HCl Cannon

Equipment

One 250-mL Erlenmeyer flask, a # 6 stopper with 3 holes, two graphite electrodes of a diameter to fit the stopper holes and a length to reach nearly to the bottom of the flask, power supply capable of approximately 6 volts and 2 amperes, 100-mL clear plastic graduated cylinder, with end spout cut off, 2-hole stopper to fit the graduated cylinder, cork to fit the graduated cylinder, plastic tubing and glass bends to channel gases and a camera flask or some other source of intense white light.
Optional: red and blue filters.

Reagents

250 mL of 6 M HCl, 250 mL of 6 M NaOH

Presentation

1. Pour the HCl solution into the flask so that the level is about 5 cm from the bottom of the stopper.
2. Place the stopper with electrodes and the gas outlet tube firmly onto the flask.
3. Place the stopper with the gas inlet and outlet tubes from the flask firmly into the graduated cylinder.
4. Place the outlet from the graduated cylinder into the NaOH solution The NaOH will react with any escaping chlorine converting it into hypochlorite.
5. Connect the output of the power supply to the electrodes and turn on the power supply.
6. Adjust the power supply to between 5 and 6 volts. This should produce a current of approximately 2 amperes. Hydrogen and chlorine gas bubbles should start forming on the surfaces of the electrodes. The amount of chlorine formation may look small at first, but the chlorine is soluble in water and so some of it is being dissolved in the solution. The solution will become pale green colored from this process.
7. Allow the electrolysis to continue until the contents of the graduated cylinder are distinctly green colored from the chlorine (probably 15-20 minutes depending upon the rate of electrolysis).
8. Turn off the power supply.
9. Quickly replace the stopper with the inlet and outlet tubes with a cork. The cork should be pressed only very gently into the graduated cylinder to prevent the cylinder from rupturing upon ignition.
10. Clamp the graduated cylinder into to a ringstand, so that the cork will travel in a high arc over the audience.
11. Wearing eye and ear protection, charge a camera flash and flash it next to the tube.
12. The filters may be placed over the light source. The red filter will not transmit light of sufficient energy to initiate the reaction. The blue filter will transmit light of sufficient energy to initiate the reaction.

Hazards

Hydrochloric acid can irritate the skin. Hydrochloric acid vapors are extremely irritating to the eyes and respiratory system. Therefore, it should be handled only in well-ventilated area.
Solid sodium hydroxide and concentrated solutions can cause severe burns to eyes, skin, and mucous membranes.
The cork is propelled with sufficient force to cause injury if someone is hit at blank range.
The cylinder may shatter if the cork is seated too firmly. The cylinder weakens with every demonstration and will usually burst after 3-4 uses.
The demonstrator should wear some type of hearing protection and the audience should be cautioned to cover their ears.

Discussion

Cl2  +  light (500nm)  ®  2Cl•         (initiation)
Cl•  +  H2  ®  HCl  +  H•           (propagation)
H•  +  Cl2  ®  HCl  +  Cl•          (propagation)
2Cl•  ®  Cl2                  (possible termination)
H•  +  O2  ®  HOO•      (possible termination)
Cl•  +  O2  ®  ClO2•      (possible termination)
radical + container wall    (possible termination)

References

1. B. Z. Shakhashiri, Chemical Demonstrations, A Handbook for Teachers of Chemistry, Wisconsin, 1983, Vol.1, p.121-123.