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A description of fire at an atomic level

  1. Aug 10, 2010 #1
    I'm trying to understand what fire is at an atomic level. I'd appreciate it if you guys could correct me where I am wrong and fill in any gaps.

    1: When you heat an object it introduces energy.
    2: This energy causes the atoms in the object to jiggle around.
    3: At some point introducing the heat will provide enough energy to overcome the electro magnetic energy holding the atoms of the object to other atoms and they break apart and fly off.

    Note: I am describing the breaking up of molecules, not the splitting of atoms.

    Do the atoms "jiggle" or does the energy cause their electrons to move to higher shells and that causes them to break apart from other atoms they are binded to? What is the process here?

    4: Some of these molecules bond with oxygenising gas atoms in the environment (such as oxygen.)

    Note: This happens because atoms are stable when their electron shells are full

    1st - 2 electrons
    2nd - 8 electrons
    3rd - 8 electrons
    4th - 18 electrons

    Oxygen is made of
    1st = 2 electrons
    2nd = 6 electrons

    So in the 2nd shell we can add 2 hydrogen atoms (1 + 1) to get water or we can add a single helium (2). Alternatively for every 2 oxygen we could add a single carbon (4 electrons in outer shell).

    How am I doing so far? This is pretty much guess work on my part to be honest so I expect most of it to be wrong.

    Also, may I ask how the smoke would be described, and also if it is possible to have fire without any smoke at all?


    Here is where I get REALLY sketchy!
    5: When certain atoms with the oxygen atoms there are too many electrons in the combined molecules so electrons are emitted.

    Is that right, do electrons get emitted? If that were the case I'd have expected electricity. I'd imagine it is more likely that electrons drop into a lower shell and then give off photons of different energy levels (depending on the atom type.)

    6: As these (electrons/photons) are emitted they hit our eyes and we see light.
    7: Photons below the visible frequency we feel as heat.
    8: Some of these photons collide with other atoms in the pre-burned material and give them extra energy, causing a chain reaction.

    Maybe understanding what happens when hydrogen is ignited would help me to understand an average garden fire?

    Thanks very much for your time!
     
  2. jcsd
  3. Aug 10, 2010 #2
    I don't know all the chemistry, but parts of your question are right and parts wrong.

    Basically fire is the release of energy when Oxygen bonds with, usually, Carbon. I believe the energy is released in a broad spectrum -- many wavelengths -- where most are in the infra-red. IR is what we sense as "heat" and slightly shorter wavelengths we "see" as light. Oxygen, and elements like Chlorine and Fluorine, are very happy to combine with other elements and release energy from their molecular bonds in the process. Regular old rust on iron is actually very slow fire as well. The iron oxidizes -- bonds with oxygen -- and releases some heat in the process (it takes a _lot_ of energy to start it really burning, thermite is an example of runaway rust). The bonding process is commonly modeled with the exchange or sharing of electrons between the subject atoms. I imagine wiki-ing Chemical Bonding will give a good introduction.

    Adding heat to an oxidation process increases it's activity, so the heat produced by the reaction tends to aid-and-abet the reaction and fire is the run-away result. Heat from nearby burning objects increases the likelyhood that the new object will catch fire.

    If things burn completely the result is just CO2 (for carbon) or H2O (for hydrogen), both of which are odorless and colorless. Smoke is actually unburned material resulting from a lack of oxygen in the actual fire, with added noxious by-products of non-carbon/hydrogen reactions. In a structure fire -- when your house burns down -- the smoke is probably more dangerous than the fire because it's A) hot, B) poisonous, and C) quite happy to explode when it gets to enough O2.
     
  4. Aug 11, 2010 #3
    Does anyone else have any feedback on this? I really would like to understand it at an atomic level.

    Thanks
     
  5. Aug 11, 2010 #4

    alxm

    User Avatar
    Science Advisor

    1 and 2 are ok.
    3: This is a correct description of a process known as pyrolysis, which is not the same thing as combustion (which is the reaction with oxygen). You can have combustion without pyrolysis going on. In a 'ordinary' fire, i.e. burning organic material, both things are going on, but at different places. At the oxygen-poor center of the flame, you have pyrolysis, and the material reacts to form gaseous intermediates, which reach complete combustion at the flame front (the 'edge' of the flame, where it meets the atmosphere)

    Both happen.

    4. Your description of the chemistry here is a bit simplistic.

    Smoke is soot (carbon) and other incompletely-combusted particles. And also ash particles. Things can burn without smoke just fine.

    Try a bunsen burner (or other butane/propane flame). It will burn without smoke and a very light blue flame, if there's sufficient oxygen mixed with the gas. If you choke the oxygen intake you get a sooty, more visible flame.

    5: No, electrons don't get emitted; there are few free electrons flying around in a fire at ordinary temperatures. You do however, have radicals and ions and other very reactive species in the fire. Electrons do get excited, and this explains some of the light you see. Other parts of it are simply due to blackbody (thermal) radiation, some of which may be in the visible part of the spectrum, depending on how hot it is.

    6-7: Ok.
    8: Heat isn't transferred just by photons, but can be directly transferred in molecular collisions. In fact, most heat is transferred this way.


    Let's straighten out how a chemical reaction works: Things don't have to decompose and then react with oxygen; they can react with oxygen directly. The reason why things don't spontaneously combust is that atoms repel at close range, and they have to get quite close before a chemical bond is formed. So there's an energetic 'hill' (aka the 'activation energy') that has to be climbed before the reaction can occur, even if the reaction gives off quite a lot of energy.
     
  6. Aug 12, 2010 #5
    Thanks alxm

    My aim is to first understand the process and then create an animation, which is why I want to understand how it looks at an atomic level.

    So now if I understand correct it goes like this

    1: Heat makes the atoms jiggle more.
    2: Once they have enough energy they jiggle so much that they break the electro magnetic force holding them together. (Is that the force which is overcome in this case?)
    3: Some fly off and collide with other atoms.
    4: If they hit each other hard enough they break the electro magnetic force and bond, 2 carbon + 1 oxygen for example. (Is that the force which is overcome to bond them?)
    4: The fusion of these two atoms causes energy to be expelled.
    5: Some of this energy excites the electrons in atoms which jump up to another shell level, before then jumping back down a shell level and emitting a photon.
    6: Some of this energy excites the other atoms causing them to jiggle and break away, causing a chain reaction.

    Is this correct?
    Why does some energy excite electrons and other energy excite the whole atom (jiggling), is it because the energy misses the electrons and hits the nucleus directly?

    Thanks for your help, I really appreciate it!
     
  7. Aug 18, 2010 #6
    Sorry,

    1: "Energy" makes the atoms jiggle more.
     
  8. Aug 21, 2010 #7
    When oxygen and carbon bond they release energy, which part of the atom loses this energy and is it lost in the form of heat?
     
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