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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 comple

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...
- twic 6y ago
  > 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 https://en.wikipedia.org/wiki/Oxygen_cycle
  - the8472 6y ago
    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 6y ago
      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.
himinlomax 6y ago
> 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 6y ago
  But more net energy released when the oxygen instantly binds much more strongly into something else.
k__ 6y ago
If this is true, why do we speak of energy density of our fuels?
- jbay808 6y ago
  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.
- fastaguy88 6y ago
  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.
  - baking 6y ago
    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
crdrost 6y ago
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.
xt00 6y ago
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..
wcp 6y ago
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 6y ago
  > 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.

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...

twic 6y ago
> 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 https://en.wikipedia.org/wiki/Oxygen_cycle
- the8472 6y ago
  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 6y ago
    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.

himinlomax 6y ago

> 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 6y ago
But more net energy released when the oxygen instantly binds much more strongly into something else.

k__ 6y ago

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

jbay808 6y ago
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.
fastaguy88 6y ago
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.
- baking 6y ago
  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

crdrost 6y ago

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.

xt00 6y ago

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..

wcp 6y ago

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 6y ago
> 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.