Protecting the Ozone Layer
What is the Ozone Layer?
Isn’t ozone hazardous to human health?
Why do we need to protect the
ozone layer?
Why is the ozone layer getting
smaller?
What can we do (if anything) to
help stop the depletion of our
ozone layer?
The Ozone Layer
• Ozone, O3, is a health hazard in the troposphere, but
essential to life on earth in the stratosphere.
The Ozone Layer
• Ozone, O3, is a health hazard in the troposphere, but
essential to life on earth in the stratosphere.
Energy + 3 O2
2 O3
Energy must be absorbed (endothermic) for this reaction to occur.
Ozone Formation
Energy + 3 O2
2 O3
Energy must be absorbed (endothermic) for this reaction to occur.
Ozone is an allotropic form of oxygen.
An allotrope is two or more forms of the same element that differ in their chemical
structure and therefore their properties.
Element
oxygen
carbon
Allotropes
O2, O3
graphite, diamond, charcoal
The Regions of the Lower Atmosphere
Atmospheric pressure changes with altitude
The Regions of the Atmosphere
Altitude record for plane flight –
31,000 m by an SR-71 on its
retirement flight.
Why does it take longer to cook an egg in Denver than it
does in New Orleans?
There is less air pressure at higher altitudes. Water
boils when the vapor pressure of the water molecules
exceeds that of the localized air pressure. Because
there is less air pressure at higher altitudes, more
energy must be supplied (longer time) to get the
temperature of the water high enough to cook the egg.
The ozone layer is a region in the stratosphere with maximum ozone
concentration.
Atomic number (Z) – The number of protons
8
(nuclear charge)
O
16.00
Mass number (A) – The sum of the
protons and neutrons
2.2
Isotopes are two or more forms of the same element
(same number of protons) whose atoms differ in
number of neutrons, and hence in mass.
Isotopes of carbon: C-12, C-13, C-14
also written as: 12C 13C 14C
Isotopes are two or more forms of the same element
(same number of protons) whose atoms differ in
number of neutrons, and hence in mass.
Hydrogen has exactly three isotopes –
1H – Majority isotope in nature. Hydrogen
2H
– Deuterium. Only about 0.0026 - 0.018% in
nature. Stable. Sometimes called heavy
hydrogen.
3H
– Tritium. Half life of only 12 years, so only
trace amounts in nature. Radioactive, but
only weakly, and not harmful to humans
externally. Used in watch dials.
Isotopes are two or more forms of the same element
(same number of protons) whose atoms differ in
number of neutrons, and hence in mass.
Uranium has three main isotopes –
238U – Majority isotope in nature. Radioactive, but
not capable of uncontrolled fission – can’t
make a bomb.
235U
– Only about 0.7% in nature. Enriching to 4%
allows nuclear power. Enriching to >50%, it
can explode.
233U
– Only about 0.005% in nature. Not as strong
a slow neutron source as 235U, but more
than 238U
Let’s go back to the Periodic Table
The Bohr Model
• This is a MODEL of the atom that links electron behavior
(microscopic) to the periodic law (macroscopic).
Let’s go back to the Periodic Table
Valence Electrons
• Valance Electrons
– Electrons in the Outermost Principal
Shell
– Which simply means the
electrons that fill the orbitals for
the main-group elements
– That means there are always
a maximum of either two
(first row) or 8 (all the
others)
– Electrons Involved in Chemical
Bonding
Valence Electrons
Valence Electrons
Li
Be
B
C
N


