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Topic: Need assistance with Thermite Demonstration  (Read 12429 times)

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500r420

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Need assistance with Thermite Demonstration
« on: May 23, 2006, 07:30:56 PM »
So I finished the AP exam, and the juniors of my class (myself included) are preparing several (teacher approved) demonstrations apiece to show some middle school kids that chemistry is cool. Ive been doing well my my demonstrations with the exception of my thermite. My mixture never reacts fully. I keep it in a double terra cotta pot on a large ringstand over a wet sand pit. I use red iron oxide Fe2O3 and powdered aluminum metal and ignite the mixture with a coiled piece of magnesium metal. The reaction only occurs near where the magnesium metal is put into the mixture. A large amount of a very dark red/orange powder is formed in the reaction which i believe is another iron oxide. The ratio i am using is 7.5 to 28 which i know has a little more aluminum than the molar ratio but an online resource recommended this. Any help making this a spectacular demonstration is appreciated.
Thanks
David

Offline mike

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Re: Need assistance with Thermite Demonstration
« Reply #1 on: May 23, 2006, 08:12:23 PM »
I would very strongly recommend you DO NOT use wet sand. There have been a souple of nasty accidents with thermite around any water so this is to be avoided, USE DRY SAND.

Your ratio doesn't look right. I use 50g iron oxide and 15g Aluminum. Not sure if you are quoting molar ratio or not though?

I use very fine Al powder and make sure the thermite mixture is very well mixed.

I place one piece of Mg strip (cleaned) into the centre of the thermite so it extends down into the mixture.

I ignite the Mg ribbon with glycerine and permanganate.

Use a shield, and fire proof mat.
Spectators should be far away from the reaction, do not let them sit/stand close to this demo!
Spectators should be wearing safety glasses.
Spectators should be wearing long sleeves, or better still lab coats.
Fire fighting equipment should be on hand to put out flames caused in any accidents (note that this will not put out the thermite itself though, merely contain surrounding fire.)

Good luck, any other questions? I do this demo for my students..
There is no science without fancy, and no art without facts.

500r420

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Re: Need assistance with Thermite Demonstration
« Reply #2 on: May 24, 2006, 07:40:50 AM »
Thanks for your help mike. Im glad someone pointed out not to use wet set. I thought people used thermite to weld underwater though? The hot iron reacts with the water i guess? Well ill give your mixture a try (the one I was using was mass ratio) and ill see how it goes. Also ive had no trouble lighting the Mg with a candle flame.

Offline pantone159

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Re: Need assistance with Thermite Demonstration
« Reply #3 on: May 24, 2006, 03:06:33 PM »
Thanks for your help mike. Im glad someone pointed out not to use wet set. I thought people used thermite to weld underwater though? The hot iron reacts with the water i guess?

I think that the hot iron does react with water, releasing potentially explosive hydrogen gas.  (As a side effect, turning the iron back into iron oxide, which may be why you ended up with so much.)  Underwater, I guess, there is no oxygen for the hydrogen to burn in, so no harm is done.  On the surface though, keep the H2O away.  (That goes for any fire-extinguishing that comes into play also.  DON'T use water, use a dry fire extinguisher.)

Offline Borek

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Re: Need assistance with Thermite Demonstration
« Reply #4 on: May 24, 2006, 03:15:53 PM »
Using common reasoning - even small amount of water in so high temperature will boil instantly, with possible splashing of molten metal into all directions. Hydrogen thing may probably happen as well.
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Offline Will

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Re: Need assistance with Thermite Demonstration
« Reply #5 on: May 24, 2006, 03:47:06 PM »
It is in fact recommended by these guys that you do it underwater.
Although I would trust mike more than 'CLEAPSS'.

Offline mike

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Re: Need assistance with Thermite Demonstration
« Reply #6 on: May 24, 2006, 07:57:23 PM »
The only reason I light the Mg ribbon with glycerine and permanganate is for a the novelty factor. I think students find it interesting that you can light the whole thing off just by adding a few drops of glycerin to the mixture and standing back, ie no flame required to produce such a spectacular reaction. Try it and you will see what I mean. Let us know how it goes.

will17: I watched your movie and their reaction seemed to work just fine. However I have heard of reports where this can go very wrong, as Borek said the molten iron flying across the room and burning people etc. I am going to continue doing it into dry sand and using flower pots as the reaction vessel.
« Last Edit: May 24, 2006, 08:22:45 PM by mike »
There is no science without fancy, and no art without facts.

500r420

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Re: Need assistance with Thermite Demonstration
« Reply #7 on: May 24, 2006, 10:27:21 PM »
First off, thanks for your advice everyone.
I am going to be testing this reaction with black iron oxide as well. I worked out a mass ratio but Im sure I made a mistake.

Fe(II) + 2e- ->Fe
[Fe(III) + 3e- ->Fe]2
8 moles of e- gained per mole of Fe3O4

Al - 3e -> Al2O3
3 moles of e- donated per mole of Al

So 8:3 molar ratio

Which leaves 2 moles of Al to 3 moles of O
matching forumla of Aluminum Oxide, Al2O3

3(molar mass of Fe3O4=231.55g/mole)=695g
8(molar mass of Al=27g/mole)=216g

Comes out to 3.22:1 ratio Fe3O4 to Al by mass

Id appreciate if anyone would point out if I messed up somewhere or reccomend another ratio. Ill be testing third period tomorrow.

Offline mike

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Re: Need assistance with Thermite Demonstration
« Reply #8 on: May 24, 2006, 10:45:08 PM »
3Fe3O4 + 8Al -----> 4Al2O3 + 9Fe

Looks right to me :)

Maybe don't use the whole 695g and 216g though! That would be one huge reaction :o I only use a couple of grams when I do it, but depends how much space you have to do it in. I do it in a fumehood so space is restricted and small scale means less likely to ruin expensive hood ;)

Also why not try my suggestion of lighting with glycerin and permanganate? Remember to add the glycerine at the very end just before you want it to light up.

