Chapter 18 Chemistry of the Environment

1. Chemistry, The Central Science, 11th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Chapter 18Chemistry of the Environment John D. Bookstaver St. Charles Community College Cottleville, MO © 2009, Prentice-Hall, Inc.

2. Atmosphere • Temperature varies greatly with altitude. • However, there is not a linear relationship between altitude and temperature. © 2009, Prentice-Hall, Inc.

3. Atmosphere Although the relationship between altitude and pressure is not linear, pressure does decrease with an increase in altitude. © 2009, Prentice-Hall, Inc.

4. Radiation The atmosphere is the first line of defense against radiation from the Sun. © 2009, Prentice-Hall, Inc.

5. Composition of the Atmosphere • Because of the great variation in atmospheric conditions, the composition of gases in the atmosphere is not uniform. • Lighter gases tend to rise to the top. © 2009, Prentice-Hall, Inc.

6. Composition of the Atmosphere • Near the Earth’s surface, about 99% of the atmosphere is composed of nitrogen and oxygen. • Oxygen has a much lower bond enthalpy than nitrogen, and is therefore more reactive. © 2009, Prentice-Hall, Inc.

7. Outer Atmosphere • The Sun emits a wide range of wavelengths of radiation. • Remember that light in the ultraviolet region has enough energy to break chemical bonds. © 2009, Prentice-Hall, Inc.

8. Photodissociation • When these bonds break, they do so homolytically. • Oxygen in the upper atmosphere absorbs much of this radiation before it reaches the lower atmosphere: O2 + h 2 O © 2009, Prentice-Hall, Inc.

9. Photoionization • Shorter wavelength radiation causes electrons to be knocked out of molecules in the upper atmosphere; very little of this radiation reaches the Earth’s surface. • The presence of these ions makes long-range radio communication possible. © 2009, Prentice-Hall, Inc.

10. Ozone • Ozone absorbs much of the radiation between 240 and 310 nm. • It forms from reaction of molecular oxygen with the oxygen atoms produced in the upper atmosphere by photodissociation. O + O2 O3 © 2009, Prentice-Hall, Inc.

11. Ozone Depletion In 1974 Rowland and Molina discovered that chlorine from chlorofluorocarbons (CFCs) may be depleting the supply of ozone in the upper atmosphere by reacting with it. © 2009, Prentice-Hall, Inc.

12. Chlorofluorocarbons • CFCs were used for years as aerosol propellants and refrigerants. • They are not water soluble (so they do not get washed out of the atmosphere by rain) and are quite unreactive (so they are not degraded naturally). © 2009, Prentice-Hall, Inc.

13. Chlorofluorocarbons • The C—Cl bond is easily broken, though, when the molecule absorbs radiation with a wavelength between 190 and 225 nm. • The chlorine atoms formed react with ozone: Cl + O3 ClO + O2 © 2009, Prentice-Hall, Inc.

14. Chlorofluorocarbons In spite of the fact that the use of CFCs in now banned in over 100 countries, ozone depletion will continue for some time because of the tremendously unreactive nature of CFCs. © 2009, Prentice-Hall, Inc.

15. Troposphere Although the troposphere is made up almost entirely of nitrogen and oxygen, other gases present in relatively small amounts still have a profound effect on the troposphere. © 2009, Prentice-Hall, Inc.

16. Sulfur • Sulfur dioxide is a by-product of the burning of coal or oil. • It reacts with moisture in the air to form sulfuric acid. • It is primarily responsible for acid rain. © 2009, Prentice-Hall, Inc.

17. Sulfur • High acidity in rainfall causes corrosion in building materials. • Marble and limestone (calcium carbonate) react with the acid; structures made from them erode. © 2009, Prentice-Hall, Inc.

18. Sulfur • SO2 can be removed by injecting powdered limestone which is converted to calcium oxide. • The CaO reacts with SO2 to form a precipitate of calcium sulfite. © 2009, Prentice-Hall, Inc.

