Protecting the Ozone Layer

Learning Objectives

• Describe the chemical nature of ozone, the ozone layer, and factors affecting its existence

• Describe the electromagnetic spectrum in terms of frequency, wavelength, and energy

• Use appropriate calculations to relate energy, wavelength, and frequency of light

• Understand how the ozone layer protects against harmful ultraviolet radiation

• Discuss the interaction of radiation with matter and changes caused by such interactions, such as biological sensitivity and the use of the UV index

But first, we need a little chemistry…

number of protons = atomic number

How many electrons and protons in these elements?

Iodine (I)

number of protons = atomic number

How many electrons and protons in these elements?

Iodine (I)

Silver (Ag)

= 53

number of protons = atomic number

How many electrons and protons in these elements?

Iodine (I)

Silver (Ag)

= 53

= 47

Valence Shells

• The first shell holds 2 valence electrons

• Most subsequent shells hold 8 valence electrons

For the “A” elements, Group number = number of outer

electrons

Electron Arrangements

*Number of outer electrons are critical. Atom is most stable when outer shell is filled.

What makes an atom happy?

• Ben and Jerry’s ice cream

(Well, maybe not ice cream… :)

• Would you believe…– having full “valence shells” of electrons

If they don’t have their own, they can share…

• So H is happy when it can share its one electron with another hydrogen

• they both “feel” like they have 2 electrons, which is a full (first shell)– While they are still also electrically neutral

• Therefore, H often goes around as H2

Single Bonds – Lewis Dot Structures

H H

H F

shared pair of electrons = covalent bond

:

: :..

..

Alternative notation

H H

H F

shared pair of electrons = covalent bond

-

-

Multiple Bonds

O O

N N

double bond = 4 shared electronstriple bond = 6 shared electrons

::

:::

.. ..

....

::

Alternative Notation

O O

N N

double bond = 4 shared electronstriple bond = 6 shared electrons

=

Occasionally a single Lewis structure does not adequately represent the structure of a molecule, so we use resonance forms.

Lewis Structure for Ozone

What’s the structure of water?

The 3-D shape of a molecule is determined by electrons….

VS.

Water

Valence Shell Electron Pair Repulsion Theory

The most stable molecular shape has the electron pairs surrounding a central atom as far away from one another as possible.

Water has two electron pairs that are bonded and two that are not bonded.

The electron pairs are tetrahedrally arranged, but the shape is described only in terms of the atoms present: So, water is said to be bent in shape.

Predicting molecule shapes

Now look at CO2

Two groups of four electrons each are associated with the central atom.

The two groups of electrons will be 180o from each other: theCO2 molecule is linear.

Group

4 common molecular shapes

• Bent– water (H2O)

• Linear– carbon dioxide (CO2)

• Pyramidal– ammonia (NH3)

• Tetrahedral– methane (CH4)

Now we need a little physics…

Waves of Light• Energy can be described as waves or particles• When described as waves, we consider three

important characteristics: wavelength, frequency, and energy

• The principal relationship is: = c

• Where– c = speed of light = 3.00 x 108 m/s = frequency (cycles per second, or Hertz) = wavelength (m)

= c (the speed of light, a constant)

Electromagnetic Spectrum

“Particles” of Energy• Planck argued in 1900 that energy distributions are not

continuous but consisted of individual ‘steps’; i.e., they are ‘quantized’

• Einstein in 1905 argued that radiation should be viewed as bundles of energy called ‘photons’

• Wave theory and particle theory are connected

• Energy per photon: E = h = hc/

where h is Planck’s constant = 6.63 x 10-34 Joule second (and 1 J is approximately the energy required to lift 1 kg 10 cm).

Energy Calculations

• Using E = h = hc/, you should be able to calculate

the energy of a photon, given the wavelength of that

energy

• Example: What is the energy of a photon with a

wavelength equal to 220 nanometers (nm)?

€

E = hc /λ = 6.63 x 10-34 Joule • s ( )3.00x108 m /s

2.20x10−7 m

⎛

⎝ ⎜

⎞

⎠ ⎟ = 9.04x10-19 Joules

Solar Spectrum

Solar Energy on Surface of Earth

(This figure shows only the UV portion of solar radiation)

UV-A = 315-400 nm

UV-B = 280-315 nm

UV-C = 100-280 nm(ref: ASTM)

Biological Sensitivity to UV

Some Effects of Exposure to UV

• vitamin D production

• skin tanning or burning

• melanoma skin cancer

• eye damage (e.g. snow blindness, cataracts)

• effects on plants and animals

crop yields in Australia (e.g. wheat, peas)

amphibian populations (frogs?)

NWS UV Index (Table 2.3)

Ozone Concentration vs Altitude

Chapman Cycle

O2 + O O3

O + O3 2 O2

O2 2 O < 240 nm

O3 O2 + O < 320 nm

(1)

(2)

(3)

(4)

OZONE FORMATION

OZONE DECOMPOSITION

Ozone Loss through Catalysis

.X + O3 XO. + O2

XO. + O .X + O2

Catalysts are not destroyed andmake the net reaction occur faster!

Net reaction: O3 + O 2 O2

Catalysts:Free Radicals in the Ozone!

Natural: .H, .OH, .NO

Anthropogenic: .NO from SSTs

.Cl from CFCs

O3

ClO.

How do CFCs interact with Ozone?

Ozone Hole over Antarctica

Ozone Depletion at Mid-Latitudes

Chemistry in Context: Applying Chemistry to Society, 3e. A Project of the American Chemical Society. Copyright © 2000 by the American Chemical Society. All Rights Reserved.

Largest extent in 2003 = 28 million km2

September 08, 2008 http://ozonewatch.gsfc.nasa.gov/daily_front.php?date=2008-09-08

Responses to the Problem

Montreal Protocol signed in 1987

Rowland and Molina win 1995 Nobel Prize

Production of CFCs halted in US in 1996

Alternatives: HFCs, HCFCs

CFCs: Properties and Uses

• Primarily used as refrigerants, foaming agents,

solvents, aerosol propellants

Usage History of CFCs• Montreal Protocol of 1987 (amended in 1990 and 1992) banned

CFC manufacturing by year 1996• See also Fig. 2.23 of the text for production through 1996

Current StatusSource: NASA TOMS data, http://jwocky.gsfc.nasa.gov/multi/oz_hole_area.jpg

Montreal Protocol of 1987

• CFC production to ½ by 1998• 1990 – ban CFC production by 2000• 1999 – added bromine compounds to ban

– BFCs

– CFCs & HCFCs eliminated by 2010

• Developing countries vs. Industrialized countries

Fig. 2.23

CFC production…

Substitutes

• Substitutes must be economic and provide the same technical benefits that CFCs provided (including toxicity, flammability, and stability)

• Most reasonable substitutes are HCFCs, which decompose faster, but still can cause problems (banned by 2030)

• HFCs

• Issues of equity for developing countries

Black Market

• Bootlegging– Russia, China, India, etc.

• 2nd only to illegal drugs– 1997: 2 million pounds of illegal CFCs

confiscatedPssst… Wanna buy some CFCs?

http://www.youtube.com/watch?v=ajHVLJG0298&feature=related

The Future

• Stratospheric chlorine peaked in the late 90s

• Even with our bans in place– 2ppb reached after 2050

• Replacement products?– toxicity (increase chlorine atoms)

– flammability (replace halogens with hydrogens)

– stability & other harmful effects (flourocarbons)

• HFCs• Pyrocool FEF