HCl Cannon Radicals

Equipment

• One 250-mL Erlenmeyer flask
• A #6 stopper with 3 holes
• Two graphite electrodes (sized to fit stopper holes and reach 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 for the graduated cylinder
• Cork for graduated cylinder
• Plastic tubing and glass bends to channel gases
• Camera flash or intense white light source
• Optional: Red and blue filters

Reagents

• 250 mL of 6 M HCl
• 250 mL of 6 M NaOH

Presentation Steps

1. Pour the HCl solution into the flask until the level is about 5 cm from the bottom of the stopper.
2. Firmly place the stopper with electrodes and gas outlet tube onto the flask.
3. Insert the stopper with the gas inlet and outlet tubes into the graduated cylinder.
4. Direct the outlet from the graduated cylinder into the NaOH solution to 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, producing approximately 2 amperes of current. Hydrogen and chlorine gas bubbles will begin to form on the electrodes’ surfaces. The solution will turn pale green due to dissolved chlorine.
7. Allow electrolysis to continue until the graduated cylinder’s contents are distinctly green (approximately 15-20 minutes).
8. Turn off the power supply.
9. Quickly replace the stopper with a cork, gently pressed into the graduated cylinder to prevent rupturing.
10. Clamp the graduated cylinder into a ring stand, positioning it for a high arc of the cork.
11. Wearing eye and ear protection, charge a camera flash and flash it next to the tube.
12. Use filters; the red filter will not initiate the reaction, while the blue filter will transmit sufficient energy to do so.

Hazards

• Hydrochloric acid can irritate skin and is extremely irritating to eyes and respiratory system; handle in a well-ventilated area.
• Solid sodium hydroxide and concentrated solutions can cause severe burns to eyes, skin, and mucous membranes.
• The cork may be propelled with sufficient force to cause injury; use caution.
• The graduated cylinder may shatter if the cork is seated too firmly and weakens after multiple uses (usually bursts after 3-4 uses).
• Hearing protection is recommended for the demonstrator, and the audience should be cautioned to cover their ears.

Chemical Reactions

Reaction	Type	Equation
Cl₂ + light (500 nm) → 2 Cl•	Initiation	Cl₂ + light (500 nm) → 2 Cl•
Cl• + H₂ → HCl + H•	Propagation	Cl• + H₂ → HCl + H•
H• + Cl₂ → HCl + Cl•	Propagation	H• + Cl₂ → HCl + Cl•
2 Cl• → Cl₂	Possible Termination	2 Cl• → Cl₂
H• + O₂ → HOO•	Possible Termination	H• + O₂ → HOO•
Cl• + O₂ → ClO₂•	Possible Termination	Cl• + O₂ → ClO₂•
Radical + container wall	Possible Termination	Radical + container wall

References

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