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ChemWiki: The Dynamic Chemistry E-textbook > Development Details > Approaches > Demos > Additional Demos > Free-Radical Reactions--The H2/Cl2 Reaction

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.
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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.

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Last Modified
10:28, 2 Oct 2013

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