Protecting the Ozone Layer
Dec 26, 2015
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
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
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
Multiple Bonds
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
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)
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)
“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
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?)
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
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.
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
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
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