Cheryl - Ozone

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S Y L L A B U S D E T A I L S OPT-SL/HL Michael Holenderski, March 2000

A.S. Obj D.9 Ozone depletion (2h)

D.9.1 1,2 Explain the dependence of O2 and O3 dissociation on the wavelength of light. The Lewis structures of O2 and O3 are O2 O3 O = O O = O − O ↔ O − O = O And they dissociate according to the following equations O2 → O ⋅ + O ⋅ O3 → O + O2 It can be seen from the Lewis structures that between the two O’s in O2 there is a double bond, in O3, however, the bond between the O’s is a resonant single/double bond, which is weaker than the double bond in O2. Therefore O2 requires more energy in order to brake the bond between the O’s and to disassociate than O3. The energy they use is the UV light coming from the sun. Its energy is given by the equation E = hf Where E is the energy, h is the planks constant and f is the frequency. But f = 1 / λ Where λ is the wavelength. The equation for the energy can now be rewritten to E = h / λ Thus the lower the wavelength of light the its higher the energy. Therefore it can be expected that the light absorbed by O2 will have a lower wavelength than the light absorbed by O3. It turns out that the exact values for the wavelengths are O2 → O ⋅ + O ⋅ O3 → O ⋅ + O2

D.9.2 1,2 Describe the steps in the catalysis of O3 depletion by CFCs and NOx. CFCs (chlorofluorocarbons) are considered to be very dangerous gases, because they act as catalyst in the stratosphere, helping in to dissociate ozone and leading to ozone depletion. Because they act as catalysts they are not used up in the reactions and thus one CFC molecule can dissociate thousands of ozone molecules. For example let us take the CFCl3. First, with the help of UV light from the sun it dissociates to produce a free radial, a Cl atom. CFCl3 → CFCl2 + Cl ⋅ Then the radical Cl dissociates an O3 molecule. Cl ⋅ + O3 → ClO ⋅ + O2 The ClO molecule then reacts with an O atom to form a Cl radical again. ClO ⋅ + O ⋅ → Cl ⋅ + O2 The Cl atom can then dissociate another ozone molecule. The nitric oxide acts as a catalyst in a very similar way to a Cl atom. First it dissociates an O3 molecule. NO ⋅ + O3 → NO2 ⋅ + O2 uv uv λ=212nm λ=330nm uv And then NO2 radical reacts with an O atom to form a NO radical again. NO2 ⋅ + O → NO ⋅ + O2

D.9.3 1,2,3 Discuss the basis for greater ozone depletion in polar regions. The level of ozone depletion in polar regions varies with seasons. During the southern winter, the air over Antarctica is cut off from the rest of the atmosphere by strong winds which sweep around the continent. They act as a wall isolating the Antarctic air from the outside world. Behind the wall the temperature of air in the stratosphere reaches –90°C which gives rise to the formation of clouds made of icy particles. These particles act as catalysts allowing reactions to take place on their surface, which produce ClO particles. These ClO particles then react to form (ClO)2 ClO ⋅ + ClO ⋅ → (ClO)2 Note that this reaction does not require any activation energy and thus will occur during the southern winter. This gives rise to a build up of (ClO)2 during winter. When the sunlight arrives, it triggers the following reaction (ClO)2 → ClOO ⋅ + Cl ⋅ which releases a Cl radical, which dissociates ozone according to the reactions in D.9.2. ClOO then reacts further to form another radical ClOO ⋅ + M → Cl ⋅ + O2 + M The M molecule stands for any particle and is used here in order to satisfy the law of conservation of energy and momentum.

D.9.4 1,2,3 Discuss the properties required for sun-screening compounds Ozone protects us from the dangerous UV light and because of the ozone depletion, there is less and less ozone to protect us. Therefore we have to use alternative protection from the sunlight, and one of them is sunscreen. A sunscreen is a compound which, like ozone, will absorb the UV light. One example is the benzene ring. With its delocalized electrons will absorb the energy from the UV light into its bonding. Thus a compound containing benzene rings (e.g. PAPA: para-dimethylaminobenzaldehyde) will do the job of protecting us from the dangerous sunlight. uv