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What makes an explosive.

  1. Nov 17, 2004 #1
    I heard that the fewer number of carbon atoms a molecule has, the faster it burns. Why?
    http://www.scottcrkps.sa.edu.au/home/jack/why.html [Broken]
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Nov 17, 2004 #2


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    Huh? That's a new one on me. Diamond and plastic both have *lots* of carbon atoms per molecule and both burn.

    Regarding explosives:
    A simple explosion might be a steam explosion where water turns to steam en masse - just a phase transition.
    Conventional explosives typically work by having a fast exothermic reaction combined with a phase transition.
    For example [itex]NH_4NO_3[/itex] ammonium nitrate is a classic explovsive. I believe the reaction goes something like [tex]2NH_4NO_3 \rightarrow 2N_2 + 2H_2O+O_2[/tex]. Two moles on the right hand side might take up a few hunderd CC's while the same material as a gas on the left hand side take up about 100 Liters.
    Last edited by a moderator: May 1, 2017
  4. Nov 17, 2004 #3
    What makes something burn so fast?
    Go to the website, to know what I'm talking about.
  5. Nov 17, 2004 #4


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    perhaps they are talking about these ease of burning hydrocarbons, or thnigs that contain hydrocarbons. if this is the case then the number of carbon atoms per mole of the substance will reflect how much oxygen gas need to combust it.
    for exmaple, methane with only 1 carbon and oxygen combusts to form carbon dioxide and water:
    CH4 + 2O2 --> CO2 + 2H2O
    needs only 2 moles/4 atoms of oxygen, but
    with pentane (5 carbons)....
    C5H12 + 8O2 --> 5CO2 + 6H2O
    it needs 8 moles/16 atoms of oxygen to combust compleatly.
    the availibility of oxygen is what slows it down, obviously the reaction does not take place instantly, the hydrocarbon needs time to mix properly with the oxygen.

    also, some stuff if just more prone to burn than others, for example, a huge tree versus a peice of paper, which is easier to catch on fire? which will burn longer?
    stuff that is allready a gas, like methane is easier to burn than a liquid like pentane, or even more dense stuff.
  6. Nov 17, 2004 #5
    Okay, now I understand. Thank you very much.
  7. Nov 20, 2004 #6
    Electrons are attracted to protons, but repell electrons. So, instead of all the electrons being bunched up right next to the nucleas, they orbit around the nucleas in shells. These shells can sometimes contain sub-shells. For example, the first shell contains only one sub-shell. As an electron gets further away from it's atom, it must have more "quantum energy." Electrons want to get as close to the nucleas as possible, but according to quantum physics, no to electrons can have the same "quantum energy." So, they orbit in shells. The electrons orbit in orbitals. The sub-shells have orbitals. For example, the 1 shell has an S orbital. Because it's an s orbital and it's the first shell it's labelled 1S. For 1-First shell-, S-S orbital. An S orbital has the shape of a sphere. An orbital wants to fill it's self. Alright, so why would the atom want to have 8 electrons in it's outer most shell, good question. The second shell has two sub-shells. One sub-shell has an S orbital, and the second has three P orbitals. The reason it has three is because they can arrange themselves according to X,Y,Z. Each orbital has only two electrons, because no two electrons can have the same "quantum energy." So, for the valence shell of an atom with two shells, one S orbital and three P orbitals. Two electrons an orbital adds to...8. Hydogen, on the other hand, only has one shell. So, to fill it's valence shell, it only needs two electrons. It already has one - Hydogen = one proton, one electron - so, it only needs to bond with one atom to fill itself. Carbon, on the other hand, has two shells, so it needs 8 to fill it's valence shell. So....

    H C H Methane!!! CH4.

    If you were to count it up everyone's filled. The carbon atom has 6 electrons. 2 in it's first shell, and 4 in it's valence shell. It needs 8 in it's valence shell. So, it shares one with hydrogen, and the hydrogen shares one of the carbons. This gives the carbon an extra electron, and the hydrogen it's desired two. The carbon, then, bonds with three more to add to 8.

    HOH Water!!! H20. Oxygen has six valence electrons, meaning it needs 2 to gain, which it does with 2 hydrogen molecules.

    O=O Oxygen!!! O2.

    You're probably wondering, why is there an equals sign between the Oxygen molecules?
    This indicated a double bond. Oxygen has six valence electrons, when it bonds with another oxygen, it gets 7. That's not the desired 8. So, it makes a double bond, and they share two electrons each. Which adds to 8.

    O O Ozone!!! O3. Each one of these atoms share with each other, making 8.

    That's covelant bonding!!!
    This "quantum energy I told you about is somewhat true. What's really true is that there are four "quantum numbers" that cannot match.
    The first is N.
    N is the energy of an electron. For example, an electron in the first shell would have an N of 1. An electron in the second shell would have an N of 2. An electron in the third shell would have an N of 3.
    N=1, means it's in the first shell.
    The second is L. It's actually a greek cursive L kind of like this. l. Okay. This sign is the orbital. L = N - 1. That's the equasion. So, if N = 1, then, L = 0. 0 is an S orbital.
    If N = 2, L can equal either 0 or 1. If it is 1, that's a P orbital. If N = 3, then that can be either 0,1 or 2. An S,P or...a D orbital.
    Now, the third quantum number is M. It is the orientation of the orbitals, you know XYZ.
    M can equal anything between -L and +L. For example if L is 1, then M can equal -1,0,1.
    This is 3 different ways of arranging the P orbital.
    Now the final one is Ms. For Spin. The spin of the electron can equal - 1/2 or 1/2.

    Okay, so let's look at the possible arrangements of some electrons.

    N L M Ms
    1 0 0 -1/2
    1 0 0 1/2 First shell, only can have two electrons.

    2 0 0 -1/2
    2 0 0 1/2
    2 1 -1 -1/2
    2 1 -1 1/2
    2 1 0 -1/2
    2 1 0 1/2
    2 1 1 -1/2
    2 1 1 1/2 Second shell, eight electrons, but none of them, nor the one's in the first shell have the same 4 quantum numbers.


    http://chemed.chem.purdue.edu/gench...h6/quantum.html [Broken]

    http://lectureonline.cl.msu.edu/~mm...od/electron.htm [Broken]
    Last edited by a moderator: May 1, 2017
  8. Nov 20, 2004 #7
    What makes something burn so fast is a large [tex]\Delta G[/tex] value. [tex]\Delta G[/tex] is called the change in Gibbs Free energy. It is a pretty fundamental concept in chemistry that you will get to if you take college-level chemistry.

    For a straight chain alkanes of the formula [tex]C_nH_{2n+2}[/tex] [tex]\Delta G[/tex] increases as n increases. You are probably already familiar with one alkane where n = 8, octane, which is the primary component of gasoline. Methane, CH[tex]_4[/tex], is another. Butane, n = 4, is commonly used in cigarette lighters. Increasingly larger chains are more powerful not only per molecule(which should be evident as you simply have more carbon atoms), but also per kg. That is why alkane based rocket and jet fuel need to be more heavily refined, as they need more power per kg. They often contains chains in the range of 12-20 carbon atoms per molecule.

    Diamond and plastic burn? Well anything burns, but they should both be endothermic at STP. Perhaps some strange plastics are not.
  9. Nov 20, 2004 #8
    Dual Op Amp, if I don't mind my asking, how old are you? That website looks like it was written by a 9 year old.
  10. Dec 24, 2004 #9
    I'm not sure what you meant by that website, I didn't make any of these websites, I just posted them. If you're talking about my entire post, well then, how rude. But, anyway. 17.
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