O


F


Ne
The group number (of the representative elements) on the
periodic table tells you the number of valence electrons.
Group 1A: 1 valence electron
1A
8A
Group 3A: 3 valence electrons
2A
3A 4A 5A 6A 7A
Representing molecules with Lewis structures:
Consider water, H2O: H
O
H
Representing molecules with Lewis structures:
Consider water, H2O: H
O
H
Representing molecules with Lewis structures:
Consider water, H2O: H
O
H
1. Find sum of valence electrons:
1 O atom x 6 valence electrons per atom = 6
+ 2 H atoms x 1 valence electron per atom = +2
8 valence electrons
Representing molecules with Lewis structures:
Consider water, H2O: H
O
H
1. Find sum of valence electrons:
1 O atom x 6 valence electrons per atom = 6
+ 2 H atoms x 1 valence electron per atom = +2
8 valence electrons
2. Arrange the electrons in pairs; use whatever electron pairs
needed to connect the atoms, then distribute the remaining
electron pairs so that the octet rule is satisfied:
lone pair
O
H
H
bonding pair
Representing molecules with Lewis structures:
Typical valence for selected atoms = the # of bonds an atom typically forms
Element
Typical
valence
1
Classification
2
divalent
N
3
trivalent
C
4
tetravalent
H,
X (X= F, Cl, Br, I)
O
monovalent
Representing molecules with Lewis structures:
Multiple bonds
O
H
O
Double bond
C
C
H
Triple bond
Occasionally, a single Lewis structure does not adequately represent
the true structure of a molecule, so we use resonance forms:
O
O
O
O
N
N
N
O
O
O
O
O
Light and Matter
• Light has a very important connection to all this.
– Light interacts with matter by either being reflected or
absorbed.
The Nature of Light
Low E
High E
Wavelength (l) = distance traveled between successive peaks (nm).
Frequency (n) = number of waves passing a fixed point in one second
(waves/s or 1/s or s–1 or Hz).
The Electromagnetic Spectrum
The various types of radiation seem different to our
senses, yet they differ only in their respective l and n.
ROY G BIV
Decreasing wavelength
Increasing Energy
Microwaves: cause molecules to rotate.
Infrared (IR): longest of the visible spectrum; heat
absorptions cause molecules to bend and stretch.
Visible: l = 700–400 nm
Short l range: includes UV (ultraviolet), X-rays, and
gamma rays.
The wavelength and frequency of electromagnetic
radiation are related by: c = ln
where c = 3 x 108 m/s (the speed of light)
The energy of a photon of electromagnetic radiation
is calculated by: E = hn
where h = 6.63 x 10–34 J.s (Planck’s constant)
Energy and frequency are directly related –
higher frequency means higher energy.
The Chapman Cycle
UV Photons
O2 
 l  242 nm   2O








UV Photons
::
:

:O O l 242 nm :O   O :
The Chapman Cycle
UV Photons
O2 
 l  242 nm   2O








UV Photons
::
:

:O O l 242 nm :O   O :
The Chapman Cycle
l≤
+
l≤
A steady state
condition
+
The Chapman Cycle
l≤
+
l≤
A steady state
condition
+
Biological Effects of Ultraviolet Radiation
The consequences depend primarily on:
1. The energy associated with the radiation.
2. The length of time of the exposure.
3. The sensitivity of the organism to that radiation.
The most deadly form of skin cancer, melanoma, is
linked with the intensity of UV radiation and the latitude
at which you live.
An Australian product uses
“smart bottle” technology;
bottle color changes from
white to blue when exposed
to UV light.
How CFCs Interact with Ozone
CFC-11
Freon 11
trichlorofluoromethane
CCl3F
CFC-12
Freon 12
dichlorodifluoromethane
CCl2F2
F
F
Cl
C
C
Cl
Cl
F
Cl
Cl
First, UV radiation breaks a carbon-halogen bond:
.CClF + Cl. (free radicals)
Photon l < 220 nm) + CCl F
2 2
2
The chlorine radical attacks an O3 molecule:
2 Cl. + 2O3
2 ClO. + 2 O2
Then two chlorine monoxide radicals combine:
2 ClO.
ClOOCl
The ClOOCl molecule then decomposes:
UV photon + ClOOCl
ClOO.
The net reaction is: 2 O3
ClOO. + Cl.
Cl. + O2
3 O2
The Cl. radicals
are free to attack
more O3
The Cl. radicals are
both consumed and
generated; they act
as catalysts
Experimental analyses show that as ClO. concentrations
increase, ozone concentration decreases.
Change in size of the ozone hole over for last decade
Another look at the ozone hole. The more blue, the worse the problem….
HCFCs are alternatives to CFCs: they decompose more
readily in the troposphere so they will not accumulate
to the same extent in the stratosphere.
HCFC-22
chlorodifluoromethane
HCFC-141b
dichlorofluoroethane
CHClF2
C2H3Cl2F
F
H
C
Cl
F
H
H
Cl
C
C
H
Cl
F
Refrigerants for Automobile Use
Freon in your car’s air conditioner is a commonly used CFC