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Thread: Stoichiometry Problem

  1. #1 Stoichiometry Problem 
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    So this problem deals with the poor man's Tannerite(NH4NO3 plus aluminum) that I take to the rifle range occasionally. The reaction when struck with a high velocity rifle bullet creates a loud report and a white vapor.

    So I initially thought the reaction looked like this: NH4NO3 + 2Al ----> Al2O3 + N2 + H2

    However, someone mentioned that when tannerite is set off it produces water vapor. I'm not sure if they are right or I am...

    So is my guess at it close to being correct or not?


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    I know almost nothing about explosives but here is a chemists point of view:

    Ammonium nitrate thermally decomposes (above 200 C) to give N2O and H2O

    NH4NO3 = N2O + 2H2O

    It can also be induced to decompose explosively by detonation. The aluminium (powder I'm guessing) acts as a catalyst to lower the activation energy of the reaction, once the reaction is started it is very difficult to stop becomes a runaway reaction causing an explosion.


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  4. #3  
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    Ok, I didn't consider that the Al powder could have been a catalyst, I just assumed it was part of the reaction.
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    Actually thinking about it the aluminium would be oxidised by the O2 in air at the temperatures involved in the explosion so you actually have two separate reactions going on, the one I gave above and the oxidation of aluminium.
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  6. #5  
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    I think in this case the aluminum acts as a fuel like fuel oil does in an ANFO explosive.
    Ammonium nitrate by itself is very hard to detonate. If it was farmers would be blowing up all the time because they use it as fertlizer.

    However ammonium nitrate is an effective oxidizer so if you add a bit of fuel to it you end up with an explosive which is cap sensitive and comparable in strength to nitroglycerine dynamites.
    Some of the ANFO mixes we used in stick form had aluminum added but I think it was to increase detonation speed. The faster the detonation wave travels through an explosive the higher the shattering effect of the explosive is. I know it was not there to just increase sensitivity.(edit: I might be wrong about it being to increase sensitivity because the sticks with aluminum in them were small diameter sticks)

    ANFO has been known to self-detonate when exposed to heat and pressure, but the fact that it is easily handled and dirt cheap has made it the preferred industrial blasting powder in most cases.
    I had to work with the stuff for a while back in the 90s, but I am not a chemist and can not give you the details of how it works chemically, and I have never seen Tannerite so I don't know how it works either.

    Edit: we used the ANFO by packing it into drill holes in the ground. The reaction didn't depend on extra air.
    Last edited by dan hunter; February 23rd, 2014 at 08:24 PM. Reason: correction.
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  7. #6  
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    Fair enough, as I said I know nothing about explosives and was guessing as to the role of the aluminium but the decomposition reaction I gave for ammonium nitrate is what it will decompose into as far as I know. From your edit it may be that the reaction in the OP is what happens if oxygen is limited and the Al is oxidised by the nitrate but that's a guess.
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  8. #7  
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    Quote Originally Posted by PhDemon View Post
    Fair enough, as I said I know nothing about explosives and was guessing as to the role of the aluminium but the decomposition reaction I gave for ammonium nitrate is what it will decompose into as far as I know. From your edit it may be that the reaction in the OP is what happens if oxygen is limited and the Al is oxidised by the nitrate but that's a guess.
    Like I said I have no idea about the chemical reaction itself because I am not a chemist. I am not sure if his compounds are right.
    Aluminum is very greedy for oxygen though. In the thermite pots for welding steel it takes the oxygen from iron oxide and the heat of the reaction leaves you with molten iron.
    When the thermite burns to the bottom of the pot it flows into a clay mold around your track rail joint and welds the two butted rails together into one rail.

    So I think he might be right but I don't know if the aluminum can pull oxygen from water molecules and leave just hydrogen molecules behind.

    Now you are getting me curious about it too.
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  9. #8  
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    Aluminium will react with steam at high temperature to give aluminium oxide and hydrogen so it may be this does happen, I not aware of any reaction of Al with N20 though so this may remain as a product.
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  10. #9  
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    Quote Originally Posted by PhDemon View Post
    Aluminium will react with steam at high temperature to give aluminium oxide and hydrogen so it may be this does happen, I not aware of any reaction of Al with N20 though so this may remain as a product.
    That seems right. I looked up a video of somebody shooting tannerite.
    It makes a very large bang and a secondary hydrogen explosion using atmospheric oxygen might do that.
    It would also leave water vapour.

