Chlorine and hydrogen gas cannon
What you can see here is a boiling tube (strengthened with sticky tape to prevent shattering) filled and corked with the explosive 50/50 mixture of chlorine and hydrogen gas. When a UV light is placed behind the boiling tube, the amount of energy generated by it is enough to ignite the mixture, suddenly increasing the pressure within the boiling tube and causing the bung to explode out with huge force.
THE SCIENCE- Free Radical Reactions
Chlorine absorbs UV light, the energy of which is equivalent to ca 400 kJ mol-1. This is more than the Cl-Cl bond strength (242 kJ mol-1) in the chlorine molecule, which consequently splits into two chlorine free radicals with one unpaired electron each:
After this initiation step, each chlorine radical can react, on collision, with a hydrogen molecule by abstracting a hydrogen atom to form hydrogen chloride and a hydrogen radical:
The hydrogen radical can then react with a chlorine molecule to produce another chlorine radical:
These two propagation steps set up a chain reaction, which terminates if two radicals combine:
The probability of each of the four steps in the chain reaction is based on the relative concentrations of the species involved. The likelihood of the termination step is improbable, owing to the much higher concentrations of H2and Cl2. Therefore, the chain reaction should propagate through a vast number of H2 and Cl2 molecules before it terminates (RSC- who could explain it far better than I ever could!).
video source for more tips and tricks of this demonstration.
Oh and this should be pretty obvious but DO NOT TRY THIS AT HOME. Chlorine is a toxic gas and the reaction can be very dangerous if not properly controlled.