[HN Gopher] Why Combustion Is Exothermic
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Why Combustion Is Exothermic
Author : jasonhansel
Score : 101 points
Date : 2021-01-18 14:51 UTC (8 hours ago)
(HTM) web link (pubs.acs.org)
(TXT) w3m dump (pubs.acs.org)
| icedistilled wrote:
| 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 wrote:
| 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!
| mncharity wrote:
| > 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, 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:)
| Gravityloss wrote:
| It 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.
| gizmo686 wrote:
| > 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.
| kurthr wrote:
| If 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.
| reggieband wrote:
| 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
| kortex wrote:
| > 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.
| srean wrote:
| Right.
|
| 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
| kortex wrote:
| Yes, kinda, with a few small exceptions. Exothermy and being
| thermodynamically spontaneous are strongly correlated, but
| you can have endothermic spontaneous reactions, such as
| dissolving urea in water in those instant ice packs.
|
| You can also drive reactions by removing the end products in
| lieu of providing external energy.
| avereveard wrote:
| > 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.
| avmich wrote:
| There were reports of using small iron pellets as a fuel, so
| does it mean rust creation can be considered combustion as
| well?
| marcosdumay wrote:
| 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.
| scsilver wrote:
| If you get small enough, you start getting into the
| territory of dust explosions.
|
| 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#:~:t
| e....
|
| I was a fire engineer in a past life, forgot the science
| but remember the cool phenomenons.
| kortex wrote:
| Right. If you increase the surface area or add more
| oxidizer, iron quite readily burns, see thermic lances,
| ferrocerium, metal powder explosions etc. But that is
| different than rusting.
| RRWagner wrote:
| https://www.amazingrust.com/Experiments/how_to/Pyrophoric_F
| e...
| avereveard wrote:
| "Iron burning in atmosphere of pure oxygen"
|
| https://www.youtube.com/watch?v=A7j14otCdyY
| PaulHoule wrote:
| 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 wrote:
| 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.
| PaulHoule wrote:
| 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 wrote:
| Wait, how do you reduce iron ore back into h2o ?
| kortex wrote:
| Hydrogen smelting.
|
| http://www.fchea.org/in-transition/2019/11/25/hydrogen-
| in-th...
| dragontamer wrote:
| 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 wrote:
| So where does the H come from?
| twic wrote:
| 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 wrote:
| 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.
| Metacelsus wrote:
| 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 wrote:
| 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.
| shawnz wrote:
| 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.
| abecode wrote:
| Where do the hydrocarbons on the asteroids come from in that
| view? I'm curious because on earth the hydrocarbons come from
| life.
| PaulHoule wrote:
| 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.
| steve76 wrote:
| 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.
| kps wrote:
| 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
|
| 2 https://en.wikipedia.org/wiki/Oxygen_Catastrophe
| swebs wrote:
| >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
| leoedin wrote:
| 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.
| crdrost wrote:
| Yeah! I remember someone talking about the methane lakes of
| Titan and remarking that if there was intelligent life on
| Titan, they would surely rule out Earth as any possible habitat
| for life, and would be absolutely shocked at the idea of these
| creatures who can only explore the rest of the universe by
| putting themselves inside of bags of rocket fuel, because that
| is what they breathe. What strange creatures those must be.
| echelon wrote:
| 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/
| wcp wrote:
| The production of heat comes from the formation of bonds, not
| from the breaking of bonds! Many people find this
| counterintuitive. The energy from combustion comes from the
| fact that all hydrocarbon combustion produces a large number of
| strong bonds in CO2 and H2O. Breaking oxygen bonds produces no
| energy and in fact requires energy - it's just that since
| oxygen bonds are weak it requires less energy than one might
| expect.
|
| Edit: slightly changed wording
| carbonguy wrote:
| > Breaking oxygen bonds produces no energy and in fact
| requires energy
|
| 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.
| himinlomax wrote:
| > but the oxygen bonds we're breaking
|
| 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.
| Tuna-Fish wrote:
| But more net energy released when the oxygen instantly binds
| much more strongly into something else.
| k__ wrote:
| If this is true, why do we speak of energy density of our
| fuels?
| fastaguy88 wrote:
| You could think of energy density as the number of CO2 or H2O
| molecules produced by combustion per gram of fuel. If a fuel
| (like gasoline) can produce more H2O+CO2 per gram than
| ethanol, it will have a higher energy density.
| jbay808 wrote:
| For air-breathing vehicles like cars, you assume the oxygen
| comes for free. For rocket fuel, you have to include the
| oxygen tanks in the energy density calculation.
| xt00 wrote:
| The thinking in this article also explains the logic behind the
| solid metal fuel -- like iron powder for example. Basically
| oxygen reacting with iron (rust formation) results in heat
| generation.
|
| 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..
| kortex wrote:
| 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.
| ralfd wrote:
| > recent photosynthesis
|
| 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...
| twic wrote:
| > AFAIK our oxygen was not made recently. It accumulated over
| billions of years.
|
| 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.
|
| [1] https://en.wikipedia.org/wiki/Oxygen_cycle
| the8472 wrote:
| That calculation is simplistic as it is based on the
| current steady-state. The oxygen catastrophe was a massive
| change away from the previous equilibrium state which first
| had to deplete all the buffers. That's how we got iron ore
| veins, by oxidizing most of the iron out of the oceans. And
| there are other sinks besides iron.
| samatman wrote:
| All true, this all depends on what question you're trying
| to answer.
|
| 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.
| whatshisface wrote:
| Actually... breaking O2 bonds and breaking hydrocarbon bonds
| _both_ consume energy. The energy "comes from" the formation
| of water and CO2.
| excannuck wrote:
| "-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.
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