    The generation of NO2 gas from AN would be a pretty powerful explosion in its own right but the bang the tannerite seemed to make in the video seemed a bit larger than I would have expected from the amount they used. It is hard to judge things on videos.
    Last edited by dan hunter; February 21st, 2014 at 08:56 AM.
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  11. #10  
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    Quote Originally Posted by PhDemon View Post
    Aluminium will react with steam at high temperature to give aluminium oxide and hydrogen so it may be this does happen, I not aware of any reaction of Al with N20 though so this may remain as a product.
    I'm almost certain that N2O will react with Al under the high temperature conditions of the reaction. However, I have no evidence that it actually does react.
    dan hunter likes this.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  12. #11  
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by PhDemon View Post
    Aluminium will react with steam at high temperature to give aluminium oxide and hydrogen so it may be this does happen, I not aware of any reaction of Al with N20 though so this may remain as a product.
    I'm almost certain that N2O will react with Al under the high temperature conditions of the reaction. However, I have no evidence that it actually does react.
    We were told in the mine Nitrogen was the main gas produced. Also that a bit of CO and NOx was normal because conditions are never really perfect, so no matter what was said about how safe the smoke after a blast was, proper venting of the drifts and stopes was still very important.
    ....I just tried looking this up for ANFO and only found a few pages that are very general. Wikipedia gives a list of the products but it does not explain the reactions.
    The chemistry of ANFO detonation is the reaction of ammonium nitrate (NH4NO3) with a long chain alkane (CnH2n+2) to form nitrogen, carbon dioxide and water. In an idealstoichiometrically balanced reaction, ANFO is composed of approximately 94.3% AN and 5.7% FO by weight. The normal ratio recommended is 2 U.S. quarts of fuel oil per 50 pounds of ammonium nitrate (80 ml/kg). In practice, a slight excess of fuel oil is added, i.e., 2.5 to 3 quarts of fuel oil per 50 pounds of ammonium nitrate, as underdosing results in reduced performance while overdosing merely results in more post-blast fumes.[10] When detonation conditions are optimal, the aforementioned gases are the only products. In practical use, such conditions are impossible to attain, and blasts produce moderate amounts of toxic gases such as carbon monoxide and nitrogen oxides (NOx).Variants of ANFO using diesel fuel, kerosene, coal dust, racing fuel, or even molasses in place of the red diesel (Nš 2 fuel oil) have been used, and finely-powdered aluminium in the mixture will sensitise it to detonate more readily.[citation needed]
    Too bad wikipedia didn't give the reaction saying which part of which compound reacts with what.
    This thread is starting to drive me a bit nuts. I am really getting curious about just how ammonium nitrate works now.
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  13. #12  
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    Ok! I may or may not be onto something...SO, the aluminum in this reaction is the reducing agent, it is oxidized by the NO3- ions. The melting point of NH4NO3 is approx ~170C. The bullet that strikes the mixture(at least .223 travelling over 2000fps) has a temperature high enough (varies but general consensus I found was 267C) to create molten AN in which it has some nitrate ions in thermal equilibrium. So the nitrate reacts with the aluminum creating aluminum oxide and releases enough energy(-1676kJ/mol) to continue the decomposition/redox reactions.

    What do you think? Feasible or way off base?
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  14. #13  
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    I am still not sure if the aluminum can take oxygen from h2o but I think you are about right. Not being a chemist I would have to open a chemistry text and check the equations to be sure. The casual references I saw mentioned the idea of oxygen balance in the proportioning.
    Last edited by dan hunter; February 24th, 2014 at 07:20 AM.
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  15. #14  
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    I did read that aluminum powder would react with steam to form the aluminum oxide and hydrogen gas...My chemistry books don't have "the" equation, just information related to different parts of the reaction. I am just a beginner level chem student though.
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