Chemistryof the

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Chapter 18Chemistry of the

Environment

Chemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten

John D. BookstaverSt. Charles Community CollegeSt. Peters, MO2006, Prentice Hall, Inc.Modified by S.A. Green, 2006

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Outline

• Atmosphere General• physical structure• chemical compositionOuter Atmosphere • ozone - photochem, Troposphere • sulfur, acid rain• CO• NOx, smog• CO2, H2O • Climate

• Water Oceans• composition, desalinationFreshwater • oxygen, water treatment

• “Green Chemistry”• principles• examples

• Ni mining/sulfide minerals • acid mine drainage

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Atmosphere

• Temperature varies greatly with altitude.

• The profile makes a Z-shape from mesosphere to the ground.

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Atmosphere

Pressure is highest at the surface and decreases with height.

Fluctuations in pressure are a driving force of weather.

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Radiation

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

Aurora Formedhere {

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Composition of the Atmosphere

• The composition of gases in the atmosphere is not uniform.

• Lighter gases tend to rise to the top.

Gases are measured in ppm volume, which is directly proportional to mole fraction.

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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.

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Outer Atmosphere

• The Sun emits radiation across the electromagnetic spectrum.

• Light in the ultraviolet region has enough energy to break chemical bonds.

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Wavelength, m Energy

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• Oxygen in the upper atmosphere absorbs much of the solar radiation before it reaches the lower atmosphere:

O2 + h 2 O

• These bonds break homolytically.

Photochemistry =1. Photodisociation2. Photoionization

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SAMPLE EXERCISE 18.2 Calculating the Wavelength Required to Break a Bond

What is the maximum wavelength of light, in nanometers, that has enough energy per photon to dissociate the O2 molecule which has a dissociation energy of 495 kJ/mol?

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SAMPLE EXERCISE 18.2 Calculating the Wavelength Required to Break a Bond

What is the maximum wavelength of light, in nanometers, that has enough energy per photon to dissociate the O2 molecule which has a dissociation energy of 495 kJ/mol?

Solution   Analyze: We are asked to determine the wavelength of a photon that has just sufficient energy to break the double bond in O2.

Plan: We first need to calculate the energy required to break the double bond in one molecule, then find the wavelength of a photon of this energy.

Chemistryof the

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SAMPLE EXERCISE 18.2 Calculating the Wavelength Required to Break a Bond

What is the maximum wavelength of light, in nanometers, that has enough energy per photon to dissociate the O2 molecule which has a dissociation energy of 495 kJ/mol?

Solution   Analyze: We are asked to determine the wavelength of a photon that has just sufficient energy to break the double bond in O2.

Plan: We first need to calculate the energy required to break the double bond in one molecule, then find the wavelength of a photon of this energy.

Solve: The dissociation energy of O2 is 495 kJ/mol. Using this value and Avogadro’s number, we can calculate the amount of energy needed to break the bond in a single O2 molecule:

We next use the Planck relationship, E = h, to calculate the frequency, , of a photon that has this amount of energy:

Finally, we use the relationship between the frequency and wavelength of light (Section 6.1) to calculate the wavelength of the light:

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• Short wavelength radiation (ionizing 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.

Photochemistry =1. Photodisociation2. Photoionization

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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 (< 242 nm).

O + O2 O3

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Ozone Depletion

In 1974 Rowland and Molina (Nobel Prize, 1995) discovered that chlorine from chlorofluorocarbons (CFCs) may be depleting the supply of ozone in the upper atmosphere.

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Chlorofluorocarbons

CFCs were used for years as aerosol propellants and refrigerants.

Mostly = CFCl3, CF2Cl2.

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 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

Movie…

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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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TroposphereAlthough 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.

This process = “scrubbing”

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Carbon Monoxide• Carbon monoxide

binds preferentially to the iron in red blood cells.

• Exposure to CO can lower O2 levels to the point of causing loss of consciousness and death.

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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 SmogSmog also contains

ozone, carbon monoxide, hydrocarbons, and particles.

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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.

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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 is causing an increase in global temperatures.

Chemistryof the

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Mount Pinatubo

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Oceans

• The vast ocean contains many important compounds and minerals.

• However, the ocean is a commercial source only 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.

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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

4. Catalysts that allow the use of safe chemicals should be employed when possible.

5. Try to use renewable feedstocks as raw materials.

6. 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.

A superior alternative might be dimethyl carbonate.

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Reagents

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Metathesis reaction

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Ni mining/sulfide minerals acid mine drainage

Sulfide minerals = FeS2, ZnS, CuS, (Ni, Fe)9S8

Sulfuric acid

Acid dissolves additional minerals, releasing metals into the watershed.

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Description: Iron hydroxide precipitate (orange) in a Missouri stream receiving acid drainage from surface coal mining.Source: Environmental Contaminants; Status and Trends of the Nations Biological Resources (Retrieved May 5, 2005)Credit: D. Hardesty, USGS Columbia Environmental Research Center.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Downstream reactions: