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How does a C4 explosion work on a molecular level

  1. May 26, 2017 #1
    I understand the fact that the detonator begins the reaction. The reaction happens many times release a ton of energy which causes the gases around it to rapidly expand and accelerate away (not the best summary). However, what causes the combustion which tends to happen as a consequence of the explosion? I'm talking about the fire that results.
  2. jcsd
  3. May 26, 2017 #2


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    I won't speak to C4 directly, i'll speak to nitroglycerine. I suspect C4 is similar but more stable.

    When the TNT molecule is jostled, its three long, unstable arms of NO2 start to wobble. With enough energy, the arms can break off. These NO2 bits are much more happy as N2 and H2O, so they reform as lower energy compounds. So:

    1] energy is released as heat.
    2] these new compounds are gaseous rather than solid, and take up much more room. So the CO2 and water and other compounds expand - very rapidly.

    So, the upshot is
    - a change in chemical compounds, resulting in
    - the release of several gases that want to expand to their equilibrium pressure, as well as
    - the release of a lot of heat which
    - is part of the expanding cloud of destruction as well as
    - jostling other nearby molecules, setting them off in a chain reaction

    That all happens very, very fast. We call this an explosion.

    There's more you can study, such as:
    - the initial shape of TNT
    - the final byproducts
    - the differences that make C4 far more stable than TNT
    but the above is the gist.
    Last edited: May 26, 2017
  4. May 26, 2017 #3


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    I came across a book many years go, called molecules something.

    It picked a hundred or so of the most common/interesting molecules and showed, in such eloquent, succinct terms how they worked.
    TNT was one of the ones that stood out. Until then, I'd known simply that an explosion "happened", but it was just magic to me.
    This book showed, like I described above, just what happens - and what had been mere mystery now became beautifully clear.

    It might actually have been this one, I'll have to check:
  5. May 27, 2017 #4


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    And where does the hydrogen come from?
  6. May 27, 2017 #5


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    Yes. I was oversimplifying for brevity. Lots to read beyond my humble little post.

    The fourth the arm is a methyl group - CH3.

    TNT's formula is actually C7H5N3O6 so plenty of Hs in there.

    It seems it breaks down into byproducts like so:

    2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C
    2 C7H5N3O6 → 3 N2 + 5 H2 + 12 CO + 2 C

    i.e. gaseous nitrogen, gaseous hydrogen, water vapor, carbon monoxide and carbon (soot).

    I guess the gaseous hydrogen and carbon monoxide suggest incomplete combustion.
    Last edited: May 27, 2017
  7. May 27, 2017 #6
    That's the way how deflagration works (e.g. explosion of gun powder). The detonation of high explosives like nitrogylcerine or TNT propagates with a supersonic shock wave.
  8. May 30, 2017 #7

    stefan r

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    Ammonium nitrate is much easier to think about IMO. You have 4 hydrogen packed around a nitrogen atom. Number two nitrogen is a nitrate molecule with 3 oxygen atoms.

    Molecules are shaped by the electron clouds. The electrons do not have exact positions. An electron stays in a molecule because it does not have enough energy to jump the barrier (usually).

    You could think of marbles inside of a soup bowl on a table. You can bump the table a little and you do not lose your marbles. If you bump the table hard enough the marble can have enough to momentum to roll over the edge of the soup bowl. The marble has less energy on the floor than it does in the bowl on the table so it stays on the floor. After that the analogy with marbles breaks down a bit. Energy released from one event would have to make the table shake more. In the case explosives the high energy molecules formed from one reaction crash into other molecules and bump outer electrons into higher energy states. Suppose you have a table with bowls and marbles in the back of a truck. Then you slam the trucks break's most of the marbles will end up off the table.

    Also partially reacted molecules like OH- pull hydrogen nuclei away from the nitrogen atom. This partial charge transfers, the attracted hydrogen atom is partially positive, the nitrogen it is bonded with is partial negative that makes anything else bonded partial positive. That makes it easier for other electrons to jump the barrier and make a new bond.
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