One of the comments to the previous post mentioned having some trouble with a procedure out of one of the lesser journals. "Trouble", in this sense, meant "vigorous unexpected fire". But when he mentioned that it involved a mixture with aluminum chloride, I knew to look out.
Chemists everywhere live by thermodynamics. And one of the basic principles is that if a reaction's starting materials are more energetic than its products, then it's favorable. It doesn't mean that it's always just going to take off spontaneously - sometimes the intermediate step is much higher in energy, and the reaction can't get over the hump. But if there's not too high a levee between the two energy states, things will indeed flow downhill for you.
It's a good thing, too, since one such reaction is burning the nitrogen in the air, thereby changing it into poisonous nitrogen oxides. (Correction: late-night brain freeze there - I had in mind the fixation of nitrogen to ammonia, which is energetically favorable but has a high activation energy. Oxidation of nitrogen itself to NO is an uphill process, but under high temperature/high pressure conditions, like those found in your car engine, it does take place). Another one of those is burning aluminum, which also has a good-sized barrier to get past (otherwise using aluminum foil in your oven would be a spectacularly bad idea). The product of that reaction, aluminum oxide (or alumina) is one of the most below-sea-level compounds I can think of, compared to the metal or many of its compounds. Give 'em a chance, and they'll take off on you.
The classic example of this is the thermite reaction: aluminum + iron oxide goes to aluminum oxide + iron. Oh, and some heat. Well, OK, a lot of heat, enough to spray molten iron all over the place. (YouTube) You have to set the reaction off with something pretty hot (burning magnesium ribbon is traditional), but once it gets going, it tosses off enough spare heat to roll right along.
So no, I'm not surprised that some aluminum chloride would take off on someone. Regard all aluminum compounds without a bond to oxygen with a little suspicion. Many of the them (and all the aluminum metal you see) came from alumina, and they're scheming to get back.
College chemistry, 1983
The 2002 Model
After 10 years of blogging. . .
1. GC on July 10, 2006 9:39 PM writes...
So no, I'm not surprised that some aluminum chloride would take off on someone. Regard all unoxidized aluminum compounds with a little suspicion.
Was this a lower than (III) Al salt??
Permalink to Comment2. Derek Lowe on July 10, 2006 10:16 PM writes...
That'd be a rare beast, wouldn't it? I just changed the text of the last paragraph to clear that up.
Permalink to Comment3. GC on July 10, 2006 10:24 PM writes...
Oop, didn't mean to be a smartass. I thought that just such an odd, angry oxidation state might be being invoked somehow.
Permalink to Comment4. secret milkshake on July 11, 2006 12:02 AM writes...
I am sorry to correct you but you cannot burn nitrogen: standard delta G of formation of all nitrogen oxides is strongly positive.
N2O +104kJ/mol
NO +86.6kJ/mol
(N2O3 unstable -> NO + NO2)
NO2 +51.5kJ/mol
N2O5 +114kJ/mol
But your can burn amonia.
As a kid, I used to enjoy burning aluminum foil wrapped in layers around powdered KMnO4 with a pinch of elementar sulfur. Molten burning aluminum flying everywhere...
Permalink to Comment5. matt netherton on July 11, 2006 12:05 AM writes...
The aluminum chloride fiasco was one of those grad school desperation moves to get a reaction (in this case a Fischer indole synthesis) to work when it is just one of those things that will never work. The 'alchemical' paper I found looked like a dream come true - tables of diverse substrates cyclizing in >80% yields. Needless to say that paper has not been cited to this day, and I have a much greater respect for Lewis acids of all flavours.
Permalink to Comment6. renan on July 11, 2006 7:45 AM writes...
``Regard all aluminum compounds without a bond to oxygen with a little suspicion.''
powder Al is pyrophorric. for that matter, so is finely powdered iron, or lead, or ...
i remember preparing pyrophorric lead by heating lead(II) tartrate when i was younger. what was i thinking? perhaps all those lead are catching up on me.
Permalink to Comment7. GA on July 11, 2006 7:53 AM writes...
Aluminum serves as the fuel for the Solid Rocket Boosters of the space shuttle. To wit (from http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html):
"The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent)."
Yes, the kinetics of burning aluminum (or aluminium) might not be trivial, but once you get past the barrier...it's a long way downhill.
Permalink to Comment8. Milo on July 11, 2006 10:36 AM writes...
My first introduction to energetic aluminum chemistry came from my first job right out of college. I was hired by Akzo Nobel as a research chemist in their metal alkyls division. My assigmemts there all revolved around working with various alkylaluminum compounds like trimethylaluminum (TMAL, which they made).
I recall going to the plant in Deer Park, Texas, in August, and seeing a demontration where a bit of neat TMAL (none of this pansy 1M stuff!) was shot from a pipe and was allowed to ignite (it is quite pyrophoric...). I have never forgotten that demo, it made me respect an appreciate what some compounds will do to get to a lower energy state.
Permalink to Comment9. Mark Leach on July 12, 2006 2:52 PM writes...
During the Falklands war, a mid-eighties UK event, it was discovered that warships built of Al underwent the thermite reaction... and nothing puts out a thermite rxn...
Permalink to Comment10. Milo on July 12, 2006 3:09 PM writes...
Mark,
I like how you had to "explain" what the Falklands war was... sign of the times...
Permalink to Comment11. Hap on July 12, 2006 3:12 PM writes...
I thought that the ship simply burned rather than undergoing a thermite reaction. The incident I read about was the impact of an Exocet with one of Britain's ships - the missile either exploded but didn't do much damage or didn't explode at all, but the remaining rocket fuel burned hot enough to ignite the aluminum superstructure, which then couldn't be extinguished, sinking the ship. I think the incident is described in one of Clancy's nonfiction books.
I assume there are good reasons for shipbuilders to use aluminum, but aluminum ships and high explosives seems like a bad combination, at least for the people on board (and only if you are hit, I guess).
Permalink to Comment12. Derek Lowe on July 12, 2006 3:23 PM writes...
There seems to be some debate about the aluminum-armor issue.
And yes, that added explanation about what the Falklands War was did make me feel like a living fossil.
Permalink to Comment13. secret milkshake on July 12, 2006 3:52 PM writes...
Nazi planes were made of a magnesium alloy that is lighter and stronger than aluminum.
It is also quite flammable, about the same as Mg turnings and that makes for the spectacular WWII footage of white-burning falling german planes.
My dad told me the magnesium scrap from german planes was a prized material for pyromaniac kids. All one has to do was to cut it into a thin piece and set it with a sparkler or Bunsen burner. A particularly memorable experiments included droping a piece of burning Mg into a jar filled with pure oxygen...
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