# The Naked Scientists Forum

### Author Topic: Thermite  (Read 62405 times)

#### ATOM_SPLITTER

• First timers
• Posts: 9
##### Thermite
« on: 15/10/2005 05:28:33 »
Its a hard to light long burning substance which you can make out of iron oxide and alumiaum powder and my question is how and why is Thermite so hard to light when it burns so long and at such high temiture? also I've been wondering thermite is made of 3 parts iron oxide and two parts alumiaum powder what does the 'part' in each formula mean?

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#### itsjustme

• Jr. Member
• Posts: 24
##### Re: Thermite
« Reply #1 on: 15/10/2005 12:08:07 »
parts mean like percentage; 3 parts iron oxide, 2 parts aluminium. add the parts, you get 5 so each part is a fifth, does this make sense? the other question hmm... maybe you need high temprature to produce molten aluminium and start a chain reaction, just guessing.

#### rosy

• Neilep Level Member
• Posts: 1018
• Chemistry
##### Re: Thermite
« Reply #2 on: 15/10/2005 13:19:34 »
2:3 is a molar ratio rather than a proportion by mass. For every 2 atoms of Al you need 3 of Fe. So you'd have to calculate the masses you'd need:
2 Al + 3 FeO -> 2 AlO3 + 3 Fe

#### pyromaster222

• Jr. Member
• Posts: 30
##### Re: Thermite
« Reply #3 on: 15/10/2005 14:18:56 »
thermite burns so hot because iron oxide is a high temperature oxidiser and so it also needs a high temperature to start (gunpowder will not have enough heat to start thermite, you need to light it with magnesium)
the ratios will be:
2 moles of aluminium which is 2(27)=54grams to 3 moles of iron(I)oxide which is 3 (56+16)=216
so ratio is 54:216 which is 1:8 Al:FeO
this applies to black iron oxide which is different to rust (Fe2O3)
the ratio for rust is
54:160 Al:Fe2O3 thats about 3:8 and it burns rather fast too

#### ATOM_SPLITTER

• First timers
• Posts: 9
##### Re: Thermite
« Reply #4 on: 15/10/2005 14:43:54 »
I've also heard It be done with sulfric acid which I don't know if thats true but yes thank you all for replying! =) and Thermite is some amazing ang forgiving stuff! Thermite burns at up to 2000 degres sometimes!and you can also light it with potassium chlorate

YAYYYYYYY!
« Last Edit: 15/10/2005 14:47:35 by ATOM_SPLITTER »

#### pyromaster222

• Jr. Member
• Posts: 30
##### Re: Thermite
« Reply #5 on: 15/10/2005 18:12:20 »
there are many different thermites such as CaSO4/Al. this is a very high temperature thermite, more so than iron oxide i believe. it's also castable but is extremely hard to light. There are also thermites with copper oxide which burn very fast (like medium grade gunpowder)

#### robotjesusbeta

• Jr. Member
• Posts: 27
##### Re: Thermite
« Reply #6 on: 16/11/2005 20:57:19 »
Thermite is great Fun , personally I find the best way to ignight it  *at a safe distance* is to use a drinking straw, first glue one end shut with hotglue, now fill about 1 inch with a pure thermite mix, then ontop of that fill it with 1Magnesium/1Hexamine/1sulphur and ontop of that an   inch of 2Potassium permanganate/2Alu/1Hexamine, and fill the rest of the space with some low grade black powder. now just submerger 1/2 of the first inch of the straw in your thermite, the reason I recomend the Addition of Hexamine is to slow the burn rate of the flash powder down because you dont want the straw to bang and go out hehehe. Also I was tweaking my Thermite one time and decided to substitute 1g of ALu for 1g of MG and mixed it in fully, thinking 1g isnt going to make a hell of a difference was where I went wrong, the thermite package (50G overall) ignited and slung molton slag in about a 2 metre radius. You may think this was because of the container I had it in but it was totally unconfined and ignited with MG ribbon! So just be careful with subsitution of Metal powders

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#### Ylide

• Moderator
• Hero Member
• Posts: 905
##### Re: Thermite
« Reply #7 on: 17/11/2005 01:01:03 »
I've seen a thermite paste used before, it basically adheres the thermite to a surface that will be burned through, like a steel plate.   (the place I saw it used was cutting through the hull of a boat)  I'm curious what medium was used for the paste.  It would have to be something organic that would not diminish the burning properties of the thermite.

