By using photons of specific wavelengths, chemists can dissociate gaseous HI to

Question

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, whereas the relatively heavy I atoms move little. Use Table 9.2 in your textbook to answer the following questions: (a) What is the longest wavelength (in nm) that can dissociate a molecule of HI? (b) If a photon of 283 nm is used, what is the excess energy (in J) over that needed for the dissociation? (c) If all of this excess energy is carried away by the H atom as kinetic energy, what is its speed (in m/s)? Table 9.2

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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