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Free-Radical Reactions - The H₂/Cl₂ Reaction

  • Page ID
    3001
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    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.

    Contributors


    Free-Radical Reactions - The H₂/Cl₂ Reaction is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by George Bodner.

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