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Why Combustion Is Exothermic

pubs.acs.org

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Re: Why Combustion Is Exothermic

#51

Post by Gravityloss »

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 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.

Re: Why Combustion Is Exothermic

#52

Post by kps »

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

Re: Why Combustion Is Exothermic

#53

Post by mncharity »

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 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, 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:)

Re: Why Combustion Is Exothermic

#54

Post by mncharity »

samatman wrote: post #46
the8472 wrote: post #27

Earlier quoted context omitted.

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.

> oldest [...] [is] about [residence time]

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?

Re: Why Combustion Is Exothermic

#55

Post by [deleted] »

Gravityloss wrote: post #51
wcp wrote: post #44

Earlier quoted context omitted.

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 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.

[deleted]

Re: Why Combustion Is Exothermic

#56

Post by ajnin »

kps wrote: post #52

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

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.

Re: Why Combustion Is Exothermic

#57

Post by Retric »

Gravityloss wrote: post #51
wcp wrote: post #44

Earlier quoted context omitted.

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 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.

Yes, in the same way an object on mercury takes more energy to reach orbit than the moon, an object falling to mercury releases more energy than it would falling to the moon.

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.

Re: Why Combustion Is Exothermic

#58

Post by baking »

fastaguy88 wrote: post #38
k__ wrote: post #29

Earlier quoted context omitted.

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.

No. This is explained in the article. That's only true for hydrocarbons which don't contain oxygen atoms. The energy comes from the O2 consumed. Sugar contains oxygen atoms, but those don't contribute to the energy released because burning sugar consumes less O2 per unit mass than burning hydrocarbons.

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

Re: Why Combustion Is Exothermic

#59

Post by iaw »

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.

Re: Why Combustion Is Exothermic

#60

Post by _carbyau_ »

echelon wrote: post #11
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.

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/

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.

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