For convenience Greek letters are NOT used in formulae and calculations, so please use ...
c = velocity of light = 3.00 x 10
8 ms
-1 w = wavelength in m (
properly!, note 1 nanometre or 1nm = 10
-9 m)
f = frequency in Hz (
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 10
23 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 CH
2 BrCl.
(a) A little conversion job! The wavelength of the uv light is
=
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w m
(b)
(i) To interconvert wavelength and frequency we use the formula:
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w = w x
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w , and so
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w /
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w (ii) The frequency of the uv light is
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w /
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w Hz
(c)
(i) Planck's Equation,
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w = h x
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w , gives us the energy of the photon.
(ii) The quantum energy of each uv photon is
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w J
(d) The energy per mole of photons ...
(i) =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w J
(ii) =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w kJ
[3sf] (e) Which bond will undergo homolytic fission when uv light photons strike the bromochloromethane molecules? C-H, C-Br or C-Cl?:
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w (f) What is the minimum energy needed to break one C-Cl bond?
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w [3sf] x
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w [4sf] =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w J for 1 mole of bonds
Energy to break one bond is
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w /
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w J
(g) What is the minimum frequency of uv light is needed to break a C-Cl bond?
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w = h x
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w , so
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w /
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w The frequency required
=
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w /
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
-34 =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w Hz
(h) What is the maximum wavelength of uv light capable of breaking a C-Cl bond?
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w = w x
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w ,
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w /
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w ,
wavelength to break C-Cl bond =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w /
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w m
=
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w x 10
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w =
1000 14 -19 23 3.00 3.07 3.46 3.90 307 -34 346 390 5 5.10 5.75 6.02 6.63 -7 7.69 8 8.67 9 -9 c C-Br E f h w nm
[3sf]