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Wavelengths and dissociation?

  1. Feb 7, 2004 #1
    Wavelengths and dissociation??

    I needed some help with this problem.

    By using photons of specific wavelengths, chemists can dissociate gaseous HI to produce H atoms with accurately known speeds. When HI dissociates, the H atoms move away rapidly, wheras the relatively heavy I atoms move little.

    a.) What is the longest wavelength that can dissociate a molecule of HI.
    b.) If a photon of 254 nm is used, what is the excess energy (in J) over that needed for the dissociation?
    c.) If all this excess energy is carried away by the H atom as kinetic energy, what is the speed(in m/s)?

    i did part b but i got the wrong answer. I used the equation E=hc/wavelength. But i got the wrong answer.

  2. jcsd
  3. Feb 7, 2004 #2


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    Bond dissociation energy=hv=h(c/wavelength), solve for wavelength.

    well, by now you know the energy required so this should be easy.
    Find the energy corresponding to the above wavelength. Subtract the bond dissociation energy from this.

    KE=.5mv(squared), solve for v, where KE is the answer to part b.
  4. Feb 8, 2004 #3
    The problem is that i dont know what the bond dissociation energy is.
  5. Feb 8, 2004 #4
    Without having seriously read this post I'll answer your last question.
    Bond dissociation energy is the energy required to break a bond. It is the energy that is holding the bonds together. It is the energy required to dissociate a bond.
  6. Feb 8, 2004 #5
    Question for GeneralChemTutor,

    Can we produce a photon with 254nm wavelength?

    What is the range that we can produce in the UV part of the spectrum?

  7. Feb 8, 2004 #6


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    I believe that the range for the UV spectrum is between 100-400nm.
  8. Feb 9, 2004 #7

    How does science work with UV values in spectral analysis since it is unseen? Energy, Temperature ?? Lyman series predicts - how do we confirm? (also looking for same idea w/ Paschen series - IR)

  9. Feb 9, 2004 #8


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    Some molecules absorb radiation within the UV range. So for instance, to ascertain between a sample which contains a certain molecule (which we know to absorb UV rays) and another (which does not), we can place both samples in a UV spectrophotometer and measure the degree of absorption. Absorption indicates that the UV rays were not reflected in any way or simply transferred throughout the molecule but that it induced a change within a molecule for instance in dislocating an electron or moving it up to a higher molecular orbital.
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