What it Shows
Different colors of LED light illuminate a small corked test tube with no reaction. Ultraviolet light causes an explosion that shoots the cork across the room.
Video of our demo in action https://youtu.be/NN82GoBG98s
Another video of our demo in action https://youtu.be/tJhVy1x9X2c
How it Works
Using a power supply, 8M hydrochloric acid solution is electrolyzed to produce a 1:1 ratio of hydrogen gas (H2) and chlorine gas (Cl2) according to the following equation:
HCl (aq) → ½ H2 (g) + ½ Cl2 (g)
The electrochemical half reactions with their standard reduction potentials are:
2H+(aq) + 2e– → H2 (g)0V
2Cl–(aq) → Cl2 (g) + 2e– -1.36 V
Though the electrolysis is not run at standard conditions, the overall standard cell potential is -1.36 V, negative indicating a nonspontaneous reaction, requiring electricity to drive the reaction. Bubbling can be seen at the graphite electrodes when the applied voltage is approximately (insert voltage here).
The reaction that causes the explosion of the cork out of the test tube is the reverse of the electrolysis reaction, forming hydrogen chloride (HCl) gas.
½ H2 (g) + ½ Cl2 (g) → HCl(g)
The gas mixture is made by the electrolysis of hydrochloric acid 8M. A water bath allows for filling the tubes under water. The tubes are quartz. There should be a video of how to fill one of these tubes, because many words otherwise.
References
* Richard Schwenz and Lynn Geiger. "Photon-Initiated Hydrogen-Chlorine Reaction." Journal of Chemical Education 76.4 (1999): 470.
* Shakhashiri, Bassam Z. "Chemical Demonstrations: A Handbook for Teachers of Chemistry." Madison: The University of Wisconsin Press, 1983. ( http://books.google.com/books?id=0rx6Cjx_l-AC&lpg=PP1&pg=PA121#v=onepage&q&f=false)