1. Kelly, H. Mechanix Illustrated 1976, 72 (577) 38—39.
Journal of Chemical Education February 1995
inventory control
Put a Little Kaboom in Your ClassroomMike Barondeau
Ever since I attended the ICE Supplements Workshop at the University of Wisconsin—Madison in the summer of
1988, I have developed a penchant for loud noises. During that time George Coulter, one of the other workshop participants, demonstrated his carbide cannon in the dorm at 2:00 a.m., and I became intrigued with it.
After I returned home I discovered an article (1) that contained plans for a bicentennial carbide cannon that was nearly six feet long. This was somewhat large for a classroom demonstration model that I might be interested in, so I decided to build it at exactly half scale. It works beautifully, and the cost for materials was considerably less than the cost of a commercial carbide cannon.
Included are plans for the cannon including several modifications that I think are improvements in design for classroom use. I made the main carriage parts and the wheels out of 2-in. x 8-in, pine for a wider old-fashioned look. (Sorry about the non-SI values, but you must purchase these items in the outside world that has not accepted SI yet.) My aiming stick has only one level on it compared to the original, and it is glued in place permanently. The most important modification is on the cannon itself. Instead of using a 4-in. end cap, I used a 3-in. x 2-in. outside reducing adapter and a 2-in. cleanout. These important modifications allow me to use chunky calcium carbide from my chemical supply room instead of the more expensive powdered calcium carbide available in craft and hobby stores.
To load the cannon, put about a half cup of water in through the touchhole using a funnel to get the water into the bottom of the 3-in, tee. The charge is about half a teaspoon of calcium carbide. I use a long handled spoon (e.g., ice cream malt type) to put the calcium carbide in through the rear cleanout. Acetylene gas forms according to the following equation:
CaC2(s) + 2H20(l) à C2H2 (g) + Ca(OH)2 (s)
By the time you reattach the cleanout, you can ignite it at the touchhole with a match, lighter, or burning tow in a linstock and.... KABOOM!
C2H9 (g) + 502(g) à 4C02 (g) + 2H20(l)
ΔHo298 = -2512 kJThere is no danger from over- or under-loading this cannon. Too little and very little, if anything, happens. Too much and you get an orange sooty flame with no noise. For re-firing, the cannon must be aired. I accomplish this by removing the rear cleanout and blow through the cannon. You could be more realistic and make a plunger-loader with a stick and a can smaller than the barrel. Running it in and out several times would air out the cannon. Removing the rear cleanout and using a bellows is even more efficient.
Demonstration Cautions• When working with any
flammable and explosive substance you should observe the normal precautions.• Do not allow anyone to stand in front of the cannon.
• Wear protective goggles and stand well back.
• Never allow the air flow to be obstructed. Do not allow any object to be placed in the barrel under any conditions.
Construction
The carriage is made out of 2-in. x 10-in, and 2-in. x 6-in.
pine and some thinner stock. Exact accuracy is not needed. The original pattern called for sharp, square corners and edges. I rounded all of mine off with a belt sander. You’ll need the following PVC sewer pieces:| (1) 3-in, tee | (1) 2-in. coupling |
| (2) 2-in. x 1.5-in, bushings | 3 in. of 3-in. o.d. pipe |
| (1) 3-in. cleanout adapter with plug | 1 foot of 2-in. o.d. pipe |
| (1) 3-in. x 2-in, inside reducer | 8 in. of 1.5-in. o.d. pipe |
| (1) 3-in. x 2-in, outside reducer | small tube of PVC cement |
| (1) 2-in. cleanout adapter with plug |
Assemble the PVC parts with the PVC cement. Work quickly, because the cement sets up quickly. Drill a 0.25-in. touchhole at the top of the tee and the 0.5-in, mounting hole for the pivot pins (trunions) after assembly. The last modification I made was to sand the ball from a roll-on deodorant bottle flat on one side and glue it on the end of the 2-in. cleanout. For a more realistic finish, spray the PVC assembly with flat black paint or mix fine sand with brush-on paint to give a textured cast iron look.
The wood can be darkened with a torch before staining.
Construction Cautions
• PVC cement vapors are harmful and very flammable.
• PVC cement should be used only outdoors or in a wel]-ventilated area.
• Spray painting should be done only in a well-ventilated area.
I use the cannon as a "great" demo to lead into my discussion of rates of reactions, heats of formation, heats of reaction, or limiting reagents. The cannon also served as the grand finale to our school’s state centennial celebration assembly in the gym several years ago.
1. Kelly, H. Mechanix Illustrated 1976, 72 (577) 38—39.
