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

= 390 x 10

-9 = 3.90 x 10

-7 m

(b)

(i) To interconvert wavelength and frequency we use the formula:

c = w x f, and so f = c / w

(ii) The frequency of the uv light is

3.00 x 10

8 / 3.90 x 10

-7 = 7.69 x 10

14 Hz

(c)

(i) Planck's Equation, E = h x f, gives us the energy of the photon.

(ii) The quantum energy of each uv photon is

6.63 x 10

-34 x 7.69 x 10

14 = 5.10 x 10

-19 J

(d) The energy per mole of photons ...

(i) = 5.10 x 10

-19 x 6.02 x 10

23 = 3.07 x 10

5 J

(ii) = 307 kJ

[3sf] (e) Which bond will undergo homolytic fission when uv light photons strike the bromochloromethane molecules? C-H, C-Br or C-Cl?: C-Br

(f) What is the minimum energy needed to break one C-Cl bond?

346

[3sf] x 1000

[4sf] = 3.46 x 10

5 J for 1 mole of bonds

Energy to break one bond is

3.46 x 10

5 / 6.02 x 10

23 = 5.75 x 10

-19 J

(g) What is the minimum frequency of uv light is needed to break a C-Cl bond?

E = h x f, so f = E /h

The frequency required

= 5.75 x 10

-19 / 6.63 x 10

-34 = 8.67 x 10

14 Hz

(h) What is the maximum wavelength of uv light capable of breaking a C-Cl bond?

c = w x f, w = c / f ,

wavelength to break C-Cl bond = 3.00 x 10

8 / 8.67 x 10

14 = 3.46 x 10

-7 m

= 3.46 x 10

-7 x 10

9 = 346 nm

[3sf] OK!