Advanced Level Chemistry PLANCK'S EQUATION Q1

Fill in all the gaps, then press CHECK at any time,as long as you are not bothered about the score, but interested in self-tuition! You can printout, do later and come back for the answers. Thoughts on style of question? Updated Doc Brown's Chemistry

For convenience Greek letters are NOT used in formulae and calculations, so please use ...
c = velocity of light = 3.00 x 108 ms-1
w = wavelength in m (lambda.gif properly!, note 1 nanometre or 1nm = 10-9m)
f = frequency in Hz (mu.gif properly!)
E = quantum energy of a single photon in J
h = Planck Constant = 6.63 x 10-34 JHz-1
Avogadro Constant = L = 6.02 x 1023 mol-1
Bond enthalpies kJ mol-1: C-H 413; C-Br 290; C-Cl 346

Answer all question parts to 2 decimal places except where indicated in [sf] or an integer power.
Q1. Bromo and chloro compounds in the atmosphere can lead to ozone depletion. In a laboratory experiment ultraviolet light (uv) of wavelength 390nm was passed through bromochloromethane CH2BrCl.

(a) A little conversion job! The wavelength of the uv light is
= x 10 = x 10 m

(i) To interconvert wavelength and frequency we use the formula:
= w x , and so = /
(ii) The frequency of the uv light is
x 10 / x 10 = x 10 Hz

(i) Planck's Equation, = h x , gives us the energy of the photon.
(ii) The quantum energy of each uv photon is
x 10 x x 10 = x 10 J

(d) The energy per mole of photons ...
(i) = x 10 x x 10 = x 10 J
(ii) = kJ [3sf]

(e) Which bond will undergo homolytic fission when uv light photons strike the bromochloromethane molecules? C-H, C-Br or C-Cl?:

(f) What is the minimum energy needed to break one C-Cl bond?
[3sf] x [4sf] = x 10 J for 1 mole of bonds
Energy to break one bond is
x 10 / x 10 = x 10 J

(g) What is the minimum frequency of uv light is needed to break a C-Cl bond?
= h x , so = /
The frequency required
= x 10 / x 10-34 = x 10 Hz

(h) What is the maximum wavelength of uv light capable of breaking a C-Cl bond?
= w x , = / ,
wavelength to break C-Cl bond = x 10 / x 10 = x 10 m
= x 10 x 10 = nm [3sf]

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