A tube filled with H 2 and Cl 2 gas is corked and covered with a black cloth. The room lights are turned off. The cloth is then removed, and, using a camera with a flash, a picture is taken close to the tube.
Nothing happens when the black cloth is removed.
The UV radiation from the flash initiated the reaction between the H 2 and Cl 2 gas. The reaction proceeds via a chain-reaction mechanism. 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 Cl 2 . This reaction is catalyzed by light toward the violet end of the visible spectrum.