Chemical Concepts Demonstrated

• Free-radical reaction mechanisms
• Photochemical reactions

Demonstration

A tube filled with H2 and Cl2 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.

Observations

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.

Contributors 

• Dr. George Bodner (Perdue University)