greater positive effect from lowering average molecular mass of combustion products than negative effect from adding an inert component. 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.
  - PaulHoule 6y ago
    You can produce oxygen from water, iron ore (O'Neill and other sci-fi writers somehow missed that astronauts brought back top quality iron ore from the moon,) stony rocks, etc. 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.
    - polishdude20 6y ago
      Wait, how do you reduce iron ore back into h2o ?
      - dragontamer 6y ago
        Iron Ore is FE + O. 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.
        polishdude20 6y ago
        So where does the H come from?
  - shawnz 6y ago
    Water is a product of combustion. It is what you are left with after the combustion has already happened, i.e. the oxygen has already been reduced.
  - twic 6y ago
    Recycling what? 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!
    - bluejellybean 6y ago
      Photosynthesis is a perfect example, I know of a handful of useful chemicals that today come from the cultivation of specific plants. Food production is the other obvious one use case for it. If you're byproduct is oxygen, well, can use it for whatever you want.
- abecode 6y ago
  Where do the hydrocarbons on the asteroids come from in that view? I’m curious because on earth the hydrocarbons come from life.
  - PaulHoule 6y ago
    A lot of carbon is made in stars and collects in dust clouds that condense into objects. 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.
excannuck 6y ago
"−418 kJ/mol" This is one of the most useful factoids to know of the top of your head. I don't remember the actual number, and in fact I remember it (incidentally far less precise) as all hydrocarbons have about the same energy mass density.
leoedin 6y ago
Wow! So the bulk of the energy we consume comes not from the hydrocarbons we're digging up, but the oxygen bonds we're breaking when we burn them. That's completely spun around my understanding of fuels. Fascinating. 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.
- echelon 6y ago
  Oxygen fuels fires. 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. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2865650/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2865650/
  - _carbyau_ 6y ago
    So, with less oxygen we have: - less fires - more reliability in our machines and structures - less people - live longer - less likely to get 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.
- kortex 6y ago
  Kinda. It's the difference in entropy, mostly driven by electron affinity. In the thermite reaction, the Fe-O bond is fairly strong - rust is quite stable. But the Al-O bond is stronger. Hence once you kick it hard enough to break that FeO bond, it drives the reaction.
- whatshisface 6y ago
  Actually... breaking O2 bonds and breaking hydrocarbon bonds both consume energy. The energy "comes from" the formation of water and CO2.
- ralfd 6y ago
  > recent photosynthesis AFAIK our oxygen was not made recently. It accumulated over billions of years. https://en.wikipedia.org/wiki/Great_Oxidation_Event 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/S209592731830375X https://www.sciencedirect.com/science/article/pii/S209592731...
kortex 6y ago
> This explains why fire is hot regardless of fuel composition. 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.
- avmich 6y ago
  There were reports of using small iron pellets as a fuel, so does it mean rust creation can be considered combustion as well?
  - marcosdumay 6y ago
    Is it hot, relatively fast, and self sustaining? 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.
    - avereveard 6y ago
      "Iron burning in atmosphere of pure oxygen" https://www.youtube.com/watch?v=A7j14otCdyY https://www.youtube.com/watch?v=A7j14otCdyY
    - scsilver 6y ago
      If you get small enough, you start getting into the territory of dust explosions. https://en.m.wikipedia.org/wiki/Dust_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#:~:text=Spontaneous%20combustion%20or%20spontaneous%20ignition,temperatures)%20and%20finally%2C%20autoignition 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.
swebs 6y ago
>These considerations show that atmospheric O2 stores energy originating from the sun, and that the heat of combustion in air can be regarded as fossil solar energy Neat
steve76 6y ago
Weak bond to strong bond, release energy. Strong bond to weak bond, store energy. Enthalpy is the outside environment, like pressure and temperature, needed per internal state requirements, coarsely being internal voltage and emissivity. As the bonds of internal state strengthen, the environment requires less to maintain that internal state and loosens it's grip on it. Heat then flows from internal state to the environment. In fusion, hydrogen is converted to helium. One proton and one electron of each hydrogen atom are free to move as they see fit. As temperatures increase, protons and electrons become free of their hydrogen atom internal state bonds. They whip around until two protons together mesh with two electrons. The freedom of movement in the plasma, and the freedom of movement from two separate hydrogen atoms grips into helium as heavier fusion requires a higher temperature. Atoms have an incredibly fine resolution, as fine as something can go. The large amount of them release substantial heat. Add neutrons, the principles of nuclear decay, nature moving heat with mass while leaving internal state properties alone, increases the ability for energy to release. Instead of thinking of little grains of matter, the internal state is very ephemeral. Before something can be brighter, it needs to be hotter. There's a capacity to hold heat and a latency to transfer heat. This capacity is natures ability to speed up and slow down it's own internal ticking clock to move with least effort. At the smallest scales possible, instead of a little building block, likely its nature's ability to "correct the record". Nothing says nature gets to change the entire environment faster than you can observe it if it's an easier path to move.
reggieband 6y ago
I first learned the idea of fire as breaking molecular bonds formed during photosynthesis from a video snippet of Richard Feynman [1]. 1. https://www.youtube.com/watch?v=N1pIYI5JQLE https://www.youtube.com/watch?v=N1pIYI5JQLE
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 ago
  I 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.
kps 6y ago
Yesterday HN had a discussion about the Fermi Paradox¹. Personally, I think there is no single Great Filter, just a product of many modest filters — and one of them is that, if early life doesn't have an oxygen catastrophe², any eventual intelligent life will have a tough time not being able to burn things. ¹ https://news.ycombinator.com/item?id=25810078 https://news.ycombinator.com/item?id=25810078 ² https://en.wikipedia.org/wiki/Oxygen_Catastrophe https://en.wikipedia.org/wiki/Oxygen_Catastrophe
- ajnin 6y ago
  Without free oxygen, animal life could hardly develop at all, let alone intelligent life, since it would not have access to the large amounts of energy it needs to exist.
iaw 6y ago
I feel like this explanation is a good step but misses the crux that any combustion process (even with a non-oxygen oxidizer, if that's still called combustion) exists because each broken bond releases enough energy to break the bond of neighboring atoms/molecules with some left over to convert to heat. 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.
laplacesdemon48 6y ago
This is a great article and I'm happy the author mentions "an explanation of combustion exothermicity in terms of Pauling electronegativities is not convincing". I was incorrectly taught this view and it took me some time to unlearn it. 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.
MaysonL 6y ago
Basically, E = MC squared.

