By using photons of specific wavelengths, chemists can dissociate gaseous H-I to
ID: 799803 • Letter: B
Question
By using photons of specific wavelengths, chemists can dissociate gaseous H-I to produce H atoms with accurately known speeds. When H-I dissociates, the H atoms move away rapidly, whereas the relatively heavy I atoms move little. Average Bond Energies
By using photons of specific wavelengths, chemists can dissociate gaseous H-I to produce H atoms with accurately known speeds. When H-I dissociates, the H atoms move away rapidly, whereas the relatively heavy I atoms move little. Average Bond Energies What is the longest wavelength (in nm) that can dissociate a molecule of H-I? nm If a photon of 218 nm is used, what is the excess energy (in J) over that needed for the dissociation? J If all of this excess energy is carried away by the H atom as kinetic energy, what is its speed (in m/s)? m/s
Explanation / Answer
a) Look up the H-I bond dissociation energy; I'll call it E. (If you find E in units of J/mol, divide by Avogadro's number for units of J/bond.) Calculate wavelength L from E = hf = h(c/L) where h is Planck's constant and c is the speed of light.
b) Calculate the energy of the photon E' = h(c/L) and then the difference E' - E between the photon and the bond energy.
c) E' - E = 1/2 m v^2, where the mass m of one atom is the atomic weight divided by Avogadro's number.
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