Chapter 18 Chemistry of the Environment Lecture Presentation John D. Bookstaver St. Charles Community College Cottleville, MO © 2012 Pearson Education,

Chapter 18

Chemistry of the Environment

Lecture Presentation

John D. BookstaverSt. Charles Community College

Cottleville, MO© 2012 Pearson Education, Inc.

Chemistryof the

Environment

Atmosphere

• Temperature varies greatly with altitude.

• However, there is not a linear relationship between altitude and temperature.

© 2012 Pearson Education, Inc.

Chemistryof the

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Atmosphere

Although the relationship between altitude and pressure is not linear, pressure does decrease with an increase in altitude.


Chemistryof the

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Radiation

The atmosphere is the first line of defense against radiation from the Sun.


Chemistryof the

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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.


Chemistryof the

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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.


Chemistryof the

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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.


Chemistryof the

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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 2O


Chemistryof the

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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.


Chemistryof the

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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


Chemistryof the

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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.


Chemistryof the

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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).


Chemistryof the

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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


Chemistryof the

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Chlorofluorocarbons

In spite of the fact that the use of CFCs is now banned in over 100 countries, ozone depletion will continue for some time because of the tremendously unreactive nature of CFCs.


Chemistryof the

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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.


Chemistryof the

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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.© 2012 Pearson Education, Inc.

Chemistryof the

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Sulfur

• High acidity in rainfall causes corrosion in building materials.

• Marble and limestone (calcium carbonate) react with the acid; structures made from them erode.


Chemistryof the

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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.


Chemistryof the

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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.


Chemistryof the

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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.


Chemistryof the

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Water Vapor and Carbon Dioxide

Gases in the atmosphere form an insulating blanket that causes the Earth’s thermal consistency.


Chemistryof the

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• Two of the most important such gases are carbon dioxide and water vapor.


Chemistryof the

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• This blanketing effect is known as the “greenhouse effect.”

• Water vapor, with its high specific heat, is a major factor in this moderating effect.


Chemistryof the

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• But increasing levels of CO2 in the atmosphere may be causing an unnatural increase in atmospheric temperatures.


Chemistryof the

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Oceans

The world’s oceans contain 97.2% of all the water on earth, compared with 0.6% for freshwater sources.


Chemistryof the

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Oceans

• The vast ocean contains many important compounds and minerals.

• However, the ocean is only a commercial source of sodium chloride, bromine, and magnesium.


Chemistryof the

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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.


Chemistryof the

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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.


Chemistryof the

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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.


Chemistryof the

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Water Purification

• Water goes through several filtration steps.

• CaO and Al2(SO4)3 are added to aid in the removal of very small particles.


Chemistryof the

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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.


Chemistryof the

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Water Purification

In third-world countries, products such as the LifeStraw can provide safe drinking water.


Chemistryof the

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Water Purification• As one sucks water

through the straw, filters remove most of the sediment and bacteria.

• Iodine-impregnated beads then kill viruses and bacteria.

• Charcoal removes the iodine smell and most remaining parasites.


Chemistryof the

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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.


Chemistryof the

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Green Chemistry Principles

1. Prevention: It is better to prevent waste than to clean it up after it has been created.


Chemistryof the

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2. Atom Economy: Methods to make chemical compounds should be designed to maximize the incorporation of all starting atoms into the final product.


Chemistryof the

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3. Less Hazardous Chemical Syntheses: Wherever practical, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4. Design of Safer Chemicals: Chemical products should be designed to minimize toxicity and yet maintain their desired function.


Chemistryof the

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5. Safer Solvents and Auxiliaries: The use of auxiliary substances should be eliminated wherever possible and, if used, should be as nontoxic as possible.

6. Design for Energy Efficiency: Energy requirements should be recognized for their environmental and economic impacts and should be minimized (e.g., carry out reactions at room temperature and pressure).


Chemistryof the

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7. Use of Renewable Feedstocks: A raw material should be renewable whenever technically and economically practical.

8. Reduction of Derivatives: Unnecessary derivatization should be minimized or avoided if possible to save reagents and waste.

9. Catalysis: Catalytic reagents improve yields within a given time and with less energy and are, therefore, preferred.


Chemistryof the

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10. Design for Degradation: Chemical products should be designed so that at the end of their function they break down into innocuous products and do not persist in the environment.

11. Real-Time Analysis for Pollution Prevention: Analytical methods should be developed to allow for real-time monitoring and control prior to the formation of hazardous substances.


Chemistryof the

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12. Inherently Safer Chemistry for Accident Prevention: Reagents and solvents used in a chemical process should be chosen to minimize the potential for chemical accidents.


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Chapter 18 Chemistry of the Environment Lecture Presentation John D. Bookstaver St. Charles Community College Cottleville, MO © 2012 Pearson Education,