CHEMISTRY THE CENTRAL SCIENCE

18 CHEMISTRY OF THE ENVIRONMENT

CHAPTER SUMMARY AND KEY TERMS

SECTION 18.1 In this section we examined the physical and chemical properties of Earth's atmosphere. The complex temperature variations in the atmosphere give rise to four regions, each with characteristic properties. The lowest of these regions, the troposphere, extends from Earth's surface up to an altitude of about 12 km. Above the troposphere, in order of increasing altitude, are the stratosphere, mesosphere, and thermosphere. In the upper reaches of the atmosphere, only the simplest chemical species can survive the bombardment of highly energetic particles and radiation from the Sun. The average molecular weight of the atmosphere at high elevations is lower than that at Earth's surface because the lightest atoms and molecules diffuse upward and also because of photodissociation, which is the breaking of bonds in molecules because of the absorption of light. Absorption of radiation may also lead to the formation of ions via photoionization.

SECTION 18.2 Ozone is produced in the upper atmosphere from the reaction of atomic oxygen with O₂. Ozone is itself decomposed by absorption of a photon or by reaction with an active species such as Cl. Chlorofluorocarbons can undergo photodissociation in the stratosphere, introducing atomic chlorine, which is capable of catalytically destroying ozone. A marked reduction in the ozone level in the upper atmosphere would have serious adverse consequences because the ozone layer filters out certain wavelengths of ultraviolet light that are not removed by any other atmospheric component. In the troposphere the chemistry of trace atmospheric components is of major importance. Many of these minor components are pollutants. Sulfur dioxide is one of the more noxious and prevalent examples. It is oxidized in air to form sulfur trioxide, which, upon dissolving in water, forms sulfuric acid. The oxides of sulfur are major contributors to acid rain. One method of preventing the escape of SO₂ from industrial operations is to react it with CaO to form calcium sulfite (CaSO₃).

Photochemical smog is a complex mixture in which both nitrogen oxides and ozone play important roles. Smog components are generated mainly in automobile engines, and smog control consists largely of controlling auto emissions.

Carbon dioxide and water vapor are the major components of the atmosphere that strongly absorb infrared radiation. CO₂ and H₂O are therefore critical in maintaining Earth's surface temperature. The concentrations of CO₂ and other so-called greenhouse gases in the atmosphere are thus important in determining worldwide climate. Because of the extensive combustion of fossil fuels (coal, oil, and natural gas), the concentration of carbon dioxide in the atmosphere is steadily increasing.

SECTION 18.3 Earth's water is largely in the oceans and seas; only a small fraction is freshwater. Seawater contains about 3.5% by mass of dissolved salts and is described as having a salinity (grams of dry salts per 1 kg seawater) of 35. Seawater's density and salinity vary with depth. Because most of the world's water is in the oceans, humans may eventually need to recover freshwater from seawater. The global water cycle involves continuous phase changes of water.

SECTION 18.4 Freshwater contains many dissolved substances including dissolved oxygen, which is necessary for fish and other aquatic life. Substances that are decomposed by bacteria are said to be biodegradable. Because the oxidation of biodegradable substances by aerobic bacteria consumes dissolved oxygen, these substances are called oxygen-demanding wastes. The presence of an excess amount of oxygen-demanding wastes in water can sufficiently deplete the dissolved oxygen to kill fish and produce offensive odors. Plant nutrients can contribute to the problem by stimulating the growth of plants that become oxygen-demanding wastes when they die.

Desalination is the removal of dissolved salts from seawater or brackish water to make it fit for human consumption. Desalination may be accomplished by distillation or by reverse osmosis.

The water available from freshwater sources may require treatment before it can be used domestically. The several steps generally used in municipal water treatment include coarse filtration, sedimentation, sand filtration, aeration, sterilization, and sometimes water softening. Water softening is required when the water contains significant concentrations of ions such as Mg²⁺ and Ca²⁺, which react with soap to form soap scum. Water containing such ions is called hard water. The lime-soda process, which involves adding CaO and Na₂CO₃ to hard water, is sometimes used for large-scale municipal water softening. Individual homes usually rely on ion exchange, a process by which hard-water ions are exchanged for Na⁺ ions.

SECTION 18.5 The green chemistry initiative promotes the design and application of chemical products and processes that are compatible with human health and that preserve the environment. The areas in which the principles of green chemistry can operate to improve environmental quality include choices of solvents and reagents for chemical reactions, development of alternative processes, and improvements in existing systems and practices.

KEY SKILLS

• Describe the regions of Earth's atmosphere in terms of how temperature varies with altitude. (Section 18.1)

• Describe the composition of the atmosphere in terms of the major components in dry air at sea level. (Section 18.1)

• Calculate concentrations of gases in parts per million (ppm). (Section 18.1)

• Describe the processes of photodissociation and photoionization and their role in the upper atmosphere. (Section 18.1)

• Use bond energies and ionization energies to calculate the minimum frequency or maximum wavelength needed to cause photodissociation or photoionization. (Section 18.1)

• Explain the role of ozone in the upper atmosphere. (Section 18.1)

• Explain how chlorofluorocarbons (CFCs) are involved in depleting the ozone layer. (Section 18.2)

• Describe the origins and behavior of sulfur oxides and nitrogen oxides as air pollutants, including the generation of acid rain and photochemical smog. (Section 18.2)

• Describe how water and carbon dioxide in the atmosphere affect atmospheric temperature via the greenhouse effect. (Section 18.2)

• Describe the global water cycle. (Section 18.3)

• Explain what is meant by the salinity of water and describe the process of reverse osmosis as a means of desalination. (Section 18.4)

• List the major cations, anions, and gases present in natural waters and describe the relationship between dissolved oxygen and water quality. (Section 18.4)

• List the main steps involved in treating water for domestic uses. (Section 18.4)

• Describe the main goals of green chemistry. (Section 18.5)

• Be able to compare reactions and decide which reaction is greener. (Section 18.5)