Chapter 18 Homework Earth And Its Surroundings Therefore Radiation Emitted

261

Chapter 18. Chemistry of the Environment

Media Resources

Figures and Tables in Transparency Pack: Section:

Figure 18.1 Temperature and Pressure in the 18.1 Earth’s Atmosphere

Atmosphere Vary as a Function of Altitude

Above Sea Level

Figure 18.6 Ozone Present in the Southern 18.2 Human Activities and Earth’s Atmosphere

Animations: Section:

Stratospheric Ozone 18.1 Earth’s Atmosphere

CFCs and Stratospheric Ozone 18.2 Human Activities and Earth’s Atmosphere

Catalytic Destruction of Stratospheric Ozone 18.2 Human Activities and Earth’s Atmosphere

Movies: Section:

Carbon Dioxide Behaves as an Acid in Water 18.2 Human Activities and Earth’s Atmosphere

3-D Models: Section:

Nitrogen 18.1 Earth’s Atmosphere

Other Resources

Further Readings: Section:

Introducing Atmospheric Reactions: A Systematic 18.1 Earth’s Atmosphere

Approach for Students

Chapter 18

262

Chemists Celebrate Earth Day 2009: Air—The 18.1 Earth’s Atmosphere

Sky’s the Limit. JCE Resources for Chemistry

and the Atmosphere: An Update

Consumption and CO2 Emissions to Illustrate

Chemical Principles

The Expiration of Respiration: Oxygen–The 18.4 Human Activities and Earth’s Water

Missing Ingredient in Many Bodies of Water

A Discovery-Based Experiment Illustrating How 18.4 Human Activities and Earth’s Water

Iron Metal Is Used to Remediate Contaminated

Groundwater

Live Demonstrations: Section:

Acid-Neutralizing Capacity of Lake Beds 18.4 Human Activities and Earth’s Water

Going Green: Lecture Assignments and Lab 18.5 Green Chemistry

Experiences for the College Curriculum

Chemistry of the Environment

263

Chapter 18. Chemistry of the Environment

Common Student Misconceptions

• Students often confuse the toxicity of ozone in the troposphere and the beneficial effects of ozone in

Teaching Tips

• This chapter gives an excellent opportunity to review Dalton’s law of partial pressure, Planck’s law

calculations, ionization energy, equilibrium, reaction mechanisms, and some acid-base chemistry.

Lecture Outline

18.1 Earth’s Atmosphere

1

,

2

• The temperature of the atmosphere varies with altitude.

• The atmosphere is divided into four regions based on the temperature profile.

• The troposphere (below an altitude of 12 km)

• The temperature decreases from 290 K to 215 K as altitude increases.

• This region is where we spend most of our time.

Composition of the Atmosphere

3

,

4

,

5

,

6

,

7

,

8

,

9

,

10

• The composition of the atmosphere is not uniform.

1

“Introducing Atmospheric Reactions: A Systematic Approach for Students” from Further Readings

2

Figure 18.1 from Transparency Pack

3

“Thermal Physics (and Some Chemistry) of the Atmosphere” from Further Readings

4

Chapter 18

264

• Temperature and pressure vary over a wide range with altitude.

• Gases in the atmosphere are bombarded by radiation and energetic particles from the sun.

• Gravity also plays a role.

• Lighter molecules and atoms are found at higher altitudes.

Photochemical Reactions in the Atmosphere

• Recall: E = h = hc/

• Thus, 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.

• Photoionization is the ionization of molecules (and atoms) caused by radiation.

• The molecule absorbs energy, causing the loss of an electron.

• Thus, the photon must have sufficient energy to remove an electron when it is absorbed by a

molecule.

• Wavelengths of light that cause photoionization and photodissociation are absorbed by the upper

atmosphere.

• This filters them out and prevents them from reaching the Earth.

Ozone in the Upper Atmosphere

11

• Ozone absorbs photons with wavelengths between 240 and 310 nm.

Chemistry of the Environment

265

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

O(g) + O2(g) → O3*(g)

• The excited ozone (O3*) can lose energy by decomposing to oxygen atoms and oxygen molecules

(the reverse reaction) or by transferring the energy to M (usually N2 or O2):

O(g) + O2(g) → O3*(g)

O3*(g) + M(g) → O3(g) + M*(g)

• Why does maximum ozone formation occur in the stratosphere?

18.2 Human Activities and Earth’s Atmosphere

The Ozone Layer and Its Depletion

12

,

13

,

14

,

15

,

16

,

17

,

18

,

19

• 1970: Cratzen demonstrated that naturally occurring nitrogen oxides can catalytically degrade ozone.

• 1974: Rowland and Molina demonstrated that chlorine atoms from chlorofluorocarbons (CFCs)

deplete the ozone layer.

• Rowland and Molina were awarded the Nobel prize in 1995.

• Hydrofluorocarbons are the main alternatives to hydrofluorocarbons.