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#### robotjesusbeta

• Jr. Member
• Posts: 27
##### Re: Thermite
« Reply #8 on: 17/11/2005 18:09:51 »
hmm i have used Silacone sealant as a binder in other pyrotechnic formulas so i would expect it would work with thermite just aslong as you used a small amount by weight i recon with a 50g batch of thermite *30g al 20g iron ox* id say start with 7g of silacone sealant possible problems are:
it will set too fast before you work out your ratios correctly
it will set before you can mix it all in
you will end up with a lump of metalic rubber that doesnt ignight

Possible improvements:
Add magnesium 2g to a 50g batch this will ensure it all ignites

(REMEMBER A thermite reaction takes less than 2 seconds to peak after ignition) with the addition of Magnesium i would say a fragment of a second but with the sealant Im thinking the 1.2 seconds scale and then  you gotta have enough to break through the hull of this boat not gonna be easy a bit of trial and error i thinks, another possible problem if you dont use enough you will warp the metal to ****

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#### Life

• Jr. Member
• Posts: 13
##### Re: Thermite
« Reply #9 on: 21/02/2006 13:16:25 »
jut a quick question..

Does thermite burn when confined? and if so, does it effect the rate at which it burns?

#### anthony

• Full Member
• Posts: 65
##### Re: Thermite
« Reply #10 on: 24/02/2006 00:28:53 »
Yes. Thermite burns when confined as it contains it's own oxidiser (iron oxide) and fuel (aluminium). The rate would be unchanged.

The, as yet, unanswered question as to why it is so difficult to light has a simple answer, it requires a very high activiation energy. The reason it gets so hot is that the reaction is highly exothermic. Classic school chemistry concepts. Why it has such a high activation energy is that the reactants and products are actually quite stable, whereas the intermediates are very unstable and require a high energy input to form.
« Last Edit: 24/02/2006 00:31:09 by anthony »

#### Ray hinton

• Sr. Member
• Posts: 476
##### Re: Thermite
« Reply #11 on: 24/02/2006 01:17:29 »
anthony
let me know when you intend testing your thermite,i will pop outside and see if i can catch your fly past(video camera is on charge).

#### DrDick

• Sr. Member
• Posts: 162
##### Re: Thermite
« Reply #12 on: 04/04/2006 20:17:09 »
quote:
Originally posted by pyromaster222

there are many different thermites such as CaSO4/Al. this is a very high temperature thermite, more so than iron oxide i believe. it's also castable but is extremely hard to light. There are also thermites with copper oxide which burn very fast (like medium grade gunpowder)

Are you sure you don't mean CuSO4/Al?  To convert a metal cation to the elemental metal, you need a more active metal (one that wants to be in the cationic state even more) to perform the conversion.  Al is more active than Fe, so Al metal will react with oxidized Fe but Fe metal won't react with oxidized Al.  Likewise, Ca is more active than Al, so oxidized Ca won't react with Al metal.  However, Cu is a pretty inactive metal, so oxidized Cu will react with Al metal.

DrDick

#### ATOM_SPLITTER

• First timers
• Posts: 9
##### Re: Thermite
« Reply #13 on: 12/07/2006 20:15:56 »
it's amazing what thermite can do,and how amazing it is

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#### lightarrow

• Neilep Level Member
• Posts: 4586
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##### Re: Thermite
« Reply #14 on: 09/08/2006 14:12:04 »
quote:
Originally posted by DrDick

quote:
Originally posted by pyromaster222

there are many different thermites such as CaSO4/Al. this is a very high temperature thermite, more so than iron oxide i believe. it's also castable but is extremely hard to light. There are also thermites with copper oxide which burn very fast (like medium grade gunpowder)

Are you sure you don't mean CuSO4/Al?  To convert a metal cation to the elemental metal, you need a more active metal (one that wants to be in the cationic state even more) to perform the conversion.  Al is more active than Fe, so Al metal will react with oxidized Fe but Fe metal won't react with oxidized Al.  Likewise, Ca is more active than Al, so oxidized Ca won't react with Al metal.  However, Cu is a pretty inactive metal, so oxidized Cu will react with Al metal.

You are right when you say that CaO cannot be reduced from Al; you could be wrong when you believe that, in CaSO4, Al must reduce CaO: it could reduce SO3; I have never tried this thermite, however.

