1. Prentice Hall © 2003Chapter 18 Chapter 18 Chemistry of the Environment CHEMISTRY The Central Science 9th Edition David P. White

2. Prentice Hall © 2003Chapter 18 The temperature of the atmosphere is a complicated function of altitude. Earth’s Atmosphere

3. Prentice Hall © 2003Chapter 18 Below an altitude of 10 km (troposphere) the temperature decreases from 290 K to 215 K as altitude increases. In the stratosphere (10 km - 50 km) the temperature increases from 215 K to 275 K. In the mesosphere (50 km - 85 km) the temperature decreases (275 K to 190 K) and in the thermosphere (> 85 km) the temperature increases. The variation of pressure with altitude is simpler: pressure decreases as altitude increases. Earth’s Atmosphere

4. Prentice Hall © 2003Chapter 18 At 80 km the pressure is about 0.01 torr. The boundaries between regions are given the suffix - pause. There is slow mixing of gases between regions in the atmosphere. Composition of the Atmosphere Earth’s atmosphere is affected by temperature and pressure as well as gravity. Earth’s Atmosphere

5. Prentice Hall © 2003Chapter 18 Composition of the Atmosphere Lighter molecules and atoms are found at higher altitudes. Two major components of the atmosphere are nitrogen, N 2, and oxygen, O 2. Earth’s Atmosphere

7. Prentice Hall © 2003Chapter 18 Photodissociation Recall –so the higher the frequency, the shorter the wavelength and the higher the energy of radiation. For a chemical reaction induced by radiation to occur, the photons must have sufficient energy to break the required bonds and the molecules must absorb the photons. Photodissociation is the rupture of a chemical bond induced by radiation. Outer Regions of the Atmosphere

8. Prentice Hall © 2003Chapter 18 Photodissociation In the upper atmosphere, photodissociation causes the formation of oxygen atoms: O 2 (g) + h  2O(g) Photoionization Photoionization is the ionization of molecules (and atoms) caused by radiation. Outer Regions of the Atmosphere

9. Prentice Hall © 2003Chapter 18 Photoionization 1924: electrons were discovered in the upper atmosphere. Therefore, cations must be present in the upper atmosphere. Photoionization occurs when a molecule absorbs a photon of sufficient energy to remove an electron. Wavelengths of light that cause photoionization and photodissociation are filtered by the atmosphere. Outer Regions of the Atmosphere

10. Prentice Hall © 2003Chapter 18 Ozone absorbs photons with a wavelength between 240 and 310 nm. Most of the ozone is present in the stratosphere (maximum ozone concentration at an altitude of 20 km). Between 30 and 90 km photodissociation of oxygen is possible: O 2 (g) + h  2O(g) Ozone and the Upper Atmosphere

11. Prentice Hall © 2003Chapter 18 The oxygen atoms can collide with oxygen molecules to form ozone with excess energy, O 3 *: O(g) + O 2 (g)  O 3 *(g) The excited ozone can loose energy by decomposing to oxygen atoms and oxygen molecules (the reverse reaction) or by transferring the energy to M (usually N 2 or O 2 ): O 3 *(g) + M(g)  O 3 (g) + M*(g) Ozone and the Upper Atmosphere

12. Prentice Hall © 2003Chapter 18 The formation of ozone in the atmosphere depends on the presence of O(g). At low altitudes, the radiation with sufficient energy to form O(g) has been absorbed. Release of energy from O 3 * depends on collisions, which generally occur at lower altitudes. Combining effects means maximum ozone formation in the stratosphere. Ozone and the Upper Atmosphere

14. Prentice Hall © 2003Chapter 18 Depletion of the Ozone Layer In October 1994 the map of ozone present in the Southern hemisphere showed a hole over Antarctica. In 1995 the Nobel Prize for chemistry was awarded to F. Sherwood Rowland, Mario Molina, and Paul Crutzen for their studies of ozone depletion. Ozone and the Upper Atmosphere

15. Prentice Hall © 2003Chapter 18 Depletion of the Ozone Layer In 1974 Rowland and Molina showed that chlorine from chlorofluorocarbons (CFCs) deplete the ozone layer by catalyzing the formation of ClO and O 2. Ozone and the Upper Atmosphere

16. Prentice Hall © 2003Chapter 18 Depletion of the Ozone Layer In the stratosphere, CFCs undergo photochemical rupture of a C-Cl bond: CF 2 Cl 2 (g) + h  CF 2 Cl(g) + Cl(g) (optimal at 30 km). –Subsequently Cl(g) + O 3 (g)  ClO(g) + O 2 (g) rate = k[Cl][O 3 ], k = 7.2  10 9 M -1 s -1 at 298 K. –In addition, the ClO generated produces Cl as well: 2ClO(g)  O 2 (g) + 2Cl(g) –giving rise to the overall reaction 2O 3 (g)  3O 2 (g). Ozone and the Upper Atmosphere

17. Prentice Hall © 2003Chapter 18 The troposphere consists mostly of O 2 and N 2. Even though other gases are present in low concentrations, their effects on the environment can be profound. Sulfur Compounds and Acid Rain Sulfur dioxide, SO 2, is produced by the combustion of oil and coal (and some other means). Chemistry of the Troposphere

18. Prentice Hall © 2003Chapter 18 Sulfur Compounds and Acid Rain The SO 2 is oxidized to SO 3 which reacts with water to produce sulfuric acid (acid rain): SO 3 (g) + H 2 O(l)  H 2 SO 4 (aq) More than 30 million tons per year of SO 2 are released into the atmosphere in the United States. Nitrogen oxides also contribute to acid rain by forming nitric acid. Normal rainwater has a pH of about 5 (due to the H 2 CO 3 produced from CO 2 ). Chemistry of the Troposphere

19. Prentice Hall © 2003Chapter 18 Sulfur Compounds and Acid Rain Acid rain has a pH around 4, whereas the pH of natural waters containing living organisms is 6.5 to 8.5. Natural waters with a pH below 4 cannot sustain life. It is too expensive to remove sulfur from oil and coal prior to its use. Therefore, the SO 2 is removed from fuel upon combustion. Chemistry of the Troposphere

20. Prentice Hall © 2003Chapter 18 Sulfur Compounds and Acid Rain SO 2 is commonly removed from fuel (oil and coal) as follows: –powdered limestone decomposes into CaO; –CaO reacts with SO 2 to form CaSO 3 in a furnace; –CaSO 3 and unreacted SO 2 are passed into a scrubber (purification chamber) where the SO 2 is converted to CaSO 3 by jets of CaO. –CaSO 3 is precipitated into a watery slurry. Chemistry of the Troposphere

21. Prentice Hall © 2003Chapter 18 Sulfur Compounds and Acid Rain Chemistry of the Troposphere

22. Prentice Hall © 2003Chapter 18 Carbon Monoxide CO is produced by incomplete combustion of fuels. About 10 14 g of CO is produced in the United States per year (mostly from automobiles). CO binds irreversibly to the Fe in hemoglobin. (CO binds to hemoglobin about 210 times more strongly than oxygen.) Chemistry of the Troposphere

23. Prentice Hall © 2003Chapter 18 Carbon Monoxide Hemoglobin is responsible for oxygen transport: –in the lungs, CO 2 is released from the Fe in the hemoglobin and O 2 binds to the Fe (forming oxyhemoglobin), –in the tissues, O 2 is released and CO 2 binds to the iron. –When CO binds to Fe in hemoglobin, carboxyhemoglobin, COHb, it cannot be displaced by O 2 or CO 2. –Therefore, in sufficient concentrations CO can stop oxygen transport in living systems. Chemistry of the Troposphere

24. Prentice Hall © 2003Chapter 18 Nitrogen Oxides and Photochemical Smog Photochemical smog is the result of photochemical reactions on pollutants. In car engines, NO forms as follows: In air At 393 nm (wavelength of sunlight), NO 2 decomposes NO 2 (g) + h  NO(g) + O(g) Chemistry of the Troposphere

25. Prentice Hall © 2003Chapter 18 Nitrogen Oxides and Photochemical Smog The O produced by photodissociation of NO 2 can react with O 2 to form O 3, which is the key component of smog O(g) + O 2 (g) + M(g)  O 3 (g) + M*(g). In the troposphere ozone is undesirable because O 3 is toxic and reactive. Water Vapor, Carbon Dioxide, and Climate There is a thermal balance between the earth and its surroundings. Chemistry of the Troposphere

26. Prentice Hall © 2003Chapter 18 Water Vapor, Carbon Dioxide, and Climate Radiation is emitted from the earth at the same rate as it is absorbed by the earth. The troposphere is transparent to visible light. However, the troposphere is not transparent to IR radiation (heat). Therefore, the troposphere insulates the earth making it appear colder from the outside than it is on the surface. Chemistry of the Troposphere

27. Prentice Hall © 2003Chapter 18 Water Vapor, Carbon Dioxide, and Climate CO 2 and H 2 O absorb IR radiation escaping from the earth’s surface. At night, the earth emits radiation. Water vapor is responsible for keeping IR on the planet. The carbon dioxide level on earth has been increasing over the years. Chemistry of the Troposphere

28. Prentice Hall © 2003Chapter 18 Water Vapor, Carbon Dioxide, and Climate Chemistry of the Troposphere

29. Water Vapor, Carbon Dioxide, and Climate Chemistry of the Troposphere

30. Prentice Hall © 2003Chapter 18 Water Vapor, Carbon Dioxide, and Climate A lot of the increase is due to the combustion of fuels. We speculate that the increased CO 2 concentration is resulting in a gradual warming of the earth’s surface. Between 2050 and 2100 the CO 2 concentration is expected to double. This will result in a global temperature increase of 1 to 3  C. Chemistry of the Troposphere

31. Prentice Hall © 2003Chapter 18 Seawater 72 % of the earth’s surface is covered with water. The volume of oceans in the world is 1.35  10 9 km 3. 97.2 % of earth’s water is in oceans. Salinity: mass of dry salts in 1 kg of seawater. Seawater averages a salinity about 35. Most elements in seawater are only present in small quantities. Commercially, NaCl, Br - and Mg 2+ are obtained from seawater. The World Ocean

