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Why Combustion Is Exothermic

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Re: Why Combustion Is Exothermic

#31

Post by avereveard »

srean wrote: post #21
kortex wrote: post #12

> 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 gl…

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

> 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.

Re: Why Combustion Is Exothermic

#32

Post by avereveard »

marcosdumay wrote: post #26
avmich wrote: post #18

Earlier quoted context omitted.

There were reports of using small iron pellets as a fuel, so does it mean rust creation can be considered combustion as well?

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.

"Iron burning in atmosphere of pure oxygen"

https://www.youtube.com/watch?v=A7j14otCdyY

Re: Why Combustion Is Exothermic

#33

Post by crdrost »

leoedin wrote: post #10

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.

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.

Re: Why Combustion Is Exothermic

#34

Post by scsilver »

marcosdumay wrote: post #26
avmich wrote: post #18

Earlier quoted context omitted.

There were reports of using small iron pellets as a fuel, so does it mean rust creation can be considered combustion as well?

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.

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#:~:te....

I was a fire engineer in a past life, forgot the science but remember the cool phenomenons.

Re: Why Combustion Is Exothermic

#35

Post by bluejellybean »

twic wrote: post #22
bluejellybean wrote: post #3

Earlier quoted context omitted.

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. Thi…

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.…

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.

Re: Why Combustion Is Exothermic

#36

Post by icedistilled »

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"

Re: Why Combustion Is Exothermic

#37

Post by polishdude20 »

dragontamer wrote: post #9
polishdude20 wrote: post #8

Earlier quoted context omitted.

Wait, how do you reduce iron ore back into h2o ?

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.

So where does the H come from?

Re: Why Combustion Is Exothermic

#38

Post by fastaguy88 »

k__ wrote: post #29
leoedin wrote: post #10

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.

If this is true, why do we speak of energy density of our fuels?

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.

Re: Why Combustion Is Exothermic

#39

Post by xt00 »

leoedin wrote: post #10

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.

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..

Re: Why Combustion Is Exothermic

#40

Post by RRWagner »

marcosdumay wrote: post #26
avmich wrote: post #18

Earlier quoted context omitted.

There were reports of using small iron pellets as a fuel, so does it mean rust creation can be considered combustion as well?

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.

https://www.amazingrust.com/Experiments/how_to/Pyrophoric_Fe...
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