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Are explosions soley the result of expanding gas?

  1. Dec 10, 2005 #1
    It's being said that the explosive property of nitrogen-containing compounds comes mainly not from release of heat, but from the fact that a few molecules of solids convert to many molecules of gas. And molecules of gas tends occupy so much more space than the solid.

    What I'm wonder about is that it would seem that energy released when the gasses expand must already be present potential energy in some form. If the molecules as a solid are held together by forces then the forces that keep the solid held together must be equal to the magnitude of the force which gives the gas a tendency to expand. So if gas is no longer "held back," so to speak, wouldn't this imply that the forces holding it back are also released as an equivalent amount of heat energy?
     
  2. jcsd
  3. Dec 10, 2005 #2

    lightgrav

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    Yes, the Energy released can be traced back to chemical Potential Energy
    (which is actually Electrical, at the molecular level).

    The Forces which were holding the molecule together before the break-up
    must be overcome by the gas molecules as they break away ...
    that is, these Forces remove Kinetic Energy from the gas molecules.
    So NOT EVERY molecule's break-up leads to an explosion.

    Explosions require large Potential Energy change so that the Kinetic Energy of the gas molecules (essentially thermal Energy) results in Pressure that is much LARGER than the Force which holds the molecule together.

    Force is NOT a conserved quantity ... it makes no sense to talk about
    "Force being released". It is the initial Potential Energy which ends up (some of it) as Thermal Energy.
     
  4. Dec 10, 2005 #3

    Tide

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    I think that is an oversimplification and a little misleading. It is the release of energy and subsequent heating that breaks the bonds holding the solid together. The thermal energy keeps the bonds from reforming and the gas pressure causes the material to expand.

    Molecules of solid are converted into a like number of gas molecules but at high pressure.
     
  5. Dec 10, 2005 #4

    lightgrav

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    A nitrated molecule typically breaks into at least half a dozen smaller molecules during an explosion.

    If these remained as condensed material , as in solid or liquid , they would occupy about the same Volume as the original molecule (within 20%). So it is ESSENTIAL that there be enough leftover Energy (as Thermal Energy) to free these small molecules from the potential Energy "wells" that they were in before the explosion.
     
  6. Dec 13, 2005 #5
    Could it be like this then? The atoms are bound by electrical forces. When the bonds break and new ones form, if the new bonds are stronger then the old, potential energy in the bonds is less and energy elsewhere is increased. And if the bonds are weaker then the potential energy is increased and energy must be added.

    When the bonds between atoms get stronger, I'm thinking, potential energy goes into weaking the bonds that hold groups molecules together as a solid.
     
  7. Dec 14, 2005 #6

    Tide

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    Essentially, the binding energy doesn't change (for the moment, I won't distinguish between ground and excited states). It's the kinetic energy that changes and if atoms have too much energy, on average, then they will not be able to form (or, rather, re-form) bonds.

    An analogous situation would be adding energy to ice. If the average kinetic energy is made large enough then the "lattice bonds" can't hold the water molecules together and the ice melts. Later, if some of that energy is removed then the bonds can once again form and the water freezes.

    In the case of the explosives, however, once the bonds are broken, the pressure is sufficient to cause the atoms to expand and separate from one another. There is no opportunity for the bonds to reform when the atoms become separated by great distances.
     
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