19. Carbon Monoxide • Carbon monoxide binds preferentially to the iron in red blood cells. • Exposure to significant amount of CO can lower O2 levels to the point that loss of consciousness and death can result. © 2009, Prentice-Hall, Inc.

20. Carbon Monoxide • Products that can produce carbon monoxide must contain warning labels. • Carbon monoxide is colorless and odorless, so detectors are a good idea. © 2009, Prentice-Hall, Inc.

21. Nitrogen Oxides • What we recognize as smog, that brownish gas that hangs above large cities like Los Angeles, is primarily nitrogen dioxide, NO2. • It forms from the oxidation of nitric oxide, NO, a component of car exhaust. © 2009, Prentice-Hall, Inc.

22. Photochemical Smog • These nitrogen oxides are just some components of photochemical smog. • Ozone, carbon monoxide, and hydrocarbons also contribute to air pollution that causes severe respiratory problems in many people. © 2009, Prentice-Hall, Inc.

23. Photochemical Smog As a result, government emission standards for automobile exhaust have become continually more stringent. © 2009, Prentice-Hall, Inc.

24. Water Vapor and Carbon Dioxide • Gases in the atmosphere form an insulating blanket that causes the Earth’s thermal consistency. • Two of the most important such gases are carbon dioxide and water vapor. © 2009, Prentice-Hall, Inc.

25. Water Vapor and Carbon Dioxide • This blanketing effect is known as the “greenhouse effect.” • Water vapor, with its high specific heat, is a major factor in this moderating effect. • But increasing levels of CO2 in the atmosphere may be causing an unnatural increase in atmospheric temperatures. © 2009, Prentice-Hall, Inc.

26. Oceans • The vast ocean contains many important compounds and minerals. • However, the ocean is only a commercial source of sodium chloride, bromine, and magnesium. © 2009, Prentice-Hall, Inc.

27. Desalination • “Water, water everywhere, and not a drop to drink.” Seawater has too high a concentration of NaCl for human consumption. • It can be desalinated through reverse osmosis. © 2009, Prentice-Hall, Inc.

28. Reverse Osmosis • Water naturally flows through a semipermeable membrane from regions of higher water concentration to regions of lower water concentration. • If pressure is applied, the water can be forced through a membrane in the opposite direction, concentrating the pure water. © 2009, Prentice-Hall, Inc.

29. Water Purification • Clean, safe fresh water supplies are of the utmost importance to society. • There are many steps involved in purifying water for a municipal water supply. © 2009, Prentice-Hall, Inc.

30. Water Purification • Water goes through several filtration steps. • CaO and Al2(SO4)3 are added to aid in the removal of very small particles. © 2009, Prentice-Hall, Inc.

31. Water Purification • The water is aerated to increase the amount of dissolved oxygen and promote oxidation of organic impurities. • Ozone or chlorine is used to disinfect the water before it is sent out to consumers. © 2009, Prentice-Hall, Inc.

32. Green Chemistry • We have become increasingly aware over the past 30 to 40 years that modern processes are not always compatible with maintaining a sustainable environment. • Promoting chemical processes that are environmentally friendly is part of the good stewardship chemists should exhibit. © 2009, Prentice-Hall, Inc.

33. Green Chemistry Principles • Rather than worry about waste disposal, it is better to avoid creating waste in the first place. • In addition to generating as little waste as possible, try to make waste that is nontoxic. • Be energy conscious in designing syntheses. © 2009, Prentice-Hall, Inc.

34. Green Chemistry Principles • Catalysts that allow the use of safe chemicals should be employed when possible. • Try to use renewable feedstocks as raw materials. • Try to reduce the amount of solvent used, and try to use environmentally friendly solvents. © 2009, Prentice-Hall, Inc.

35. Solvents Solvents such as supercritical CO2 are great “green” alternatives. © 2009, Prentice-Hall, Inc.