By using photons of specific wavelengths, chemists can dissociate gaseous HI to
ID: 1055965 • Letter: B
Question
By using photons of specific wavelengths, chemists can dissociate gaseous HI to produce H atoms with accurately known speeds When Hl dissociates, the H move away rapidly, whereas the relatively heavy I atoms move little. Use table 9.2 in your textbook to answer the following questions: What it the longest wavelength (in nm) that can dissociate a molecule of HI? If a photon of 284 nm is used, what is the excess energy (in J) over that needed for the dissociation? If all of this excess energy is earned away by the H atom as kinetic energy, what is its speed (in m/s)?Explanation / Answer
Bond energy of HI= 255 Kj/mol =295*1000/ (6.023*1023) Joules/photon=4.89*10-19 joules/phoron
from E= hC/Wave length, E = 4.223*10-19 joules/photon
wave length = hC/E
h= plancks constant =6.627*10-34 J.s, c=3*108 m/s E =4.89*10-19 joules/photon
wave length = 6.627*10-34*3*108/ (4.89*10-19) =4.06*10-7 m= 406 nm
the longest wave length is 406 nm
b) when a photon of 280 nm is used wave legnth = 280*10-9 m
E= 6.627*10-34* 3*108/280*10-9 =7.1*10-19 joules
Excess energy needed = 7.1*10-19 -4.89*10-19 joules =2.2*10-19 joules
from Kinetic eneergy= (1/2)mV2, m= 1.673*10-27 kg
2.2*10-19*2= 1.673*10-27 V2 ,, V =16255.4 m/sec
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