22.10 Free-Radical Reactions--The H2/Cl2 Reaction


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.