The Foundations of Chemistry

CC The Environment

HEMISTRY IN USE

Ozone

Ozone, O 3 , is such a powerful oxidizing agent that in signif-

icant concentrations it destroys many plastics, metals, and

rubber, as well as both plant and animal tissues. We there-

fore try to minimize exposure to ozone in our immediate

environment. In the upper atmosphere, however, ozone plays

a very important role in the absorption of harmful radiation

from the sun. Maintaining appropriate concentrations of

ozone—minimizing its production where ozone is harmful

and preventing its destruction where ozone is helpful—is an

important challenge in environmental chemistry.

Ozone is formed in the upper atmosphere as some O 2

molecules absorb high-energy electromagnetic radiation

from the sun and dissociate into oxygen atoms; these then

combine with other O 2 molecules to form ozone.

O 2 (g)
UV radiation88n2O(g) (step 1—occurs once)

O 2 (g)
O(g)88nO 3 (g) (step 2—occurs twice)

3O 2 (g)
UV radiation88n2O 3 (g) (net reaction)

Although it decomposes in the upper atmosphere, the ozone

supply is continuously replenished by this process. Its con-

centration in the stratosphere (7–31 miles above the earth’s

surface) is about 10 ppm (parts per million), whereas it is only

about 0.04 ppm near the earth’s surface.

The high-altitude ozone layer is responsible for absorb-

ing much of the dangerous ultraviolet light from the sun in

the 20–30 Å wavelength range.

O 3 (g)
UV radiation 88nO 2 (g)
O(g)

(step 1—occurs once)

O 2 (g)
O(g) 88nO 3 (g) (step 2—occurs once)

No net reaction

We see that each time this sequence takes place, it absorbs

one photon of ultraviolet light; however, the process regen-

erates as much ozone as it uses up. Each stratospheric ozone

molecule can thus absorb a significant amount of ultraviolet

light. If this high-energy radiation reached the surface of the

earth in higher intensity, it would be very harmful to plants

and animals (including humans). It has been estimated that

the incidence of skin cancer would increase by 2% for every

1% decrease in the concentration of ozone in the stratos-

phere.

Chlorofluorocarbons (CFCs)are chemically inert, non-

flammable, nontoxic compounds that are superb solvents and

have been used in many industrial processes; they are excel-

lent coolants for air conditioners and refrigerators. Two

CFCs that have been widely used are Freon-11 and Freon-

12 (Freon is a DuPont trade name).

Cl

Cl

Cl CF

Cl

F

Cl CF

Freon-11 is CCl 3 F Freon-12 is CCl 2 F 2

1.0

0.5

0

63 ̊S

0.5

1.0

1.5

2.0

2.5

Ozone, ppm

Chlorine oxide, ppb

Approximate latitude 72 ̊S

O 3

ClO

A plot that shows the decrease in [O 3 ] as [ClO]

increases over Antarctica.

The compound known as HCFC-134, a

fluorocarbon currently used in home and

automobile air conditioners.