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Why Combustion Is Exothermic

pubs.acs.org

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Re: Why Combustion Is Exothermic

#61

Post by CamperBob2 »

gizmo686 wrote: post #48
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'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.

Or, put another way: strong bonds require the input of more energy to break them apart, so it's not too surprising if they emit energy in the form of heat at the time of formation.

Re: Why Combustion Is Exothermic

#62

Post by laplacesdemon48 »

This is a great article and I'm happy the author mentions "an explanation of combustion exothermicity in terms of Pauling electronegativities is not convincing". I was incorrectly taught this view and it took me some time to unlearn it.

I keep seeing comments about how certain science topics are initially presented in an overly complicated fashion. But there are two forces at play here: correctness versus accessibility. The theory presented in this paper "predicts most heats of combustion with an error of only a few percent". This is good enough for most practical applications, especially if your goal is to introduce students to this topic.

But this is not the most "correct" description of reality that we currently have. A better model to decrease the error would incorporate information about the 3D structures/conformations of the molecules and some funky terms related to the quantum mechanics.

Anybody publishing literature about/teaching something like organic chemistry is stuck between this balancing act of correctness versus accessibility. This difficulty is compounded by the fact that you basically have to unlearn some of the stuff you picked up during the path to "accessibility" in order to properly move into the "correctness" phase. I genuinely sympathize with anybody dealing with the balancing act.

After studying organic chemistry for an extended period of time, it dawned on me that the well-thought-out explanations in my textbooks were just post-hoc rationalizations the field uses to avoid delving into the true quantum mechanical nature of the reactions. I'm happy I sacrificed some correctness for the huge amount of accessibility I got. But I'm also happy to unlearn some of the stuff on my path to correctness.

Re: Why Combustion Is Exothermic

#63

Post by esperent »

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…

Granted it's been a few years since I studied chemistry, but isn't the word "combustion" defined as "exothermic reaction"? At least that's what wiki states. If a reaction doesn't create heat, we don't call it combustion.

Re: Why Combustion Is Exothermic

#64

Post by galaxyLogic »

gizmo686 wrote: post #48
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'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.

It's counter intuitive because when we burn something say a wooden house it turns into ashes. Where are the stronger bonds then?

Re: Why Combustion Is Exothermic

#66

Post by mikewarot »

avmich wrote: post #18
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?

A cutting torch for steel is really just an oxy-acetylene torch. Once the flame gets the steel hot enough, you cut back the acetylene, and the pure oxygen burns the steel away. It's a surprisingly effect method of cutting.

Re: Why Combustion Is Exothermic

#67

Post by Gravityloss »

Retric wrote: post #57
Gravityloss wrote: post #51

Earlier quoted context omitted.

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 i…

With reversible reactions the energy to break it or make it should be equal and opposite.

Re: Why Combustion Is Exothermic

#68

Post by thunderbong »

galaxyLogic wrote: post #64
gizmo686 wrote: post #48

Earlier quoted context omitted.

> 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.

It's counter intuitive because when we burn something say a wooden house it turns into ashes. Where are the stronger bonds then?

The strong bonds are in CO2 and H2O i.e. carbon dioxide and water vapour, both of which dissipate into the air.

Re: Why Combustion Is Exothermic

#69

Post by galaxyLogic »

thunderbong wrote: post #68
galaxyLogic wrote: post #64

Earlier quoted context omitted.

It's counter intuitive because when we burn something say a wooden house it turns into ashes. Where are the stronger bonds then?

The strong bonds are in CO2 and H2O i.e. carbon dioxide and water vapour, both of which dissipate into the air.

Right. It is true but counter-intuitive. For us humans strong bonds in a gas don't look like strong bonds.
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