136 points by jasonhansel 6y ago

Considering space resources people have a hard time recognizing that hydrocarbons are a structural material but not an energy source w/o molecular O2. (e.g. it's thought that many asteroids are like Saudi Arabia but with the relative proportions of sand and hydrocarbons reversed)

bluejellybean 6y ago
I don't understand your point, if you are claiming that O2 is not abundant in space, I think the idea is that we also find an ice asteroid. Where there is water, there is oxygen. 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.
- Metacelsus 6y ago
  Yeah but you have to expend energy to split water into hydrogen and oxygen. So burning hydrocarbons using water doesn't make much sense.
  - avmich 6y ago
    Imagine you're making a rocket engine, and both fuel and combustion products have high molecular weight. A rough example is burning CO (carbon monoxide) in O2, with reasoning being "both of them can be obtained from martian athmosphere". So the fuel isn't great, and specific impulse - Isp - is low, but if you add a relatively low-molecular weight component to fuel - like water - you can hope to have greater positive effect from lowering average molecular mass of combustion products than negative effect from adding an inert component. 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.
- PaulHoule 6y ago
  You can produce oxygen from water, iron ore (O'Neill and other sci-fi writers somehow missed that astronauts brought back top quality iron ore from the moon,) stony rocks, etc. 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.
  - polishdude20 6y ago
    Wait, how do you reduce iron ore back into h2o ?
    - dragontamer 6y ago
      Iron Ore is FE + O. 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.
      - polishdude20 6y ago
        So where does the H come from?
- shawnz 6y ago
  Water is a product of combustion. It is what you are left with after the combustion has already happened, i.e. the oxygen has already been reduced.
- twic 6y ago
  Recycling what? 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!
  - bluejellybean 6y ago
    Photosynthesis is a perfect example, I know of a handful of useful chemicals that today come from the cultivation of specific plants. Food production is the other obvious one use case for it. If you're byproduct is oxygen, well, can use it for whatever you want.
abecode 6y ago
Where do the hydrocarbons on the asteroids come from in that view? I’m curious because on earth the hydrocarbons come from life.
- PaulHoule 6y ago
  A lot of carbon is made in stars and collects in dust clouds that condense into objects. 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.

excannuck 6y ago

"−418 kJ/mol" This is one of the most useful factoids to know of the top of your head. I don't remember the actual number, and in fact I remember it (incidentally far less precise) as all hydrocarbons have about the same energy mass density.

leoedin 6y ago

Wow! So the bulk of the energy we consume comes not from the hydrocarbons we're digging up, but the oxygen bonds we're breaking when we burn them. That's completely spun around my understanding of fuels. Fascinating. 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.