12

“Understanding Ozone” from Further Readings

13

Figure 18.6 from Transparency Pack

Chapter 18

266

Sulfur Compounds and Acid Rain

20

,

21

,

22

,

23

,

24

• Sulfur dioxide, SO2, is produced by natural events (volcanic gases, bacterial action, forest fires).

• The major source is linked to human activities such as the combustion of sulfur-containing fuels.

• Combustion of coal accounts for approximately 65% of the SO2 released in the United States.

• The amount of SO2 produced depends on the sulfur content of the coal or oil, which varies with

the source of the coal or oil.

• The SO2 can be oxidized to SO3, which dissolves in water to produce sulfuric acid (a component

of acid rain): SO3(g) + H2O(l) → H2SO4(aq)

• The acids in acid rain are problematic.

• They react with metals and promote corrosion.

• They react with carbonates (such as the calcium carbonate in marble and limestone).

• How can we reduce the amount of SO2 produced from fuel combustion?

• It is too expensive to remove sulfur from oil and coal prior to its use.

Nitrogen Oxides and Photochemical Smog

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

• Oxides of nitrogen are the primary components of smog.

• In car engines, NO forms as follows:

2NO(g) + O2(g) → 2NO2(g) ∆H = –113.1 kJ

• In air NO is rapidly oxidized: NO2(g) + h → NO(g) + O(g)

20

Figure 18.9 from Transparency Pack

21

“Acid Rain Effects on Stone Monuments” from Further Readings

Chemistry of the Environment

267

Greenhouse Gases: Water Vapor, Carbon Dioxide, and Climate

25

,

26

,

27

,

28

,

29

,

30

• There is a thermal balance between the Earth and its surroundings.

• Therefore, 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.

fossil fuels in the present manner.)

FORWARD REFERENCES

• Other methods of energy production, such as direct methanol fuel cells, will be discussed in

Chapter 20 (section 20.7).

18.3 The Earth’s Water

• 72% of the Earth’s surface is covered with water.

• Water plays an important role in our environment.

• The properties of water are important.

The Global Water Cycle

• All water on Earth is connected in a global water cycle.

25

Figure 18.11 from Transparency Pack

26

Figure 18.12 from Transparency Pack

Chapter 18

268

• Water is warmed by the sun.

• Liquid water in the oceans evaporates into the atmosphere as water vapor.

• The water vapor condenses into water droplets.

• Water falls to the ground as water, snow, or rain.

Saltwater: Earth’s Oceans and Seas

31

• The volume of oceans in the world is 1.35  109 km3.

Freshwater and Groundwater

• Freshwater : natural waters that have low concentrations of dissolved salts and solids.

• Includes waters of lakes, rivers, ponds, and streams.

• An adult needs about 2 L of water a day for drinking.

• In the United States, the average person uses about 300 L of freshwater per day.

• Industry uses even more freshwater than this (e.g., about 105 L of water is used to make 1000 kg of

steel).

18.4 Human Activities and Earth’s Water

Dissolved Oxygen and Water Quality

32

,

33

• Water fully saturated with air at 1 atm and 20°C 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.

Chemistry of the Environment

269

Water Purification: Desalination

• Seawater has a salt concentration too high for drinking.

• Water used for drinking should contain less than 500 ppm dissolved salts (United States

municipal water).

• Desalination is the removal of salts from seawater or brackish water.

• Common methods for isolation of drinking water include distillation (small scale) and reverse

Water Purification: Municipal Treatment

34

,

35

• There are five steps:

• Coarse filtration

• This occurs as water is taken from a lake, river, or reservoir and passed through a screen.

• Sedimentation

• Water is allowed to stand so that solid particles (e.g., sand) can settle out.

• To remove small components (like bacteria), CaO and Al2(SO4)3 are added.

Chapter 18

270

Water Softening

36

• Water containing a high concentration of Ca2+ and Mg2+ and other divalent cations is called hard

water.

• The presence of these ions may cause the water to be unsuitable for some uses.

• For example, soaps form an insoluble soap “scum” and water in water heaters forms a deposit

FORWARD REFERENCES

• The role of water in redox reactions will be discussed throughout Chapter 20.

• Teflon will be mentioned in Chapter 22 (section 22.4).

• Lead chemistry, lead poisoning, and chelating agents will be mentioned in Chapter 23.

18.5 Green Chemistry

37

,

38

,

39

,

40

,

41

,

42

• 12 basic principles of green chemistry: an initiative that promotes the design and application of

chemical products and processes that are compatible with human health and that preserve the

environment:

• 1. Prevention

• 2. Atom Economy

• 3. Less Hazardous Chemical Syntheses

• 4. Designing Safer Chemicals

• 5. Safer Solvents and Auxiliaries

Demonstrations

Chemistry of the Environment

271

Supercritical Solvents

• The release of toxic volatile solvents to the atmosphere must be avoided.

• Alternative environmentally friendly methods need to be developed.

Greener Reagents and Processes

• Earth-friendly alternatives are being developed for many processes important to modern society.

• Where possible, synthetic routes are adjusted to result in a high level of ‘atom economy”:

• A high percentage of the atoms from the starting materials end up in the product.

Chapter 18

272