1. Yr 10 Chemistry Environmental Chemistry: Ozone

2. Starter With the person sitting next to you: you have 1 min to come up with an answer to these questions………… What is a catalyst? Give me an example of a catalyst. Be prepared to give an answer………..

3.  TO KNOW what is ozone, where it is found in the atmosphere and why it is important.  TO BE ABLE to find ways to reduce ozone destroying chemicals.  TO UNDERSTAND how certain chemicals are destroying ozone and its consequences. Objectives

4.  Draw the Lewis structure for ozone.(O 3 )  Draw the Lewis structure for oxygen. (O 2 )  Describe how they are different………..

5.  Discuss the reasons why the following statement invalid:  "CFCs are not reactive and so do not contribute to stratospheric ozone depletion".  Include in your answer:  Why is atmospheric ozone good and how does it protect us? (include chemical reactions in your answer)  How do increases in CFC’s affect the stratospheric ozone?(include chemical reactions in your answer)  What ways can we reduce our effect on ozone depletion? (marks will be given for quality and structure of your answer as well) Exam Question

6.  Ozone is an unstable form of oxygen (O 3 ) not the more stable O 2  Lewis Structure ?  Although unstable it exists naturally in our upper atmosphere  We also need to know about a HARMFUL part of the Electromagnetic spectrum call Ultraviolet Radiation produced by the Sun.  (We have already discussed harmless Infra Red Radiation when examining Global Warming) What is Ozone?

8. Regions of the atmosphere  The lower atmosphere is known as the troposphere. In this region most of the common weather phenomena occur.  Ozone that occurs in the troposphere can be a significant air pollution problem. Ozone is a health hazard - it is a major constituent of photochemical smog.

9. The Stratosphere  In the stratosphere, ozone (O 3 ) plays a vital role by absorbing potentially harmful ultra-violet (UV) radiation from the sun.  At wavelengths between 240 and 320 nm ultraviolet radiation can cause skin cancer and damage vegetation.

11.  Stratospheric ozone is continually formed and decomposed, forming a dynamic equilibrium with oxygen (called the Chapman Cycle).  It is created when ultraviolet radiation from the sun strikes oxygen molecules in the stratosphere.  This occurs in four steps:

12. Step One O 2 (g) + ultraviolet Radiation  2 O (this occurs with high energy UV light) Step Two O + O 2 + Ultraviolet Radiation  O 3 (g) (this occurs with lower energy UV light) Step 3  O 3 + Ultraviolet Radiation  O 2 + O (very low energy UV – reverse of step 2) Step 4  O + O 3  2O 2  (Ozone is destoyed with very low energy UV waves).... This continuing process called the ozone- oxygen cycle (or the Chapman Cycle).

13.  Why is UV harmful for us? What Would Happen If…… Although ozone is a reactive and poisonous gas, it protects us from harmful UV radiation which would affect life on earth. UV radiation can cause skin cancer.

14. BUT breakdown is easier in the presence of chlorofluorocarbons (CFC's)

15. CFC’s Apparent benefits were offset by unexpected side effects. GOOD created in 1928 as a non-toxic, non- flammable refrigerant also used as solvents and in air conditioners low reactivity and volatility

16. CFC’s Apparent benefits were offset by unexpected side effects. GOOD created in 1928 as a non-toxic, non- flammable refrigerant also used as solvents and in air conditioners low reactivity and volatility BAD UV light in the upper atmosphere easily breaks the C-Cl bonds free radicals formed speeded up the depletion of the ozone layer

17.  The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sydney Chapman in 1930.Sydney Chapman  Ozone in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O 2 ), splitting them into individual oxygen atoms (atomic oxygen)ultraviolet lightoxygenmoleculesatoms  The atomic oxygen then combines with unbroken O 2 to create ozone, O 3. The ozone molecule is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O 2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle (or the Chapman Cycle).ozone-oxygen cycle  Chemically, this can be described as:  O 2 + ℎ ν uv → 2O  O + O 2 ↔ O 3  O 3 + O ↔ O 2 Also  O 3 + ℎ ν uv → O + O 2 About 90% of the ozone in our atmosphere is contained in the stratosphere.Ozone concentrations are greatest between about 20 and 40 kilometres (66,000 and 131,000 ft), where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimeters thick

19. Other O-O Bonds  We can obtain experimental data on bond energies of other molecules in the same way. Molecular oxygen, O 2, is photolyzed by light of 241 nm and has a bond energy of 498 kJ/mol. Why do we see such large difference in the strength of oxygen-oxygen bonds in these molecules. Let's look at the Lewis structures. The bond energy correlates with the bond order.

