Why Combustion Is Exothermic • Hacker News Board

Re: Why Combustion Is Exothermic

#11

Post by echelon » Mon, Jan 18, 2021, 4:39 PM UTC

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/

Re: Why Combustion Is Exothermic

#12

Post by kortex » Mon, Jan 18, 2021, 4:46 PM UTC

> 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.

Re: Why Combustion Is Exothermic

#13

Post by kortex » Mon, Jan 18, 2021, 4:51 PM UTC

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.

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.

Re: Why Combustion Is Exothermic

#14

Post by whatshisface » Mon, Jan 18, 2021, 4:52 PM UTC

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.

Actually... breaking O2 bonds and breaking hydrocarbon bonds both consume energy. The energy "comes from" the formation of water and CO2.

Re: Why Combustion Is Exothermic

#15

Post by kortex » Mon, Jan 18, 2021, 4:53 PM UTC

polishdude20 wrote: post #8

PaulHoule wrote: post #5

Earlier quoted context omitted.

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 fro…

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

Hydrogen smelting.

http://www.fchea.org/in-transition/2019/11/25/hydrogen-in-th...

Re: Why Combustion Is Exothermic

#16

Post by swebs » Mon, Jan 18, 2021, 4:53 PM UTC

>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

Re: Why Combustion Is Exothermic

#17

Post by ralfd » Mon, Jan 18, 2021, 5:13 PM UTC

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.

> 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...

Re: Why Combustion Is Exothermic

#18

Post by avmich » Mon, Jan 18, 2021, 5:15 PM UTC

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…

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

Re: Why Combustion Is Exothermic

#19

Post by himinlomax » Mon, Jan 18, 2021, 5:21 PM UTC

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.

Re: Why Combustion Is Exothermic

#20

Post by twic » Mon, Jan 18, 2021, 5:23 PM UTC

ralfd wrote: post #17

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.

> 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 Pa…

> 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