[pistaciolow]

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.
A. If a photon of 256 nm is used, what is the excess energy (in J) over that needed for the dissociation?
E = hc/lambda
(1.9875 e-25)/(2.56 e-7) = 7.76e-19 J

Your answer differs from the correct answer by orders of magnitude.

B.If all of this excess energy is carried away by the H atom as kinetic energy, what is its speed (in m/s)?
no idea

Help?

[Borek]

What is formula for kinetic energy?

[pistaciolow]

KE = ½mv². I'm thinking KE = the answer of part A and m=1.673 x 10^-27 kg, but I still haven't figured out the correct value for part A yet.

[plankk]

Look for value of energy, which is needed to dissociate HI.

[pistaciolow]

Bond dissociation energy?

This was also apart of the question: a.) What is the longest wavelength that can dissociate a molecule of HI.
406 nm

Bond dissociation energy=hv=h(c/wavelength)
BE=6.626e^-34 (3e^8*2.56 e-7)=7.76e^-19
BE=6.626e^-34 (3e^8*4.06 e-7)=4.896e^-19
7.76e^-19
-(4.896e^-19)
=2.86e^-19?

...And KE = ½mv².
=2.86e^-19=(1.673e^-24)v^2
                                  2
=2.86e^-19=(1.673e^-24)v^2
                                 2
=2.86e^-19= (8.365e^-24)v^2
=2.86e^-19          =v^2
   (8.365e^-24)         
           
v=584.72 m/s, 585 m/s (adjusted for sig figs)?         

[pistaciolow]

Nevermind, I figured it out.