About 3CuO + 2Al = 3Cu + Al2O3, I have tried it several times. If powders are very fine, the reaction is almost as quick as flash powder! Delta H of formation of CuO is much lower (in absolute value) than that of Fe2O3, for example, so the heat of reaction is Very high. Probably, some of this heat decomposes CuO producing oxigen gas, and this makes the reaction explosive. Don't confine this thermite at all!

#### lightarrow

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##### Re: Thermite
« Reply #15 on: 05/09/2006 11:34:14 »
Two days ago I made thermite with CaSO4 and Al. It really works! Before having read in more than one site that this is the right formula, I would have thought it was wrong too.
It's incredible what Al powder is able to reduce!

Assuming the reaction is: CaSO4 + 2Al --> Al2O3 + CaO + S, I added iron to combine with S (I thought hot gaseous S released could make the reaction dangerous and lose heat) , so that the reaction should have been:
CaSO4 + 2Al + Fe --> Al2O3 + CaO + FeS. I'm not sure about these equations, however; anyway the reaction was fast and very hot! I have ignited it with a little amount of normal thermite.

I used: 11.5grams of CaSO4, 4.6g Al, 4.7g Fe.
Al: fine, grey powder.
CaSO4: dried with kitchen burner.
Fe: powder.
« Last Edit: 05/09/2006 19:41:51 by lightarrow »

#### Mr Andrew

• Sr. Member
• Posts: 206
• God was primitive man's attempt at Physics.
##### Re: Thermite
« Reply #16 on: 12/09/2006 19:18:13 »
How is the Fe oxidized to combine with the sulphur(which must be reduced again to combine with the Fe)?  I don't think the Fe really does anything but then again the heat of the reaction could be enough for the reaction to form FeS to occur.

Either way it is very interesting an I think it is worth looking into more. (Yay, we've actually got a discussion on an experiment!!!)
I think I'm going to run this experiment sometime and see what happens.

Does anyone know the standard enthalpy of formation of S from 6+? What about [Al]3+ from Al metal? Those figures would help in determining how much material to use to make sure it doesn't kill somebody.

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#### lightarrow

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##### Re: Thermite
« Reply #17 on: 13/09/2006 07:09:41 »
Quote
How is the Fe oxidized to combine with the sulphur(which must be reduced again to combine with the Fe)?  I don't think the Fe really does anything but then again the heat of the reaction could be enough for the reaction to form FeS to occur.

Does anyone know the standard enthalpy of formation of S from S6+? What about Al3+ from Al metal? Those figures would help in determining how much material to use to make sure it doesn't kill somebody.
DeltaH°(f) FeS = -101.67
(data from:
http://webbook.nist.gov/cgi/cbook.cgi?Formula=FeS&NoIon=on&Units=SI&cTC=on&cTR=on)
so, actually, the formation of FeS releases heat.

S (that is: SO42-) is reduced from Al (it's this reaction that releases the most of the heat) and then S combines with Fe.
« Last Edit: 02/08/2007 19:58:28 by lightarrow »

#### Mr Andrew

• Sr. Member
• Posts: 206
• God was primitive man's attempt at Physics.
##### Re: Thermite
« Reply #18 on: 15/09/2006 22:21:14 »
I think I get it. Just to make sure, if you take S and Fe (that is the elements) and heat them together do you get FeS? If you do then I think it makes sense.

I think, for safety sake, I am going to run the reaction in a galvanic cell so the energy produced is in the form of electricity and that can be easily controlled (i.e. disconnecting the electrodes). Also, I think I'll leave out the Fe to make sure that SO2 is really formed (the S is reduced). I've thought about this and come up with the following equation:

2Al + 3[Ca]2+ + 3[SO4]2- --> 2[Al]3+ 3SO2 + 3[Ca]2+ + 6
• 2-

I can't see why the S might go all the way from 6+ to 2- (the ox-state needed to make FeS) when it could stop at 4+ and make the molecule SO2 which is not extremely active and smells oh-so-wonderful. Maybe in the presence of Fe it does that, I don't know for sure.  The fact that O2 can be used from the air to mix with the Ca or Al makes all that much harder to determine the mechanism. The oxygen ions left over might form O2 molecules and reduce the S more (though oxygen is an oxidizing agent-imagine that- so I don't know that this would be probable).

