Protecting the Ozone Layer

Isn’t ozone hazardous to human health?

What can we do (if anything) to

help stop the depletion of our ozone layer?

Why do we need to protect the ozone layer?

Why is the ozone layer getting smaller?

What is the 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

Ozone is an allotropic form of oxygen.

Energy must be absorbed (endothermic) for this reaction to occur.

Element Allotropesoxygen O2, O3

carbon graphite, diamond, charcoal

An allotrope is two or more forms of the same element that differ in their chemicalstructure and therefore their properties.

The Regions of the Lower Atmosphere

Atmospheric pressure changes with altitude

The Regions of the Atmosphere

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.

Altitude record for plane flight – 31,000 m by an SR-71 on its retirement flight.

The ozone layer is a region in the stratosphere with maximum ozone concentration.

O8

16.00

Atomic number (Z)

Mass number (A)

– The number of protons (nuclear charge)

– 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.

1A

2A 3A 4A 5A 6A 7A

8A

Group 1A: 1 valence electron

Group 3A: 3 valence electrons

Representing molecules with Lewis structures:

Consider water, H2O: OH H

Representing molecules with Lewis structures:

Consider water, H2O: OH H

Representing molecules with Lewis structures:

Consider water, H2O:

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

OH H

Representing molecules with Lewis structures:

Consider water, H2O:

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

electrons2. 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:

OH H

lone pair

bonding pair

OH H

Representing molecules with Lewis structures:Typical valence for selected atoms = the # of bonds an atom typically forms

Element Typical valence

Classification

H, X (X= F, Cl, Br, I)

1 monovalent

O 2 divalent

N 3 trivalent

C 4 tetravalent

Representing molecules with Lewis structures:

Multiple bonds

O O H C C H

Triple bondDouble bond

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

N

O

O ON

O

O ON

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.

Visible: l = 700–400 nm

Infrared (IR): longest of the visible spectrum; heat absorptions cause molecules to bend and stretch.

Microwaves: cause molecules to rotate.

Short l range: includes UV (ultraviolet), X-rays, and gamma rays.

R O Y G B I VDecreasing wavelength

Increasing Energy

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)

The wavelength and frequency of electromagnetic radiation are related by: c = ln where c = 3 x 108 m/s (the speed of light)

Energy and frequency are directly related –higher frequency means higher energy.

The Chapman Cycle

2 242

242

2

:: : :: :

UV Photonsnm

UV Photonsnm

O O

O O O O

l

l

The Chapman Cycle

2 242

242

2

:: : :: :

UV Photonsnm

UV Photonsnm

O O

O O O O

l

l

The Chapman Cycle

A steady state condition

l ≤

l ≤

+

+

The Chapman Cycle

A steady state condition

l ≤

l ≤

+

+

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.

An Australian product uses “smart bottle” technology; bottle color changes from white to blue when exposed to UV light.

The most deadly form of skin cancer, melanoma, is linked with the intensity of UV radiation and the latitude at which you live.

First, UV radiation breaks a carbon-halogen bond:

Photon l < 220 nm) + CCl2F2 .CClF2 + Cl. (free radicals)

How CFCs Interact with Ozone

CFC-11 CFC-12Freon 11 Freon 12

trichlorofluoromethane dichlorodifluoromethane

C

F

Cl

Cl

Cl C

F

F

Cl

Cl

CCl3F CCl2F2

2 Cl. + 2O3 2 ClO. + 2 O2

2 ClO. ClOOCl

The chlorine radical attacks an O3 molecule:

Then two chlorine monoxide radicals combine:

UV photon + ClOOCl ClOO. + Cl.

ClOO. Cl. + O2

The ClOOCl molecule then decomposes:

The net reaction is: 2 O3 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-22chlorodifluoromethane

CHClF2

HCFC-141bdichlorofluoroethane

C2H3Cl2F

C

F

F

Cl

H C

Cl

F

Cl

C

H

H

H

Refrigerants for Automobile Use

Freon in your car’s air conditioner is a commonly used CFC