prentice hall © 2003chapter 18 chapter 18 chemistry of the environment chemistry the central...

Prentice Hall © 2003 Chapter 18

Chapter 18Chapter 18Chemistry of the EnvironmentChemistry of the Environment

CHEMISTRY The Central Science

9th Edition

David P. White


• The temperature of the atmosphere is a complicated function of altitude.

Earth’s AtmosphereEarth’s Atmosphere


• Below an altitude of 10 km (troposphere) the temperature decreases from 290 K to 215 K as altitude increases.

• In the stratosphere (10 km - 50 km) the temperature increases from 215 K to 275 K.

• In the mesosphere (50 km - 85 km) the temperature decreases (275 K to 190 K) and in the thermosphere (> 85 km) the temperature increases.

• The variation of pressure with altitude is simpler: pressure decreases as altitude increases.



• At 80 km the pressure is about 0.01 torr.• The boundaries between regions are given the suffix -pause.

• There is slow mixing of gases between regions in the atmosphere.

Composition of the Atmosphere• Earth’s atmosphere is affected by temperature and

pressure as well as gravity.



Composition of the Atmosphere• Lighter molecules and atoms are found at higher

altitudes.• Two major components of the atmosphere are nitrogen,

N2, and oxygen, O2.



Photodissociation• Recall

– so the higher the frequency, the shorter the wavelength and the higher the energy of radiation.

• For a chemical reaction induced by radiation to occur, the photons must have sufficient energy to break the required bonds and the molecules must absorb the photons.

• Photodissociation is the rupture of a chemical bond induced by radiation.

Outer Regions of the Outer Regions of the AtmosphereAtmosphere

hc

hE


Photodissociation• In the upper atmosphere, photodissociation causes the

formation of oxygen atoms:

O2(g) + h 2O(g)

Photoionization• Photoionization is the ionization of molecules (and

atoms) caused by radiation.



Photoionization• 1924: electrons were discovered in the upper atmosphere.

Therefore, cations must be present in the upper atmosphere.

• Photoionization occurs when a molecule absorbs a photon of sufficient energy to remove an electron.

• Wavelengths of light that cause photoionization and photodissociation are filtered by the atmosphere.



• Ozone absorbs photons with a wavelength between 240 and 310 nm.

• Most of the ozone is present in the stratosphere (maximum ozone concentration at an altitude of 20 km).

• Between 30 and 90 km photodissociation of oxygen is possible:

O2(g) + h 2O(g)

Ozone and the Upper Ozone and the Upper AtmosphereAtmosphere


• The oxygen atoms can collide with oxygen molecules to form ozone with excess energy, O3*:

O(g) + O2(g) O3*(g)

• The excited ozone can loose energy by decomposing to oxygen atoms and oxygen molecules (the reverse reaction) or by transferring the energy to M (usually N2 or O2):

O3*(g) + M(g) O3(g) + M*(g)


O(g) + O2(g) O3*(g)


• The formation of ozone in the atmosphere depends on the presence of O(g).

• At low altitudes, the radiation with sufficient energy to form O(g) has been absorbed.

• Release of energy from O3* depends on collisions, which generally occur at lower altitudes.

• Combining effects means maximum ozone formation in the stratosphere.



Depletion of the Ozone Layer• In October 1994 the map of ozone present in the

Southern hemisphere showed a hole over Antarctica.• In 1995 the Nobel Prize for chemistry was awarded to F.

Sherwood Rowland, Mario Molina, and Paul Crutzen for their studies of ozone depletion.



Depletion of the Ozone Layer

• In 1974 Rowland and Molina showed that chlorine from chlorofluorocarbons (CFCs) deplete the ozone layer by catalyzing the formation of ClO and O2.



Depletion of the Ozone Layer• In the stratosphere, CFCs undergo photochemical rupture of a C-Cl

bond:

CF2Cl2(g) + h CF2Cl(g) + Cl(g) (optimal at 30 km).

– Subsequently

Cl(g) + O3(g) ClO(g) + O2(g)

rate = k[Cl][O3], k = 7.2 109 M-1s-1 at 298 K.

– In addition, the ClO generated produces Cl as well:

2ClO(g) O2(g) + 2Cl(g)

– giving rise to the overall reaction

2O3(g) 3O2(g).



• The troposphere consists mostly of O2 and N2.