echelon 6y ago
Oxygen fuels fires. 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. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2865650/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2865650/
- _carbyau_ 6y ago
  So, with less oxygen we have: - less fires - more reliability in our machines and structures - less people - live longer - less likely to get 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.
kortex 6y ago
Kinda. It's the difference in entropy, mostly driven by electron affinity. In the thermite reaction, the Fe-O bond is fairly strong - rust is quite stable. But the Al-O bond is stronger. Hence once you kick it hard enough to break that FeO bond, it drives the reaction.
whatshisface 6y ago
Actually... breaking O2 bonds and breaking hydrocarbon bonds both consume energy. The energy "comes from" the formation of water and CO2.
ralfd 6y ago
> recent photosynthesis AFAIK our oxygen was not made recently. It accumulated over billions of years. https://en.wikipedia.org/wiki/Great_Oxidation_Event 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/S209592731830375X https://www.sciencedirect.com/science/article/pii/S209592731...

kortex 6y ago

> This explains why fire is hot regardless of fuel composition. 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.

avmich 6y ago
There were reports of using small iron pellets as a fuel, so does it mean rust creation can be considered combustion as well?
- marcosdumay 6y ago
  Is it hot, relatively fast, and self sustaining? 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.
  - avereveard 6y ago
    "Iron burning in atmosphere of pure oxygen" https://www.youtube.com/watch?v=A7j14otCdyY https://www.youtube.com/watch?v=A7j14otCdyY
  - scsilver 6y ago
    If you get small enough, you start getting into the territory of dust explosions. https://en.m.wikipedia.org/wiki/Dust_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#:~:text=Spontaneous%20combustion%20or%20spontaneous%20ignition,temperatures)%20and%20finally%2C%20autoignition 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.

swebs 6y ago

>These considerations show that atmospheric O2 stores energy originating from the sun, and that the heat of combustion in air can be regarded as fossil solar energy Neat

steve76 6y ago

Weak bond to strong bond, release energy. Strong bond to weak bond, store energy. Enthalpy is the outside environment, like pressure and temperature, needed per internal state requirements, coarsely being internal voltage and emissivity. As the bonds of internal state strengthen, the environment requires less to maintain that internal state and loosens it's grip on it. Heat then flows from internal state to the environment. In fusion, hydrogen is converted to helium. One proton and one electron of each hydrogen atom are free to move as they see fit. As temperatures increase, protons and electrons become free of their hydrogen atom internal state bonds. They whip around until two protons together mesh with two electrons. The freedom of movement in the plasma, and the freedom of movement from two separate hydrogen atoms grips into helium as heavier fusion requires a higher temperature. Atoms have an incredibly fine resolution, as fine as something can go. The large amount of them release substantial heat. Add neutrons, the principles of nuclear decay, nature moving heat with mass while leaving internal state properties alone, increases the ability for energy to release. Instead of thinking of little grains of matter, the internal state is very ephemeral. Before something can be brighter, it needs to be hotter. There's a capacity to hold heat and a latency to transfer heat. This capacity is natures ability to speed up and slow down it's own internal ticking clock to move with least effort. At the smallest scales possible, instead of a little building block, likely its nature's ability to "correct the record". Nothing says nature gets to change the entire environment faster than you can observe it if it's an easier path to move.

reggieband 6y ago

I first learned the idea of fire as breaking molecular bonds formed during photosynthesis from a video snippet of Richard Feynman [1]. 1. https://www.youtube.com/watch?v=N1pIYI5JQLE https://www.youtube.com/watch?v=N1pIYI5JQLE

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 ago
I 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.

kps 6y ago

Yesterday HN had a discussion about the Fermi Paradox¹. Personally, I think there is no single Great Filter, just a product of many modest filters — and one of them is that, if early life doesn't have an oxygen catastrophe², any eventual intelligent life will have a tough time not being able to burn things. ¹ https://news.ycombinator.com/item?id=25810078 https://news.ycombinator.com/item?id=25810078 ² https://en.wikipedia.org/wiki/Oxygen_Catastrophe https://en.wikipedia.org/wiki/Oxygen_Catastrophe

ajnin 6y ago
Without free oxygen, animal life could hardly develop at all, let alone intelligent life, since it would not have access to the large amounts of energy it needs to exist.

iaw 6y ago

I feel like this explanation is a good step but misses the crux that any combustion process (even with a non-oxygen oxidizer, if that's still called combustion) exists because each broken bond releases enough energy to break the bond of neighboring atoms/molecules with some left over to convert to heat. 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.

laplacesdemon48 6y ago

This is a great article and I'm happy the author mentions "an explanation of combustion exothermicity in terms of Pauling electronegativities is not convincing". I was incorrectly taught this view and it took me some time to unlearn it. 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.

MaysonL 6y ago

Basically, E = MC squared.