I have attached a pdf of the procedure I roughly follow for doing the demo, although the mass of thermite is different and you can change this to suit your requirements.
« Last Edit: May 24, 2006, 10:55:42 PM by mike »
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Offline billnotgatez

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Re: Need assistance with Thermite Demonstration
« Reply #9 on: May 25, 2006, 01:43:41 AM »
There is an axiom that says; “You know just enough to be dangerous”. This implies that you have the technical knowledge but not the practical knowledge.

You do the mathematics and end up with 695 grams when you should be using 6.95 grams. Even with the smaller amount you might end up with a reaction that will be so grandiose as to overwhelm any safety precautions you take. 6.95 grams still seems like too much for a classroom demonstration.

My understanding is that Thermit reactions are very dependent on concentration of each of the constituents.

I would hope you practice any classroom demonstration before hand with someone watching at a distance in case of problems.

I might invoke images of the China syndrome, but that would be a bit of hyperbole. And yet, you notice that when you see any demonstrations done all the participants run a good distance away, even when it is a small amount of reactants. By the way, do they call it the China syndrome in Australia or is it the American syndrome?  Albany, Australia is almost opposite Albany, New York USA on the earth.

It seems we have had a rash of request to discuss this for a classroom demonstration. It would be boring to have 40 students doing the same thing.


Offline mike

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Re: Need assistance with Thermite Demonstration
« Reply #10 on: May 25, 2006, 02:26:17 AM »
billnotgatez: I hadn't heard of the China syndrome until today, *lol* that is funny!

Before I started the thermite reaction as a demonstration here is what I did:

*Researched the theory
*Read how other places do it
*Researched the reported incidences of accidents during thermite reactions (there have been quite a few)
*Picked what I thought would be the most suitable/safe method
*tested that method on 1/10 of the scale suggested
*first 6 tests were outside
*next 10 tests were inside in the fumehood.
*once I was happy it was reproducible in my fume hood I then took all the equipment to where the demo was to be performed and without any audience did another 5 or so test in this environment

We ran about 12 demos of the thermite reaction a few weeks back in front of students without incident.

Thermite was in the fumehood.
Spectators all wore safety glasses, lab coats and gloves as well as the obligatory closed shoes, tied back hair etc etc.
The scale was still small (<5grams).
No water was used at all.
Fire fighting apparatus were on hand = fire blanket, fire sand, extinguishers, fire hose reel, shower, eye wash, exhaust fan.

So as you can see I took this as a very serious exercise and I strongly recommend anyone else doing this experiment does at least the same preparation if not more. :) :)
There is no science without fancy, and no art without facts.

500r420

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Re: Need assistance with Thermite Demonstration
« Reply #11 on: May 25, 2006, 10:11:04 AM »
Hold up a minute folks, you forgot that this is a highschool chem lab. its not as though my AP Chem teacher isnt as obsessive about safety as any other. The number 695 was the mass for 3 moles of the iron oxide, which I used to get a ratio, thats it. I have been practicing with about 15g of mixture and will be using around 80 when I do the demonstration. I perform it on the blacktop counter at the front of the room which has at least 25 feet distance from anything flammable. When I go to the schools, I will perform it outside the school.

Also Ive been having some problems with another demo of mine, water synthesis. I make H2 and O2 gas with water hydrolysis, ignite the H2 gas in a test tube then restart the fire on the splint with the O2. The next part doesnt work so well, where I produce the Reactants and store them in a balloon with chemical reactions. My goal is to make one balloon of H2 gas double the size of an O2 balloon, but my H2 reaction creates the gas with much less pressure. I am disintegrating Al foil in Strong base and decomposing Hydrogen Peroxide with Manganese Dioxide. When I set up the balloons (the O2 one usually being larger) the explosion of the H2 which is very small never bursts the O2 balloon. How do you chem experts do this one?

Offline mike

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Re: Need assistance with Thermite Demonstration
« Reply #12 on: May 25, 2006, 07:48:43 PM »
Quote
Hold up a minute folks, you forgot that this is a highschool chem lab. its not as though my AP Chem teacher isnt as obsessive about safety as any other. The number 695 was the mass for 3 moles of the iron oxide, which I used to get a ratio, thats it. I have been practicing with about 15g of mixture and will be using around 80 when I do the demonstration. I perform it on the blacktop counter at the front of the room which has at least 25 feet distance from anything flammable. When I go to the schools, I will perform it outside the school.

I just added all the safety stuff for other people who may read this post.... I hope it all goes really well :)

Quote
How do you chem experts do this one?

We actually just fill the balloons from cylinders.
There is no science without fancy, and no art without facts.

500r420

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Re: Need assistance with Thermite Demonstration
« Reply #13 on: May 25, 2006, 09:53:45 PM »
The thermite went spectacularly today. I ignited about 30g and the iron actually poured out the bottom of the pot. The Water synthesis went, how say, less than well. Its either school, or county, or state policy that we cant keep compressed gas so I have to make it. I tried using powdered aluminum to speed up the process and ended up with hot 4 molar KOH all over my hands and wrists. No damage done, but I think Ive got a better idea. I will use a stopper with a funnel hose attached to the balloon, perform the reaction, and then just switch the hose to the other vessel.

Offline mike

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Re: Need assistance with Thermite Demonstration
« Reply #14 on: May 25, 2006, 09:58:59 PM »
Quote
The thermite went spectacularly today.

Well done, sounds like a success, I presume the audience was suitably impressed :)

Good on you!
There is no science without fancy, and no art without facts.

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