Chemical Concepts Demonstrated: Free-radical reaction mechanisms, photochemical reactions
Nothing happens when the black cloth is removed. Once the "picture" is taken, the gases within the tube react.
Explanation (including important chemical equations):
The UV radiation from the flash initiated the reaction between the H2 and Cl2 gas. The reaction proceeds via a chain-reaction mechanism.
overall reaction: H2 + Cl2 ---> 2 HCl
initiation: Cl2 + hv ---> 2 Cl ·
propagation: Cl · + H2 ---> HCl + H·
H· + Cl2 ---> HCl + Cl ·
termination: 2 H· ---> H2
2 Cl · ---> Cl2
H· + Cl · ---> HCl
The enthalpy for the overall reaction is -184.6 kJ per two moles of HCl. However, the enthalpy for the initial step is 243.36 kJ per mole of Cl2. This corresponds to the energy carried by photons with a wavelength of 491.5 nm This reaction is catalyzed by light toward the violet end of the visible spectrum.
It should be noted that, because a camera's flash bulb initiates the reaction, a certain amount of showmanship could be employed. For example, one could take the cloth off and, when nothing happens, take a photograph of the demonstration under the ruse that it actually failed and such a picture is a necessary bit of documentation in such an event.