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Post by jaydeehess on Dec 17, 2005 2:22:11 GMT -4
Just watched an episode of "Numbers" in which a car lot booth explodes when a guy opens the door. Backdraft is blamed but our intrepid mathimaticians prove that it had to have had an accellerant that was not detectable. They conclude that the accellerant was high concentration hydrogen peroxide. The theory is that a smoldering fire is lit in the booth, H2O2 is poured out in the booth too. The smoldering fire heats the interior up, evaporating the H2O2 and adding heated gasses from the fire. When the booth door is opened the inrush of oxygen then allows all of the gasses and H2O2 to ignite at once.
However, it seems to me that H2O2 IS an oxidizer not a fuel. This senario would not the require the door to be opened, oxygen would be supplied by the H.peroxide.
Or do I have my head in a well known dark place on this?
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Bob B.
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Post by Bob B. on Dec 17, 2005 4:31:11 GMT -4
What you describe sounds fishy to me as well. Hydrogen peroxide is most definitely an oxidizer and easily decomposes into water and oxygen. If the H2O2 was heated enough to vaporize, I think it would have decomposed and supplied oxygen to the smoldering fire causing it to burn more aggressively. I don't see why this mixture would have sat there all stable waiting for someone to open the door.
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Post by jaydeehess on Dec 17, 2005 19:36:21 GMT -4
Ah ha, then the lesson is to never send a mathamatician to do a chemist's job. ;D or worse yet let a TV writer put words into a supposed genius' mouth.
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Bob B.
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Post by Bob B. on Dec 18, 2005 1:53:22 GMT -4
Of course I could be wrong. I believe PhantomWolf is a chemist. What do you think, PW?
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Post by PhantomWolf on Dec 18, 2005 7:18:05 GMT -4
I hadn't really bothered to answer because you're both right. Heating Hydrogen Peroxide causes it to decompose into water and oxygen as shown below. 2 H2O2 -> 2 H20 + O2 It can cause explosions if it is highly concentrated and then reacted with certain metals or organic compounds, but in those reactions it is indeed the oxidiser in the reaction, and this isn't the case in the example given above. So, yes it is bad chemistry, but since you'd already come to that conclusion before I got to this thread I didn't see it worth seconding at the time.
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Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
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Post by Al Johnston on Dec 18, 2005 8:48:19 GMT -4
The catalysed decomposition of hydrogen peroxide torpedo propellant is believed to be responsible for the explosions that caused the losses of HMS Thetis and the Kursk
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Bob B.
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Post by Bob B. on Dec 18, 2005 11:32:51 GMT -4
Heating Hydrogen Peroxide causes it to decompose into water and oxygen as shown below. 2 H2O2 -> 2 H20 + O2 I'd like to add one more thing to your equation, 2 H2O2 --> 2 H20 + O2 + HeatThe decomposition releases heat, which then increases the rate of decomposition. It is therefore a self-sustaining reaction that can lead to explosions. There are many commonplace substances that can act as a catalyst to start the decompostition of hydrogen peroxide.
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Post by PhantomWolf on Dec 18, 2005 21:05:32 GMT -4
Well, yes it can lead to an explosion, but it's not the H2O2 that explodes, the increased O2 in the enviroment makes normally benign items highly explosive. You can see this with a neat experiment. Get a jar of oxygen and then heat up some steel wool. In air it's totally unreactive, but open the jar and plunge the hot steel wool into it. The result is VERY impressive. This is the same thing with the heating of H2O2. It increases the O2 levels to the point where things just burn really well, like what happened with Apollo 1. (see and you were wondering where we'd get an Apollo reference in here. )
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Post by PhantomWolf on Dec 18, 2005 21:17:02 GMT -4
I think a way of making it work would be to have a fuel in the booth, making sure it's airtight, and a non-ignitive source of heat for the H202, this would raise the levels of 02 in the booth to dangerous levels without anything igniting. Then have an ignition source on the door so that when it opens it produces a spark. If your fuel is a gas as well even better, it'll make a big bang. (though this will most likely be an implosion rather then an explosion.)
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Bob B.
