The Acetylene-Powered Trash Can

Check out the Experiment Pictures (16 K)

The following materials will be needed:

When solid calcium carbide is added to water, flammable acetylene gas is produced.

CaC2(s) + 2H2O(l) ---> Ca(OH)2(aq) + C2H2(g)

Provided initiation, the acetylene gas will burn in the presence of oxygen.

2C2H2(g) + 5O2(g) ---> 4CO2(g) + 2H2O(g)     Heat of reaction = -1300 KJ/mol C2H2

Making Predictions:
This is a chance to show off how powerful a balanced equation and a good counting unit can be. We can predict what nature is going to do before it happens. What you want to predict is how much calcium carbide you will need to get a good explosion. To do this, you will have to work backwards starting with the size of your container.

volume of trash can = 30 gal = 113 L
mass of trash can = 9.0 lb = 4.1 Kg

The combustion of acetylene will work best when acetylene and oxygen are present in a 1 to 2.5 ratio as shown by the recipe above. If we assume your local air supply is 20% oxygen than we have a reacting ratio of 1 part C2H2 to 12.5 parts air (5 x 2.5 to account for the unhelpful nitrogen molecules in the air). This requires 1/13.5 of the total volume of the trash can to be filled with the energy-rich acetylene. This would represent about 8.4 L out of our 113 L trash can. Now we can determine how many counting units of C2H2 will occupy 8.4 L of space.

PV = nRT
(1.00 atm)(8.4 L) = n(0.0821 L x atm/mol x K)(283 K)   *it was cold out there
n = 0.36 mol C2H2 gas

Now we must use the recipe of the initial reaction to predict how much calcium carbide with which to begin. Since nature creates 1 C2H2 from each CaC2, than you need 0.36 mol of CaC2. A counting unit of CaC2 requires 64.1 g, therefore 0.36 mol represents about 23 g. Due to some expected losses during the first stage of the reaction, we chose to use close to 30 g. The water molecules in the coffee can are in excess supply.

One can also predict how high the trash can might rise if we assume that 100% of the potential energy stored in those triple bonds is transferred into launching the trash can.

0.36 mol C2H2 x -1300 KJ/mol = -468 KJ of energy
E = mgh
468,000 J = (4.1 Kg)(9.8 m/s2)h
h ~ 11,650 m ~ 38,222 ft ~ 7.2 miles!

Students immediately think that there is a miscalculation somewhere, but it is legit. The problem is that we assumed 100% of the energy would contribute to the flight pattern of our trash can. Of course, this assumption is like most assumptions, it doesn't hold a full cup of validity. Where did some of our potential energy go astray? You can exercise their minds pre- or post-observation. If you are curious about the height attained, you can predict it using a stopwatch to time how long the trash can takes to reach the peak of its journey.

height = 1/2(9.8 m/s2)(time/2)2

Pre-mass several trials worth of calcium carbide into plastic baggies. Find an open field near school. Pick a spot that is not near an abundance of tall, dry grass or other materials that will cause this experiment to more famous that you want it to be. Drive a stake into the ground. Make sure the trash can will rest against the ground when inverted over the set-up. Tape the ignitor to the stake and attach the wire. Set the coffee can and water beneath the stake. Unroll your wire to a distance of about 25 yards. Do not have the wire pulled taut so that when the trash can is lowered it will rip the ignitor off the stake. One chemist should be holding the trash can upside down over the set-up while another chemist dumps the calcium carbide into the water. Immediately lower the trash can. Now we are to the part that we haven't quite figured out yet. We have had good and bad trials from both short wait times (~1 min.) and long wait times (~3-5 min.). Send some electrons when you're ready. You can experiment with wait time and also with the launch container. We also tried a large plastic laundry soap bucket. We scaled down the reaction and the results were impressive (see pictures, 16 K). Experiment and enjoy some good, safe observations of atoms doing the exceptional. Let me know how it works.

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