Oh man, I really need to run this experiment in the lab!! Oh, that reminds me...my chem teacher needs documentation of this experiment and what should happen (kind of like a formal lab report of this reacton being run previously or a demo designed around this reaction...etc.) so he can use the school's equipment and can't be fired/sued/well, you get the idea. If he doesn't have something that he can pick up (like a book of demos) and have a procedure and materials and expected results right there in front of him he is not willing to let me do it :(.

Any help would be appreciated greatly!

"His mind is the ultimate weapon!"-MacGyver television series

#### lightarrow

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##### Re: Thermite
« Reply #19 on: 16/09/2006 08:12:56 »
quote:
Originally posted by Mr Andrew

I think I get it. Just to make sure, if you take S and Fe (that is the elements) and heat them together do you get FeS? If you do then I think it makes sense.
Yes, it's a classical experiment for high school lessons, showing the fact two different elements (Fe and S) after a chemical reaction becomes a completely different substance (Fe is not present anymore, because the compound it's not affected from a magnet anymore). You simply mix together very well 56g of Fe powder, 32g S powder, put the mix inside a test tube and then heat it with a flame. After a while, the mix becomes very hot and the reaction proceeds by itself.
quote:

2Al + 3[Ca]2+ + 3[SO4]2- --> 2[Al]3+ 3SO2 + 3[Ca]2+ + 6
• 2-

I can't see why the S might go all the way from 6+ to 2- (the ox-state needed to make FeS) when it could stop at 4+ and make the molecule SO2 which is not extremely active and smells oh-so-wonderful.
DeltaH°(formation) SO3 = -395.77 KJ/mol ; DeltaH°(f) Fe2O3 = -825.5 KJ/mol (data from NIST); it means that, if Al reduces Fe2O3 to Fe, it reduces much better SO3 to S. Certainly we should also consider the heat necessary to decompose CaSO4 to CaO, I couldn't find it, but it's surely much less than those previous values.
I don't know what could happen in an electrolitycal cell, however.
« Last Edit: 16/09/2006 08:15:50 by lightarrow »

#### Mr Andrew

• Sr. Member
• Posts: 206
• God was primitive man's attempt at Physics.
##### Re: Thermite
« Reply #20 on: 16/09/2006 19:46:59 »
Ok, that makes sense.

[:(!]I still can't find any official documentation on the CaSO4/Al Thermite reaction so I can run it in school. Please help me!

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#### Mr Andrew

• Sr. Member
• Posts: 206
• God was primitive man's attempt at Physics.
##### Re: Thermite
« Reply #21 on: 16/09/2006 19:46:59 »
Ok, that makes sense.

[:(!]I still can't find any official documentation on the CaSO4/Al Thermite reaction so I can run it in school. Please help me!

"His mind is the ultimate weapon!"-MacGyver television series

#### Xare

• First timers
• Posts: 2
##### Re: Thermite
« Reply #22 on: 21/09/2006 02:24:53 »

This is ONE of many steel collums from the WTC.

It has been cut by a THERMITE Shaped Charge Demolition Device.

INFOWARS.COM

9-11 was a False Flag Shadow Government Operation.  It was an Inside Job to set a Pretext for War.

#### Mr Andrew

• Sr. Member
• Posts: 206
• God was primitive man's attempt at Physics.
##### Re: Thermite
« Reply #23 on: 21/09/2006 03:24:58 »
quote:
Originally posted by Xare

9-11 was a False Flag Shadow Government Operation.  It was an Inside Job to set a Pretext for War.

This is a Chemistry site, not a political thread.  Yeah, so thermite's a chemical reaction, fine.  But you don't need to go further than that.  Spare the rest of us who are interested in chemistry--PLEASE!

"His mind is the ultimate weapon!"-MacGyver television series

#### Mr Andrew

• Sr. Member
• Posts: 206
• God was primitive man's attempt at Physics.
##### Re: Thermite
« Reply #24 on: 21/09/2006 03:25:28 »
quote:
Originally posted by Xare

9-11 was a False Flag Shadow Government Operation.  It was an Inside Job to set a Pretext for War.

This is a Chemistry site, not a political thread.  Yeah, so thermite's a chemical reaction, fine.  But you don't need to go further than that.  Spare the rest of us who are interested in chemistry--PLEASE!

"His mind is the ultimate weapon!"-MacGyver television series

#### The Naked Scientists Forum

##### Re: Thermite
« Reply #24 on: 21/09/2006 03:25:28 »