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Chemistry

of the

Environment

Chapter 18

Chemistry of theEnvironment

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.

Modified by S.A. Green, 2006

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Chemistry

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Outline

AtmosphereGeneral

physical structure

chemical composition

Outer Atmosphere ozone - photochem,

Troposphere

sulfur, acid rain

CO

NOx, smog CO2, H2O

Climate

WaterOceans

composition, desalination

Freshwater

oxygen, water treatment

Green Chemistry principles

examples

Ni mining/sulfide minerals

acid mine drainage

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Chemistry

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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 withheight.

Fluctuations in

pressure are a

driving force ofweather.

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Chemistry

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Radiation

The atmosphere is

the first line of

defense againstradiation from the

Sun.

AuroraFormed

here

{

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Chemistry

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Composition of the Atmosphere

The composition of

gases in the

atmosphere is notuniform.

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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Chemistry

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Composition of the Atmosphere

Near the Earths

surface, about 99% of

the atmosphere iscomposed of nitrogen

and oxygen.

Oxygen has a much

lower bond enthalpythan nitrogen, and is

therefore more reactive.

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Chemistry

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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.

Numbe

rof

photons

Wavelength, m

Energy

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Chemistry

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Oxygen in the upper atmosphereabsorbs much of the solar radiation

before it reaches the lower atmosphere:

O2 + h

2 O These bonds break homolytically.

Photochemistry =1. Photodisociation

2. Photoionization

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Chemistry

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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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Chemistry

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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?

SolutionAnalyze: We are asked to determine the wavelength of a photon that has just sufficient energyto 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.

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Chemistry

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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?

SolutionAnalyze: We are asked to determine the wavelength of a photon that has just sufficient energyto 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 O2is 495 kJ/mol. Using this value and Avogadros 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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Chemistry

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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 Earths surface.

The presence of these ions makes long-range

radio communication possible.

Photochemistry =1. Photodisociation

2. Photoionization

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Chemistry

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Ozone

Ozone absorbs much of the radiationbetween 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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Chemistry

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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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Chemistry

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Chlorofluorocarbons

CFCs were used for years as aerosolpropellants and refrigerants.

Mostly = CFCl3, CF2Cl2.

They are not water soluble (so they do notget washed out of the atmosphere by

rain)and are quite unreactive (so they are not

degraded naturally).

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Chemistry

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Chlorofluorocarbons

The CCl bond is easily broken,

though, when the molecule absorbs

radiation with a wavelength between190 and 225 nm.

The chlorine atoms formed react with

ozone:Cl + O3 ClO + O2

Movie

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Chemistry

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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 sometime because of the tremendously

unreactive nature of CFCs.

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Chemistry

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Troposphere

Although the troposphere is made up almostentirely of nitrogen and oxygen, other gases

present in relatively small amounts still have

a profound effect on the troposphere.

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Chemistry

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Sulfur

Sulfur dioxide is a by-product of the burningof coal or oil.

It reacts with moisturein the air to form

sulfuric acid. It is primarily

responsible for acidrain.

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Chemistry

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Sulfur

High acidity in rainfallcauses corrosion in building

materials.

Marble and limestone

(calcium carbonate) reactwith the acid; structures

made from them erode.

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Chemistry

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Sulfur

SO2 can be

removed by

injecting powdered

limestone which isconverted to

calcium oxide.

The CaO reacts

with SO2 to form aprecipitate of

calcium sulfite.

This process = scrubbing

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Chemistry

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Carbon Monoxide

Carbon monoxidebinds preferentially to

the iron in red blood

cells.

Exposure to CO can

lower O2 levels to the

point of causing loss

of consciousness anddeath.

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Chemistry

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Carbon Monoxide

Products that can

produce carbon

monoxide must containwarning labels.

Carbon monoxide is

colorless and odorless,

so detectors are a goodidea.

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Chemistry

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Nitrogen Oxides

What we recognize assmog, that brownish

gas that hangs above

large cities like Los

Angeles, is primarilynitrogen dioxide, NO2.

It forms from the

oxidation of nitric oxide,NO, a component of

car exhaust.

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Chemistry

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Photochemical Smog

Smog also containsozone, carbon

monoxide,

hydrocarbons, and

particles.

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Chemistry

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Water Vapor and Carbon Dioxide

Gases in the atmosphere form aninsulating blanket that causes the

Earths thermal consistency.

Two of the most important such

gases are carbon dioxide andwater vapor.

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Chemistry

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Water Vapor and Carbon Dioxide

This blanketing effect isknown as the

greenhouse effect.

Water vapor, with its high

specific heat, is a major

factor in this moderating

effect.

But increasing levels ofCO2 in the atmosphere is

causing an increase in

global temperatures.

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Chemistry

of the

EnvironmentMeelh, 2005.

Mount Pinatubo

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Chemistry

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Oceans

The vast ocean

contains many

important compoundsand minerals.

However, the ocean is

a commercial source

only of sodium chloride,bromine, and

magnesium.

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Chemistry

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Desalination

Water, water

everywhere, and not a

drop to drink. Seawaterhas too high a

concentration of NaCl for

human consumption.

It can be desalinatedthrough reverse osmosis.

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Chemistry

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Reverse Osmosis

Water naturally flows through asemipermeable 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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Chemistry

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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 municipalwater supply.

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Water Purification

Water goesthrough several

filtration steps.

CaO and

Al2(SO4)3 are

added to aid in the

removal of very

small particles.

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Chemistry

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Water Purification

The water is aeratedto 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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Chemistry

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Green Chemistry

We have become increasingly aware overthe past 30 to 40 years that modern

processes are not always compatible withmaintaining a sustainable environment.

Promoting chemical processes that areenvironmentally friendly is part of thegood stewardship.

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Chemistry

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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 solventused, and try to use environmentally

friendly solvents.

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Chemistry

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Solvents

Solvents such as supercritical water and CO2are great green alternatives.

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Chemistry

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Reagents Phosgene, COCl

2, is

commonly used as astarting material for plasticpolymers.

Phosgene is a highly toxic

substance, and the by-products of many of itsreactions are undesirable.

A superior

alternative might be

dimethyl carbonate.

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Chemistry

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Reagents

QuickTime and a

TIFF (Uncompressed) decompressorare needed to see this picture.

Metathesis reaction

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Chemistry

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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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Chemistry

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Description: Iron hydroxide precipitate (orange) in a Missouri streamreceiving acid drainage from surface coal mining.Source: Environmental Contaminants; Status and Trends of the Nations Biological Resources(Retrieved May 5, 2005)

QuickTime and a

TIFF (Uncompressed) decompressorare needed to see this picture.

Downstream reactions: