Chapter 18 Chemistry of the Environment

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Chapter 18Chemistry of the Environment
Chemistry, The Central Science, 10th
edition Theodore L. Brown H. Eugene LeMay, Jr.
and Bruce E. Bursten
John D. Bookstaver St. Charles Community
College St. Peters, MO ? 2006, Prentice Hall, Inc.
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Atmosphere

• Temperature varies greatly with altitude.
• However, there is not a linear relationship
  between altitude and temperature.

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Atmosphere

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

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Radiation

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

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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.

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Composition of the Atmosphere

• Near the Earths 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.

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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.

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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? ??? 2 O

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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 Earths surface.
• The presence of these ions makes long-range radio
  communication possible.

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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

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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.

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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).

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Chlorofluorocarbons

• The CCl 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

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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.

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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.

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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.

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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.

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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.

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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.

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Carbon Monoxide

• Products that can produce carbon monoxide must
  contain warning labels.
• Carbon monoxide is colorless and odorless, so
  detectors are a good idea.

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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.

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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.

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Photochemical Smog

• As a result, government emission standards for
  automobile exhaust have become continually more
  stringent.

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Water Vapor and Carbon Dioxide

• Gases in the atmosphere form an insulating
  blanket that causes the Earths thermal
  consistency.
• Two of the most important such gases are carbon
  dioxide and water vapor.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Green Chemistry Principles

1. Rather than worry about waste disposal, it is
   better to avoid creating waste in the first
   place.
2. Try to generate as little waste as possible, and
   try to make waste that is nontoxic.
3. Be energy conscious in designing syntheses.

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Green Chemistry Principles

1. Catalysts that allow the use of safe chemicals
   should be employed when possible.
2. Try to use renewable feedstocks as raw materials.
3. Try to reduce the amount of solvent used, and try
   to use environmentally friendly solvents.

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Solvents

• Solvents such as supercritical water and CO2 are
  great green alternatives.

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Reagents

• Phosgene, COCl2, is commonly used as a starting
  material for plastic polymers.
• Phosgene is a highly toxic substance, and the
  by-products of many of its reactions are
  undesirable.

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Reagents

• A superior alternative might be dimethyl
  carbonate.