• Even though other gases are present in low concentrations, their effects on the environment can be profound.

Sulfur Compounds and Acid Rain

• Sulfur dioxide, SO2, is produced by the combustion of oil and coal (and some other means).

Chemistry of the Chemistry of the TroposphereTroposphere



• The SO2 is oxidized to SO3 which reacts with water to produce sulfuric acid (acid rain):

SO3(g) + H2O(l) H2SO4(aq)

• More than 30 million tons per year of SO2 are released into the atmosphere in the United States.

• Nitrogen oxides also contribute to acid rain by forming nitric acid.

• Normal rainwater has a pH of about 5 (due to the H2CO3 produced from CO2).



Sulfur Compounds and Acid Rain• Acid rain has a pH around 4, whereas the pH of natural

waters containing living organisms is 6.5 to 8.5.• Natural waters with a pH below 4 cannot sustain life.• It is too expensive to remove sulfur from oil and coal

prior to its use. Therefore, the SO2 is removed from fuel upon combustion.




• SO2 is commonly removed from fuel (oil and coal) as follows:– powdered limestone decomposes into CaO;

– CaO reacts with SO2 to form CaSO3 in a furnace;

– CaSO3 and unreacted SO2 are passed into a scrubber (purification chamber) where the SO2 is converted to CaSO3 by jets of CaO.

– CaSO3 is precipitated into a watery slurry.



Carbon Monoxide• CO is produced by incomplete combustion of fuels.• About 1014 g of CO is produced in the United States per

year (mostly from automobiles).• CO binds irreversibly to the Fe in hemoglobin. (CO binds

to hemoglobin about 210 times more strongly than oxygen.)



Carbon Monoxide• Hemoglobin is responsible for oxygen transport:

– in the lungs, CO2 is released from the Fe in the hemoglobin and O2 binds to the Fe (forming oxyhemoglobin),

– in the tissues, O2 is released and CO2 binds to the iron.

– When CO binds to Fe in hemoglobin, carboxyhemoglobin, COHb, it cannot be displaced by O2 or CO2.

– Therefore, in sufficient concentrations CO can stop oxygen transport in living systems.



Nitrogen Oxides and Photochemical Smog

• Photochemical smog is the result of photochemical reactions on pollutants.

• In car engines, NO forms as follows:

• In air

• At 393 nm (wavelength of sunlight), NO2 decomposes

NO2(g) + h NO(g) + O(g)


N2(g) + O2(g) 2NO(g) H = 180.8 kJ

2NO(g) + O2(g) 2NO2(g) H = -113.1 kJ


Nitrogen Oxides and Photochemical Smog

• The O produced by photodissociation of NO2 can react with O2 to form O3, which is the key component of smog

O(g) + O2(g) + M(g) O3(g) + M*(g).

• In the troposphere ozone is undesirable because O3 is toxic and reactive.

Water Vapor, Carbon Dioxide, and Climate• There is a thermal balance between the earth and its

surroundings.



Water Vapor, Carbon Dioxide, and Climate

• Radiation is emitted from the earth at the same rate as it is absorbed by the earth.

• The troposphere is transparent to visible light.• However, the troposphere is not transparent to IR

radiation (heat).• Therefore, the troposphere insulates the earth making it

appear colder from the outside than it is on the surface.




• CO2 and H2O absorb IR radiation escaping from the earth’s surface.

• At night, the earth emits radiation. Water vapor is responsible for keeping IR on the planet.

• The carbon dioxide level on earth has been increasing over the years.



Chemistry of the TroposphereChemistry of the Troposphere



• A lot of the increase is due to the combustion of fuels.

• We speculate that the increased CO2 concentration is resulting in a gradual warming of the earth’s surface.

• Between 2050 and 2100 the CO2 concentration is expected to double. This will result in a global temperature increase of 1 to 3 C.



Seawater• 72 % of the earth’s surface is covered with water.• The volume of oceans in the world is 1.35 109 km3.• 97.2 % of earth’s water is in oceans.• Salinity: mass of dry salts in 1 kg of seawater. Seawater

averages a salinity about 35.• Most elements in seawater are only present in small

quantities.• Commercially, NaCl, Br- and Mg2+ are obtained from

seawater.

