Initially different LEDs of distinct wavelengths are shined over a tube half filled with chlorine gas and half with hydrogen gas.
Nothing appears to happen at the beginning because the molecules have not enough kinetic energy at room temperature to collide with enough energy to overcome the activation energy of the reaction until that is, the right wavelength (energy) is shined on. In this case, a UV torch.
A free radical chain reaction is then initiated photochemically by the cleavage of chlorine molecules to give chlorine free radical atoms.
Cl2 + (photon, energy hʋ) → 2 Cl∙
The reaction is extremely fast and loud sending the cork flying several metres away. Oxygen present in the tubes can delay or inhibit the reaction by creating additional/different reaction pathways.
Using the bond energy of the Cl-Cl bond, it is possible to calculate the minimum wavelength in order to initiate the reaction using the following equation:
E= hʋ = hc/λ
The result: only blue light or shorter wavelengths (UV) can be used to start this reaction, but no longer wavelength such as red or yellow.
More details on this reaction can be found in the references below:
* 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=0rx6...)
THIS IS A DANGEROUS DEMONSTRATION with production of highly toxic chlorine and hydrogen chloride gases. Wear ear defenders and protective clothing/equipment and fill the tubes in a fume hood. Use only a boiling tube covered with cellotape and DO NOT SCALE UP the experiment.
Be aware that the hydrogen-chlorine mixture can explode unexpectedly even in cloudy days and can be easily triggered by dust, rubber bits, etc. which act as catalysts.