Thursday, May 3, 2007

Torturing gummi bears and molten iron

Tomorrow is the last day of the semester. I'm running out of steam. The tank is on "E." So, instead of trying to cram a few more things into the semester, I decided to do a couple of visually stimulating demos for my General Chemistry class.

Since the weather is finally nice, I did a couple of demos that are best done outdoors. Both are redox reactions.

The first one I did was the reaction of molten potassium chlorate and gummi bears. The set up is simple. In a large test tube, about 5 grams of potassium chlorate is melted with a propane torch. A gummi bear is dropped into the test tube and the demonstrator runs away. The sugar in the gummi bears get oxidized quite rapidly and dramatically. Colored flames are typical.

The second demo I did is easily my favorite demo of all time: the thermite reaction. Here is the procedure I typically use: 100 grams of aluminum powder mixed with 350 grams of iron (III) oxide. This is the thermite mixture.

This reaction has a high activation energy. To get the reaction going, some people use an ignited magnesium ribbon as a fuse. I prefer to use glycerol and potassium permanganate. I like the KMnO4 because it adds another oxidation reaction to the mix that doesn't involve elemental oxygen. The drawback is that it take a while to get going, and sometimes the permanganate/glycerol reaction doesn't provide enough heat to get the thermite going.

That's what happened today. I poured the thermite mixture into a flower pot with a paper towel blocking the hole. I made a depression in the thermite and filled it with
KMnO4 and poured glycerol on top of it. Then we waited.... and waited... and waited. Finally, after about 2 minutes the glycerol and KMnO4 reacted, but the thermite didn't go. I added more glycerol and KMnO4. Still nothing.

My students were disappointed and ready to leave, when I had a flash of brilliance (or stupidity). I decided to add some potassium chlorate (from the gummi bear demo) to the top. There was plenty of glycerol left over. Nothing happened. So I heated it very briefly with the propane torch... That is all it took!!!

The thermite reaction took off like crazy. It was great. The molten iron came out of the bottom and landed right on top of the dandelion I was aiming at. Sparks were flying every where. My students were most impressed.

From now on I will use potassium chlorate to initiate the reaction.


Ψ*Ψ said...

The thermite reaction is nothing short of awesome. I think they use glycerol/KMnO4 here as well, though I might be thinking of the wrong oxidiser. Oddly enough, the last time I saw it (a week ago), there were initiation problems too. Eventually it worked.
The best part was a couple of high school kids in the back headbanging through the whole thing. Not that I understood why.

TacomaChem said...

I've always activated my thermite reaction with a pair of "4th of July" sparklers turned upside-down into the thermite mixture. light the top with a lighter and then you have plenty of time to move away. For the students there is a good amount of anticipation that builds as the sparklers slowly burn down to the thermite, then voila! ignition. Works every time.