The World OceanThe World Ocean


Desalination• Water used for drinking should contain less than 500

ppm dissolved salts (United States municipal water).• Desalination: removal of salts from seawater.• Common method: reverse osmosis (energy intensive).

– Osmosis: transport across a semipermeable membrane.

– Osmosis: movement of solvent molecules. No good for desalination.

– Reverse osmosis: under applied pressure, solvent moves from more concentrated solution to more dilute solution.



Desalination– In a reverse osmosis desalination plant, Each cylinder is called

a permeator.

– Seawater is introduced under pressure and water passes through the fiber walls, into the fibers and is separated from the ions.



• An adult needs about 2 L of water a day for drinking.• In the United States, the average person uses about 300 L

per day of freshwater.• Industry uses even more freshwater than this (e.g. about

105 L of water are used to make enough steel for one car.• As the water flows over the earth it dissolves many

substances.• Freshwater usually contains some ions (Na+, K+, Mg2+,

Ca2+, Fe2+, Cl-, SO42-, and HCO3

-) and dissolved gases (O2, N2, and CO2).

FreshwaterFreshwater


Dissolved Oxygen and Water Quality• Water fully saturated with air at 1 atm and 20C has 9

ppm of O2 dissolved in it.

• Cold water fish require about 5 ppm of dissolved oxygen for life.

• Aerobic bacteria consume oxygen to oxidize biodegradable organic material.

• Biodegradable materials are oxygen-demanding wastes. E.g., sewage, industrial waste from food-processing plants and paper mills, and effluent from meat packing plants.



Dissolved Oxygen and Water Quality

• Aerobic bacteria oxidize organic material into CO2, HCO3

-, H2), NO3-, SO4

2-, and phosphates.

• Once the oxygen level has been depleted, aerobic bacteria cannot survive.

• Anaerobic bacteria complete the decomposition process forming CH4, NH3, H2S, PH3, and other foul smelling products.

• Eutrophication is the increase in dead and decaying plant matter resulting from excessive plant growth.



Treatment of Municipal Water Supplies• There are five steps:

– Coarse filtration. Occurs as water is taken up from lake, river or reservoir.

– Sedimentation. Water is allowed to stand so that solid particles (e.g. sand) can settle out. To remove components like bacteria, CaO and Al2(SO4)3 are added. This causes a gelatinous precipitate of Al(OH)3, which settles slowly carrying small particles with it.



Treatment of Municipal Water Supplies• Sand Filtration. Water is filtered through a sand bed to remove

Al(OH)3 and anything it trapped.

– Aeration. Air oxidizes any organic material.

– Sterilization. Chlorine is usually used because it forms HClO(aq) in solution, which kills bacteria.



Treatment of Municipal Water Supplies



• Chemical industry has recognized that it is important to use environmentally friendly chemicals and processes.

• There are several goals for green chemistry:• Prevent waste rather than subsequently cleaning it.

• Synthesize new products minimizing waste.

• Design energy efficient processes.

• Use catalysts as much as possible.

• Raw materials should be renewable feedstocks.

• Eliminate solvents as far as possible.

Green ChemistryGreen Chemistry


Solvents and Reagents• Many organic molecules that are used as solvents are

volatile and can do environmental damage.• Also, many solvents are toxic.

• Liquid or supercritical CO2 is a non-toxic solvent that has many potential applications.

• Du Pont uses supercritical CO2 to make Teflon™ instead of the environmentally damaging chlrofluorocarbon solvents.



Solvents and Reagents• Supercritical water can be used to make the plastic and

polyester fiber PET.• Lexan™ polycarbonate, the coatings on CDs, could be

made from dimethyl carbonate instead of the very toxic phosgene, which is currently used.

Other Processes

• Liquid CO2 is currently used in the dry cleaning industry as an alternative to Cl2C=CCl2, which is toxic.



Other Processes• Yttrium is being used in place of lead for automobile

coatings.

Water Purification

• When Cl2 is used to treat water, some trihalomethanes (THM) are often produced, which can go undetected.

• The THMs are suspected carcinogens.

• Ozone and ClO2 could be used as alternatives, but they are not completely safe.

• Green water purification is an open problem.



End of Chapter 18:End of Chapter 18:Chemistry of the EnvironmentChemistry of the Environment

prentice hall © 2003chapter 18 chapter 18 chemistry of the environment chemistry the central...

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