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Developer Pixie
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10 + 48 points

But... HOW!? by Developer Pixie, Sir Shplankton The Crabula, Polutropos Rathazaxx

June 3rd, 2011 12:06 AM

INSTRUCTIONS: Complete this task. Post pictures on how you did it. This task will self-destruct in one month (June 21st).

"What's better then making your own lava?
Making lava in a pirate hat and petty coat"


Not only did we post pictures on how we did it, but also a video!

Gaaa! Also I had another writeup, but it got deleted. Here’s another one, enjoy!

Summary: Whilst equipped with pirate and princes attire, we destroyed a SF0 artifact – in magma. Furthermore, we created the magma through SCIENCE!


Details: Science

Useful resources: hbcpnetbase
my chem 101 – 102 textbook

So thermite is a mixture of some kind of iron oxide and pure aluminum powder (just the element). The reaction requires a significant amount of activation energy to start (just like how a leaf won’t burn without enough energy to get it to combust). However, when started, the reaction produces iron and aluminum oxide. Aluminum oxide has a really high enthalpy of formation (all shall be described, dear reader) – enough to melt the iron that is produced by the reaction. Yay molten iron!


Ok so before we start with the specifics of our reaction, let’s lay down some chem conventions.

names (normally these numbers would be subscripts, they are subscripts in the attached word document)
Fe: iron
Al: aluminum
FeO: iron (II) oxide
Fe2O3: iron (III) oxide
Fe3O4: iron (II, III) oxide
Al2O3: aluminum oxide

Here’s an example reaction:
Fe2O3 + 2Al → Al2O3 + 2Fe
This means that one particle of Fe2O3 could react with two particles of Al to produce one particle of Al2O3 and two particles of Fe. In order to scale up this interaction to quantities of particles that are interesting and fiery, we use the mole. A mole is a number of things, like a dozen or a gross, except a mole is 6.022*10^23 things (this can also be written as 6.022E23). (There might be .5 moles of sand grains on earth.) So we can say that one mole (6.022*10^23 particles) of Fe2O3 could react with two moles of Al to produce one mole of Al2O3 and two moles of Fe.


There are a few different types of thermite reaction, all involving some form of iron oxide:
(not including sates because this reaction can be carried out in many states of matter, i.e. solid, liquid, gas, aqueous)
3FeO + 2Al → 3Fe + Al2O3
Fe2O3 + 2Al → 2Fe + Al2O3
3Fe3O4 + 8Al → 9Fe + 4Al2O3
Since aluminum powder is significantly more expensive than any iron oxide, I wanted to find out which reaction unleashed the most energy per mole of aluminum power consumed.


So I was going to do this large-ish section on enthalpy of formation and Gibbs free energy and why this reaction is as hot as it is. It was going to be a lot more work than is necessary for this.
Long story simplified: Aluminum oxide has a very high enthalpy of formation at standard state. This means that the reaction that forms aluminum oxide from its constituent elements at standard state is highly exothermic, meaning much heat is released into the environment as that reaction takes place. In this case, aluminum oxide isn’t being formed only from its constituent elements at standard state (the standard state of an element differs from element to element, but it’s the HTFJYGUHJK^%&* natural form at which an element exists. For Al it is Al, for Fe it is Fe, and for O it is O2). Rather, it’s being formed from Al in its standard state and O not in its standard state. There is a calculation that lets us know how much energy should be unleashed by the reaction, but in each of the above reactions, the energy output per mole is relatively large. For a cool thermite reaction, any could be used.


Here is an equation:
ΔG° = ΔH° - TΔS°
ΔG°: the change in the amount of free energy of a system when a compound is formed from its constituent elements at standard state
ΔH°: the change in the amount of heat of a system when a compound is formed from its constituent elements at standard state
T: temperature in Kelvin
ΔS°: the change in entropy (organization of matter) of a system when a compound is formed from its constituent elements at standard state

So reactions can be either exothermic or endothermic. An endothermic reaction absorbs heat from its environment, while an exothermic reaction releases heat into its environment. We refer to this as the change in enthalpy, or ΔH, of a reaction.
Not all of this heat can be used to do work in the surrounding environment. Some of it is (usually) consumed by the change in molecular structure. We refer to this as ΔS, or the change in entropy. ΔG is the amount of energy left over available for work.
Ehhhhh this isn’t going in the right direction.

So now that we’re using 3Fe3O4 + 8Al → 9Fe + 4Al2O3, we’ll need some Fe3O4, Al (both in powder form), and a way to start the reaction. Ordering the iron oxide and aluminum was easy enough (the particle size of both powders was like 30 microns or so?). The one minor qualm about this whole project was that I purchased thermite starters to start the reaction. (They were small quantities of some substance that burned with adequate heat at the end of a wick.) Typically, I’d use a sparkler (much more home-grown feel).
Aluminum came in 5-pound increments. I decided to buy 10 pounds. Next I would have out figure out how much iron oxide I would need. Also know ‘mole’ is abbreviated ‘mol.’
10 pounds = 4.54 kg
molecular mass of Al: 26.98 g / mol (that is to say, 1 mole of Al particles has a mass of 26.98 grams)
4536g (1 mol / 26.98g) = 168.1 mol
168.1 mol (3/8) = 63.047 mol
molecular mass of Fe3O4: 231.53 grams / mole
63.047 mol (231.53 g/mol) = 14597 g
14597 g = 32.108 lbs of Fe3O4 required for a complete reaction
∴mass ratio Fe3O4 : Al = 3.218
Fe3O4 was also offered in 5 lb increments, so I just went for 30 lbs.
Even if I did have 10 lbs of Al and 30 (instead of 32.1) lbs of iron oxide, the reaction would work quite well. However, that’s a lot of thermite and I didn’t want to do that much all at once. Figuring out how much to mix by volume seemed to be the most practical way to prepare thermite in the field.
density of Fe3O4: 5.17 g / cm^3
density of Al: 2.70 g / cm^3
14597 g (1 cm^3 / 5.17 g) = 2823.4 cm^3
4536 g (1 cm^3 / 2.70 g) = 1680.0 cm^3
volume ratio Fe3O4 : Al = 2823.4 / 1680.0
2823.4 / 1680.0 = 1.681
So you need 1.681 times more Fe3O4 than Al (by volume) for an ideal reaction.
We filled approximately ¾ of a fairly large flower pot. I don’t know exactly what the volume of thermite was, but we endeavored to add reagents as the volume ratio stated.

Just mixed, plopped in the starter, lit it, and hunkered down! And got spotted by a truck leaving the power plant where we did this.

Also, it’s scary to think of the number of options you have when walking around with that kinda stuff. For example, we had train tracks to ourselves for many solid, potentially destructive minutes … !

*interesting note about Fe3O4:
It’s actually FeO complexed to Fe2O3. Back when I remembered how to draw basic atomic structures (back in chem. 101), I couldn’t draw Fe3O4 with the rules they gave us – there’s some complex stuff going on there. (?)
*attention to sig figs: eeeeehhhhhhhh whatever. When actually out there, I might’ve gotten volumes accurate to 2 significant figures (so I probably used a volume ratio of 1.7 instead of 1.681). Objective: molten iron, not extreme efficiency.

+ larger

Do it with LAVA
Pettycoats and knee socks and LAVA!
also intrepid.jpg
Site 001 (NOT railroad tracks, mind you).jpg
MATH! One of the best things ever.jpg
Fe3O4 is really strange .jpg
though appetizing, do not eat.jpg
Special guest ROAD SCHOLAR!
We were going to burn Plato's Republic too, but l sold it back to the Reed bookstore instead

10 vote(s)


watchthevideo, noreallywatchthevideo

4 comment(s)

(no subject)
posted by N Mutans on June 3rd, 2011 9:33 AM

Thermite would get a vote from me even if the rest weren't awesome.

(no subject) +1
posted by Juliette on June 3rd, 2011 9:37 PM

I. You. What. Agh. How can I even begin...


(no subject)
posted by Amithy Ilexa on June 6th, 2011 11:44 PM



(no subject)
posted by Dead Cube on June 8th, 2011 8:12 PM