33. Prentice Hall © 2003Chapter 18 Desalination Water used for drinking should contain less than 500 ppm dissolved salts (United States municipal water). Desalination: removal of salts from seawater. Common method: reverse osmosis (energy intensive). –Osmosis: transport across a semipermeable membrane. –Osmosis: movement of solvent molecules. No good for desalination. –Reverse osmosis: under applied pressure, solvent moves from more concentrated solution to more dilute solution. The World Ocean

34. Prentice Hall © 2003Chapter 18 Desalination –In a reverse osmosis desalination plant, Each cylinder is called a permeator. –Seawater is introduced under pressure and water passes through the fiber walls, into the fibers and is separated from the ions. The World Ocean

35. Prentice Hall © 2003Chapter 18 An adult needs about 2 L of water a day for drinking. In the United States, the average person uses about 300 L per day of freshwater. Industry uses even more freshwater than this (e.g. about 10 5 L of water are used to make enough steel for one car. As the water flows over the earth it dissolves many substances. Freshwater usually contains some ions (Na +, K +, Mg 2+, Ca 2+, Fe 2+, Cl -, SO 4 2-, and HCO 3 - ) and dissolved gases (O 2, N 2, and CO 2 ). Freshwater

36. Prentice Hall © 2003Chapter 18 Dissolved Oxygen and Water Quality Water fully saturated with air at 1 atm and 20  C has 9 ppm of O 2 dissolved in it. Cold water fish require about 5 ppm of dissolved oxygen for life. Aerobic bacteria consume oxygen to oxidize biodegradable organic material. Biodegradable materials are oxygen-demanding wastes. E.g., sewage, industrial waste from food-processing plants and paper mills, and effluent from meat packing plants. Freshwater

37. Prentice Hall © 2003Chapter 18 Dissolved Oxygen and Water Quality Aerobic bacteria oxidize organic material into CO 2, HCO 3 -, H 2 ), NO 3 -, SO 4 2-, and phosphates. Once the oxygen level has been depleted, aerobic bacteria cannot survive. Anaerobic bacteria complete the decomposition process forming CH 4, NH 3, H 2 S, PH 3, and other foul smelling products. Eutrophication is the increase in dead and decaying plant matter resulting from excessive plant growth. Freshwater

38. Prentice Hall © 2003Chapter 18 Treatment of Municipal Water Supplies There are five steps: –Coarse filtration. Occurs as water is taken up from lake, river or reservoir. –Sedimentation. Water is allowed to stand so that solid particles (e.g. sand) can settle out. To remove components like bacteria, CaO and Al 2 (SO 4 ) 3 are added. This causes a gelatinous precipitate of Al(OH) 3, which settles slowly carrying small particles with it. Freshwater

39. Prentice Hall © 2003Chapter 18 Treatment of Municipal Water Supplies Sand Filtration. Water is filtered through a sand bed to remove Al(OH) 3 and anything it trapped. –Aeration. Air oxidizes any organic material. –Sterilization. Chlorine is usually used because it forms HClO(aq) in solution, which kills bacteria. Freshwater

40. Prentice Hall © 2003Chapter 18 Treatment of Municipal Water Supplies Freshwater

41. Prentice Hall © 2003Chapter 18 Chemical industry has recognized that it is important to use environmentally friendly chemicals and processes. There are several goals for green chemistry: Prevent waste rather than subsequently cleaning it. Synthesize new products minimizing waste. Design energy efficient processes. Use catalysts as much as possible. Raw materials should be renewable feedstocks. Eliminate solvents as far as possible. Green Chemistry

42. Prentice Hall © 2003Chapter 18 Solvents and Reagents Many organic molecules that are used as solvents are volatile and can do environmental damage. Also, many solvents are toxic. Liquid or supercritical CO 2 is a non-toxic solvent that has many potential applications. Du Pont uses supercritical CO 2 to make Teflon™ instead of the environmentally damaging chlrofluorocarbon solvents. Green Chemistry

43. Prentice Hall © 2003Chapter 18 Solvents and Reagents Supercritical water can be used to make the plastic and polyester fiber PET. Lexan™ polycarbonate, the coatings on CDs, could be made from dimethyl carbonate instead of the very toxic phosgene, which is currently used. Other Processes Liquid CO 2 is currently used in the dry cleaning industry as an alternative to Cl 2 C=CCl 2, which is toxic. Green Chemistry

44. Prentice Hall © 2003Chapter 18 Other Processes Yttrium is being used in place of lead for automobile coatings. Water Purification When Cl 2 is used to treat water, some trihalomethanes (THM) are often produced, which can go undetected. The THMs are suspected carcinogens. Ozone and ClO 2 could be used as alternatives, but they are not completely safe. Green water purification is an open problem. Green Chemistry

45. Prentice Hall © 2003Chapter 18 End of Chapter 18: Chemistry of the Environment