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Combustion/explosion reactions

  1. Jun 24, 2010 #1
    combustion/explosion reactions - anyone have an explanation?

    I am a layperson, desperately seeking enlightenment!

    I have been thinking about combustion/explosion reactions and have been trying to build a mental model of the physical processes involved.

    I can imagine two atoms, let's say carbon and oxygen snapping together. Where does the increase in temperature come from?

    The only thing I have come up with is, maybe "other" atoms are attached to the carbon and/or oxygen before the reaction occurs. They get pulled along until the C and O snap hard together, and break loose from the collision.

    Like people getting thrown from crashing cars.

    So these "thrown" atoms contain all of the new released energy.

    Thanks in advance anybody who can help!
    Last edited: Jun 24, 2010
  2. jcsd
  3. Jun 24, 2010 #2
    All chemical bonds have a property called "enthalpy", which can be thought of as how badly two atoms "want" to be together. Carbon and oxygen really want to be together, so it takes energy to separate them. Energy must be conserved, so the energy that it would take to separate them is equal to the potential energy they have when they're apart, which is also equal to the energy they release when they come together. The released energy is in the form of heat.

    Now, I have to admit, I haven't thought a lot about this on a molecular level. The thought of two atoms coming together and forming a molecule of higher kinetic energy is spooky to me. Maybe I can pass the torch off to someone at this point...
  4. Jun 24, 2010 #3
    Ok, I already know about that principle. I have considered it, and I know it will be important to our problem, but we have still not answered the question. What actually happens to transfer this energy that will be released to other atoms nearby.

    Another idea I have thought of is, maybe the collision/snapping together of the C and O alone causes their electrons to vibrate in their "orbits". Would that emit enough EM radiation to shake the nearby atoms sufficiently to explain a fire.

    For some reason i doubt it.

    Interestingly, this is one of the most common phenomena we experience in life (fire), yet I have never heard a complete physical explanation.

    Is it possible that virtually no one really knows?

    Thanks Archosaur, I appreciate your input. I just don't think we are there yet.
  5. Jun 24, 2010 #4


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    For a mental picture, think of bonds as little springs between magnets. When you bring two magnets close to each other, attraction snaps them together. The spring in between would prevent them coming completely together, but all that energy is still there, and it will make the two magnets bounce around quite a bit on that spring.

    Same thing happens in a chemical reaction. After the two particles came together, they don't just stop, they keep vibrating on that "spring" of the bond between them. And since there is no friction, they keep vibrating until something mechanically hits them.

    Now imagine a CO2 molecule, vibrating like crazy after being formed, encountering another CO2 molecule, equally vibrating. When they hit each other, odds are, all that vibration will result in the two parting much faster than they came together.

    As a physics note, there is actually a theorem, called Equipartition Theorem, which states, in simple term, that left to their own devices, energy will distribute itself between particle motion, vibration, and rotation in a specific proportion. And motion of particles, of course, is related to temperature.

    So basically, all that vibrational energy gets shared with mechanical energy. It works backwards too, if you heat a gas, part of the energy goes into vibration of the molecules.

    Net result, of course, is the increase in gas temperature after combustion took place.
  6. Jun 24, 2010 #5
    K2 - I have actually pictured that as a possible theory. It seems to make some sense without running the math.

    The problem I had with that explanation was:

    -The fact that on the original bonded pair (C-O), the oscillation from the bond would be in and out -giving a 50% chance of making contact with another atom while moving outward, and also a 50% chance of making contact with another atom while moving inward.

    Wouldn't that result in zero net increase in energy for the environment?

    Is this idea verified anywhere?
  7. Jun 24, 2010 #6


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    How do you picture collision when it's moving inward? Unless the other particle is moving at speed higher than that of vibration, in which case, that particle is already "hotter" than the one you're talking about.

    And yes. You can look up Equipartition Theorem and see where it comes from with all related notes.
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