20. Summary  UV light provides sufficient energy to cleave an atom of oxygen from ozone.  Since we know that breaking bonds is an endothermic process, this process absorbs the wavelength of the electromagnetic spectrum and hence limits the surface of the Earth to its exposure.

21.  O 2 + ℎ ν uv → 2O endothermic process and requires energy from UV. The bond energy of the O2 molecule (498 kJ mol-1) corresponds to the energy of a 240 nm UV photon; only photons of wavelengths less than 240 nm can photolyse the O2 molecule  O + O 2 ↔ O 3 exothermic process and creates a source of heat, which actually forms the stratosphere itself (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the Earth's atmosphere.  O 3 + O ↔ O 2 Also  O 3 + ℎ ν uv → O + O 2 Provides additional protection by absorbing UV at different wavelength. Because the bonds in the O 3 molecule are weaker than those in the O 2 molecule, photolysis is achieved with higher wavelength (lower-energy) photons (340nm)

22. GREEN CHEMISTRY – EXAMPLES CFC’s Apparent benefits were offset by unexpected side effects. GOOD created in 1928 as a non-toxic, non-flammable refrigerant also used as solvents and in air conditioners low reactivity and volatility BAD UV light in the upper atmosphere easily breaks the C-C l bonds free radicals formed speeded up the depletion of the ozone layer CFC’s very stable and unreactive will remain in atmosphere for a very long time. It is only exposed to cosmic radiation in the stratosphere does it become reactice. It is a gas so can drift in the atmosphere

23. DEPLETION OF THE OZONE LAYER EFFECT OF CFC’S There is a series of complex reactions but the basic process is :- CFC's break down in the presenceCCl 2 F 2 —> C l + CC l F 2 of UV light to form chlorine radicals Chlorine radicals react with ozone O 3 + C l —> C l O + O 2 Chlorine radicals are regenerated C l O + O —> O 2 + C l Overallchlorine radicals are not used up so a small amount of CFC's can destroy thousands of ozone molecules before the termination stage. THIS COMPETING REACTION UPSETS THE CHAPMAN CYCLE AND REDUCES THE AMOUNT OF OZONE PRESENT TO ABSORB UV.

24. DEPLETION OF THE OZONE LAYER OXIDES OF NITROGEN NOx Oxides of nitrogen, NOx, formed during thunderstorms or by aircraft break down to give NO (nitrogen monoxide) which also catalyses the breakdown of ozone. nitrogen monoxide reacts with ozone O 3 + NO —> NO 2 + O 2 nitrogen monoxide is regenerated NO 2 + O —> O 2 + NO

25. Environmental Effect  In the first instance there is an increase in ultraviolet radiation that reaches the Earth's surface.  Ultraviolet light has sufficient energy to damage biological molecules, especially proteins.  The immediate impact of increased ultraviolet radiation on humans would be an increase in sunburn, followed by an increase in melanomas and non-melanoma skin cancers.  It could also cause an increase in cataracts and blindness

26. Effect on Plants  The impact of increased ultraviolet radiation on plants would result in growth and photosynthesis inhibition.  Plants could become more susceptible to disease.  Marine ecosystems may endure a loss in phytoplankton. Because these phytoplankton produce biomass, there would be less food for other marine organisms and a loss of he carbon dioxide sink.

27. Solutions To the Problem  CFC-s are now a banned substance in household products.  Most replacements are hydrofluorocarbon compounds such as CHF 2 Cl.  These compounds decompose more readily and therefore they are less likely to build up in the stratosphere.  Unfortunately many of these compounds are also flammable.

28.  Discuss the reasons why the following statement invalid:  "CFCs are not reactive and so do not contribute to stratospheric ozone depletion".  Include in your answer:  Why is atmospheric ozone good and how does it protect us? (include chemical reactions in your answer)  How do increases in CFC’s affect the stratospheric ozone?(include chemical reactions in your answer)  What ways can we reduce our effect on ozone depletion? (marks will be given for quality and structure of your answer as well) Exam Question

29. Create a mind map of the depletion of the ozone layer. Include: Causes Effects Solution