Chapter 2: Protecting the Ozone Layer
Why do we need to do to protect
the ozone layer?
Isn’t ozone hazardous to human health?
Why is the ozone layer getting smaller?
What can we do (if anything) to
help stop the depletion of our
ozone layer?
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, buckminister fullerenes
2.1
The Regions of the Lower Atmosphere
Atmospheric pressure changes with altitude
2.1
The Regions of the Atmosphere
Have you ever been in a plane landing in Denver, CO?
You may have experienced being thrust forward as the
plane uses more energy to stop. There are fewer air
molecules at that height, reducing the amount of
friction, so a greater amount of energy is needed to
stop the plane.
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.
2.1
The ozone layer is a region in the stratosphere with maximum ozone concentration.
2.1
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
2.2
The electrons in the outermost energy levels are called
valence electrons.
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
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
2.2
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
2.3
Representing molecules with Lewis structures:
Typical valence for selected atoms = the # of bonds an atom typically forms
Element
Typical
valence
1
Classification
O
2
divalent
N
3
trivalent
C
4
tetravalent
H,
X (X= F, Cl, Br, I)
monovalent
2.3
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
2.3
Try these; draw valid Lewis structures for:
O
HNO3
N
O
CO2
H2S
H
O
O
C
O
Sulfur is under oxygen;
think of H2O
S
H
Sulfur is in the 3rd period: it
can have an expanded octet
H
O
H2SO4
Can you draw other valid Lewis
structures for HNO3?
S
H
O
O
O
H
O
S
H
O
O
O
H
O
O
S
H
O
H
O
2.3
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).
2.4
The Electromagnetic Spectrum
The various types of radiation seem different to our
senses, yet they differ only in their respective l and n.
2.4
Visible: l = 700 - 400 nm
ROY G BIV
Decreasing wavelength
Infrared (IR) : longest of the visible spectrum, heat ray
absorptions cause molecules to bend and stretch.
Microwaves: cause molecules to rotate.
Short l range: includes UV (ultraviolet), X-rays,
and gamma rays.
2.4
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 (Plank’s constant)
Energy and frequency are directly relatedhigher frequency means higher energy.
2.5
What is the energy associated with a photon of light with a wavelength of 240 nm?
C = ln
E=hn
n =C
l
E = (6.63 x 10-34 J.s) (1.3 x 1015 s-1)
E = 8.6 x 10-19 J
n = 3 x 108 m/s
-9 m = 1.3 x 1015 s-1
10
240 nm x
nm
UV radiation has sufficient energy to cause molecular bonds to break
2.5
2.6
The Chapman Cycle
A steady state
condition
2.6
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.
2.7
How CFCs Interact with Ozone
CFC-11
Freon 11
trichlorofluoromethane
CCl3F
CFC-12
Freon 12
dichlorodifluoromethane
CCl2F2
F
F
Cl
C
Cl
Cl
Cl
C
F
Cl
First, UV radiation breaks a carbon-halogen bond:
.CClF + Cl. (free radicals)
Photon (l < 220 nm) + CCl2F2
2
2.9
The chlorine radical attacks an O3 molecule:
2Cl. + 2O3
2ClO. + 2O2
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
3O2
The Cl. radicals
are free to attack
more O3
The Cl. radicals are
both consumed and
generated; they act
as catalysts
2.9
Experimental analyses show that as ClO. concentrations
increase, ozone concentration decreases.
2.9
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
2.12
Refrigerants for Automobile Use