chapter 18 chemistry of the environment - …web.uvic.ca/~chem102/brolo/chapter...

Chapter 18 Chemistry of the Environment

• Earth’s Atmosphere

• Outer Regions of the Atmosphere

• Ozone in the Upper Atmosphere

• Chemistry of the Troposphere

• You will need to read the Course Pack to compliment the Textbook it is available at the UVic Bookstore

• Ozone hole over Antarctica Acid Rain

• Global Warming and The Greenhouse Effect

The temperature of the atmosphere varies in a complex manner as altitude increases

18.1 Earth’s Atmosphere

The atmosphere is divided into 4 regions

The pressure of the atmosphere decreases in a regular manner as altitude increases

The atmospheric pressure declines more rapidly at lower elevations - Due to the ‘compressibility’ of the atmosphere

The sun bombards us with radiation and energetic particles

The atmosphere is our first line of defense!

The aurora borealis (Northern Lights) is caused by collisions of high speed electrons and protons from the Sun with air molecules

The charged particles are channeled towards the polar regions by Earth’s magnetic field

The Sun emits radiant energy over a wide range of wavelengths

18.2 Outer Regions of the Atmosphere

As the radiation and high energy particles pass through the outer atmosphere it causes two types of chemical change:

Photons of sufficient energy are absorbed by a molecule causing Homolyticsplitting

Photodissociation

99% of the oxygen molecules in the upper atmosphere are dissociated

Very few Nitrogen molecules are dissociated

Nitrogen has a very high bond enthalpy and does not readily absorb photons

Molecules absorb radiation (high energy photons) causing a valence electron to be lost, (ejected)

Photoionization

Ozone absorbs much of the radiation in the 240 -310nm wavelength range (UV)

18.3 Ozone in the Upper Atmosphere

The rate at which ozone forms depends on the factors that vary in opposite directions as the altitude increases

http://ozonewatch.gsfc.nasa.gov/http://www.atm.ch.cam.ac.uk/tour/index.html

The cycle of ozone formation and dissociation is responsible for the temperature peak observed at the stratosphere boundary

Ozone cycle

•Used as coolant gases and propellants

•Un-reactive and insoluble so they are able to survive extended periods in the atmosphere

Depletion of the Ozone Layer

It has been known for 25 years that the ozone layer can be depleted due to reaction with chlorine from Chlorofluorocarbons (CFCs)

The C-Cl bond is easily broken as the molecule diffuses to higher altitudes and absorbs photons with wavelengths between 190 and 225nm

This reaction generating Cl(g) occurs at the greatest rate in the region of highest ozone concentration.

The rate at which ozone is destroyed increases with increasing chlorine concentration

The dark area represents the area of lowest O3 concentration - The ‘Hole’centered over Antarctica

The Chlorine monoxide generated is photo-dissociated back to Cl(g) and O(g), this generates more Cl(g) that can react with more ozone!

Overall these 3 equations can be combined (eliminate the like species) to give the following reaction:

In this catalytic mechanism, 1Cl destroys 50 O3 on a spring day over Antarctica * Course Pack!

http://www.ldeo.columbia.edu/edu/dees/V1003/lectures/ozone/index.html

Because of the great variation in atmospheric conditions, the composition of gases in the atmosphere is not uniform.

Composition of the Atmosphere

Parts per million (ppm) are commonly used when referring to the trace constituents

For substances in aqueous solution 1ppm is equivalent to 1 mg in 1 kg

Parts per million

PV = n RT so Volume is proportional to n

So 1 mole in 1 million moles of gas = 1 ppm

99% of the Troposphere is made up of N2 and O2, however minor constituent gases can have a profound effect on the Troposphere

18.4 Chemistry of the Troposphere

Many of the minor constituents are present naturally

However Human activity, primarily combustion, can increase the concentration of these constituents causing adverse effects.

Oxides of Sulfur and Nitrogen cause acidification of rain

Acid Rain

Sulfur Compounds

Sulfur Compounds occur naturally due to decay of organic matter and volcanic activity, however Human activity accounts for an approximate three fold increase in Sulfur compounds, primarily sulfur dioxide (SO2)

The high temperatures associated with these processes results in NO(g)

Nitrogen Compounds

Nitrogen Compounds, primarily Nitrogen monoxide (NO), also occur naturally, being formed due to lightening. However NO(g) can also be formed during hydrocarbon combustion (transportation), and in Industrial processes involving fossil fuel combustion.

