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Why Combustion Is Exothermic

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Re: Why Combustion Is Exothermic

#41

Post by Tuna-Fish »

himinlomax wrote: post #19
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.

> 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.

But more net energy released when the oxygen instantly binds much more strongly into something else.

Re: Why Combustion Is Exothermic

#42

Post by kortex »

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.

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.

Re: Why Combustion Is Exothermic

#43

Post by kortex »

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

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.

Re: Why Combustion Is Exothermic

#44

Post by wcp »

icedistilled wrote: post #36

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 appa…

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!

Re: Why Combustion Is Exothermic

#45

Post by wcp »

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 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

Re: Why Combustion Is Exothermic

#46

Post by samatman »

the8472 wrote: post #27
twic wrote: post #20

Earlier quoted context omitted.

> 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 bi…

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.

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.

Re: Why Combustion Is Exothermic

#47

Post by carbonguy »

wcp wrote: post #45
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 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 ex…

> 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.

Re: Why Combustion Is Exothermic

#48

Post by gizmo686 »

wcp wrote: post #44
icedistilled wrote: post #36

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 appa…

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…

> 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.

Re: Why Combustion Is Exothermic

#49

Post by PaulHoule »

abecode wrote: post #25
PaulHoule wrote: post #2

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)

Where do the hydrocarbons on the asteroids come from in that view? I’m curious because on earth the hydrocarbons come from life.

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.

Re: Why Combustion Is Exothermic

#50

Post by kurthr »

wcp wrote: post #44
icedistilled wrote: post #36

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 appa…

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…

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.

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