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Post by Bob B. on Dec 18, 2005 22:05:35 GMT -4
The decomposition releases heat, which then increases the rate of decomposition. It is therefore a self-sustaining reaction that can lead to explosions. Well, yes it can lead to an explosion, but it's not the H2O2 that explodes, the increased O2 in the enviroment makes normally benign items highly explosive. I wasn't referring to an explosion in the normal sense of a fuel and oxidizer burning. If the hydrogen peroxide is in a sealed contained and it begins to decompose, the pressure build up can cause the container to burst. I've heard the whole thing can go whoosh into a cloud of superheated steam and oxygen.
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Post by PhantomWolf on Dec 18, 2005 22:28:49 GMT -4
Ahhh, though of course that depends on the container and how much pressure it can take, but then you can do that with heating any volitile liquid really.
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Post by PhantomWolf on Dec 18, 2005 22:54:18 GMT -4
Oh, just to show how much energy this reaction produces.
In the reaction, we have to break an 4 O-H bonds and 2 O-O bonds, then create one O=O bond and 4 O-H bonds. Obviously the O-H bonds cancel out, so we have 2 O-O bonds on one side and one O=O on the other.
the O-O bond requires an energy of 145 kJ/mol to break while the O=O bond releases 498 kJ/mol.
Thus we get
2 O-O - O=O = (2 x 145) - 498 = 290 - 498 = -208 kJ/mol
This would indicate that it is an exothermic reaction, producing energy of 208 kJ/mol.
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Bob B.
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Post by Bob B. on Dec 19, 2005 11:23:04 GMT -4
...but then you can do that with heating any volitile liquid really. Ah... but often that heat must come from an external source. Hydrogen peroxide generates its own heat. One mole of substance is the number of grams equal to its molecular weight, correct? Thus one mole of H2O2 is 34 grams. Using your figures, the amount of energy released by the decomposition of H2O2 is, 208 kJ/34 g = 6.12 kJ/g. Let's say we have a liter of hydrogen peroxide, i.e. 1,440 grams. If 5% of this decomposes, then the energy generated is, 1440 g x 0.05 x 6.12 kJ/g = 441 kJ. How much will this amount of energy raise the temperature of the H2O2? I should know the answer to this but its been 28 years since my last chemistry class. I know that 441 kJ (105,400 calories) will raise 1,440 g of water 73 o C, but I don't know about H2O2. It seems that once the decomposition begins, it's possible to get a run away reaction.
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Post by jaydeehess on Dec 19, 2005 13:53:29 GMT -4
The short answer though is that this could not have taken place as it was said to have in the show. The whole basis of this show is the genius math whiz who, along with his mentor and his brother(an FBI agent) use science to find the bad guy.
In the booth, if the temp was raised such that the H2O2 was decomposing and raising both the heat and the O2 levels in that confined space then it would most likely have gone up on its own since the smoldering fire(a cigarette under a cushion) woiuld have provided the ignition source and the contents of the booth would include many combustible items that would do to this booth what high O2 levels and (amoung other things) velcro did to Apollo 1.
In fact since the booth had not reached this point yet, opening the dorr would immediatly cool the volume and lower the O2 concentration in the booth thus alleviating the danger.
There have been other assertions and conclusions reached by the hero on this show in the few episodes that I have seen that I would question. This is the first one that really jumped out at me as being false.
Then there is "CSI" and its spin offs and their playing fast and loose with science. But its all entertainment and if it spurs a kid to actually learn about the true science and puts scientifically savvy persons in the role of hero I am all for it. Beats the hell out of Bruce Willis, "Die Hard" role models.
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Post by nomuse on Dec 19, 2005 16:35:46 GMT -4
I laud CSI because they (mostly) walk the walk. They make observations, take them back to the lab, make a hypothesis based on those observations and test it, either through experiment or fresh observations. And they disprove (falsify) as many hypothesis as they prove (if not more). They even set up cases of different investigators supporting competing theories -- until the weight of evidence finally comes down to one.
Of course, they get a lot wrong -- starting from the (necessary) series premise, pretty much the same "So the Captain and the Chief Engineer beam down..." problem. And as the episodes unfold as a scientific puzzle they must educate the audience as well, which involves lots of variations of the "idiot lecture."
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