Rainwater is naturally slightly acidic, due to the formation of carbonic acid (H2CO3) from carbon dioxide and water

The presence of sulfuric and nitric acid in rainwater can have drastic effects on the pH of the rainwater

http://people.eku.edu/ritchisong/317notes8.html

Effects of Acid Rain

Acid Rain causes the acidification of freshwater

The pH of freshwater sites around the US

Don’t think the West coast is OK: Rain fall and fog near LA has been measured to have a pH less than 2 !

Acid Rains attacks metals and stone (calcium carbonate)

This stone statue shows evidence of acid rain damage

Acid rain also damages forests

acid rain causes forest soils to lose valuable nutrients such as calcium and magnesium. It also increases the concentration of aluminum in the soil, which interferes with the uptake of nutrients by the trees

Removing SulfurPowdered limestone (CaCO3) can be used to remove SO2 from the gases formed by combustion of coal and oil

Carbon Monoxide

CO is relatively unreactive and has little effect on vegetation and materials.

Carbon monoxide (CO) is formed by the incomplete combustion carbon compounds and is present in unpolluted air at levels of 0.5ppm.

Photochemical Smog

What we recognize as smog, the brownish haze that hangs above major urban areas is largely NO2, nitrogen dioxide

NO2(g) is formed by oxidation of the NO(g) produced by the reaction of nitrogen and oxygen at the very high temperatures in combustion engines.

Nitrogen oxides and ozone are just some of the components of photochemical smog

Catalytic Convertors use precious metals Rh and Pt to catalyze the conversion of NO and CO to CO2 and N2

http://auto.howstuffworks.com/catalytic-converter2.htm

Global warming and the Greenhouse effect

The greenhouse effect describes the trapping of heat (infra-red (IR) radiation) near the Earth’s surface by gases, primarily carbon dioxide and water vapor. The effect gives the earth thermal consistency.

IR is only absorbed by molecules with polar bonds

All molecules vibrate with a specific energy, the vibrations of molecules possessing polar bonds causes the dipole moment to change during the course of vibration

The excited molecules lose the excess energy through collisions with other molecules

Although the level of water vapor in our atmosphere has not changed appreciably the levels of other greenhouse gases, especially CO2 have increased since the 1900’s.

Greenhouse gases include all molecules with polar bonds, CH4, NOx, CFCs etc

Carbon Dioxide

Carbon dioxide is naturally present in the atmosphere, the carbon cycle, describes the mechanism of how carbon is transferred to and from the atmosphere.

Increase in Greenhouse gas emissions disrupt the earth’s delicate thermal balance and causes “Global Warming”

http://cdiac.ornl.gov/trends/co2/sio-mlo.html

Global Warming

Increases in greenhouse gases have caused the earths surface temperature to increase abnormally over the past 150 years

This graph (from Wiki) plots the temperature change during this time period

The small increase may seem insignificant but has had very noticeable effect on global weather patterns with related adverse consequences.

9 out of the 10 hottest years recorded occurred during 1994 to 2004

Shorter WintersOver the last 3 decades winters have become 11 days shorter (on average) and warmer, there are fewer “frost” days

Warmer BC winters are failing to control the population of the mountain pine beetleMosquito borne diseases such as malaria, dengue fever and West Nile virus are becoming prevalent in areas where they were previously not viable

Signs of global warming

More extreme weather

Total annual rainfall increased in the last century, however, typically dry areas north and south of the equator became even drier, while cooler climates became wetter, hurricanes and storms have become more frequent

Size

Melting Ice cover

Antarctica OK? Contains 90% of the worlds ice.

Glaciers, Arctic and Greenland all in danger, changing fragile eco-systems

Mt. Baker

http://earthtrends.wri.org/

Warmer oceans and rising sea levels

A warmer ocean is the dominant factor in rising sea levels. Water is most dense at 4oC. Sea levels are predicted to rise between 15 and 95cm over the next 100 years.

The Pacific island of Tuvalu is already falling victim to rising sea levels

Warmer oceans are also killing coral and threatening sea-life

A quarter of the worlds coral has disappeared and another 30% is expected to be gone in 30 years

Warmer water contains less dissolved oxygen stressing fish populations and adversely effecting marine life