Tuesday, December 13, 2011

Thermite




Thanks to my good friend and fellow science teacher, Josh, who makes the high speed videos.
Who doesn’t love the thermite reaction?  This reaction is exciting, dramatic, and a bit scary.  Shakhashiri describes it like this, “Flame, flying sparks, smoke, and dust are produced.  Molten iron runs through the hole in the pot into the sand bath.”  I pulled out this demo today as part of the “5-types of chemical reactions” unit and a great example of an exothermic reaction.  Thermite is a single displacement reaction with a high activation energy.  The thermite mixture, aluminum powder and iron oxide, can sit on the shelf quietly without much concern because it takes another reaction to get it started. 

Inserting the ignition stick into the thermite mixture.
I tried to use the potassium permanganate reaction with glycerin (as described by Shakhashiri) to get the reaction going.  I did the potassium permanganate reaction yesterday in class, so it seemed like a nice follow up today.  (By the way, it filled up the room with smoke and we all poured out of the classroom coughing as the bell rang at the end the period.  I’ll do it outside next time!) The permanganate reacted vigorously with the glycerin, but it did not produce enough heat to spark the thermite.  (You can watch this “dud” reaction at the end of the “Thermite Balls” video.)

Take a look at the chunk of iron that we collected from the water bath.
A close-up of the iron drops that formed from the reaction.
So I went back to the stand-by:  thermite ignition sticks that we bought from the chemical supply company.  These sticks are essentially really fat sparklers.  I used a Bunsen burner to light the stick, and then I ran outside with my burning sparkler.  I held it in the thermite mixture until the reaction started.  Once it got going, all that was left to do was stand back and enjoy the show.  Watch for the molten iron dropping out of the bottom of the clay pot, making the water in the tank boil.  I fished out the iron pellet from the tank to show the kids the iron drops that formed.  


I followed up the thermite reaction with what I call “hand held thermite” or “thermite balls”.  I can make the same reaction on a smaller scale with two rusty cannon balls, one covered with aluminum foil.  With enough activation energy, provided by hitting the balls together at a high speed, sparks fly and there’s a loud pop.  The high speed video doesn’t do justice to the reaction without the sound effects, but you get to see the classic facial expressions I make when I do the demo.  I never realized how much I flip my hair until I started this blog project.

Ready for the hand held thermite reaction.
 

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