Homeschool Learning Network
Making Molecules Tasty
The Chemistry of Ozone--Making and Breaking Ozone
Name ___________________________
Date ___________________________
In this lesson you will learn about the chemical reactions that underlie the formation and
breakdown of ozone in the different layers of Earth’s atmosphere. As you read through the lesson,
try to imitate the chemical reactions using different colored jellybeans for the atoms and toothpicks
for the chemical bonds.
Use the following table to help you define the chemical symbols and create the jellybean and
toothpick models.
Chemical Symbol
N
Definition
Nitrogen atom
Chemical Model
White jellybean = N
O
Oxygen atom
Blue jellybean = O
Cl
Chlorine atom
Black jellybean = Cl
O2
Molecular Oxygen
O–O
O3
Ozone
O—O—O
NO2
Nitrogen Oxide
O—N—O
Low-Level Ozone Reactions:
Ozone (O3) is made from three oxygen atoms, is blue in color and has a strong odor (its name in
Greek, ozein, means “to smell".) The oxygen molecule that we breathe (O2) has two oxygen
atoms and is colorless and odorless. In the troposphere, ozone is much less common than normal
molecular oxygen. Out of each 10 million air molecules, about 2 million are normal oxygen, but
only 3 are ozone.
Question A: What are the percentages of O2 and of O3 in the air that we breathe?
The series of chemical reactions that lead to the increase in the levels of ozone in the troposphere
starts when Nitrogen Oxides (NOx), along with other unstable chemical compounds, are produced
from our motor vehicle exhaust, industrial emissions, gasoline vapors, and chemical solvents. The
interaction of these pollutant precursors with sunlight, heat, and available oxygen molecules
creates ozone pollution or “smog.” The sequence of chemical reactions that underlies the
production of ozone in the troposphere is:
(1) breakdown of nitrogen oxide by sunlight, releasing an oxygen atom:
NO2 + sunlight + heat Æ NO + O,
© 2001 The Homeschool Learning Network, all rights reserved. The Homeschool Learning Network permits teachers
and parents to reproduce this page for non-profit and educational purposes only.
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Homeschool Learning Network
Name ___________________________
Date ___________________________
(2) combination of molecular oxygen with the highly reactive oxygen atom to form ozone:
O2 + O Æ O3.
Question B: Why smog is worse in the summer months than in the winter months?
High-Level Ozone Reactions:
Building the Ozone Layer
At any given time, ozone molecules are constantly being formed and destroyed in the
stratosphere. The total amount, however, remains relatively stable. Normal ozone synthesis and
breakdown in stratosphere uses and absorbs harmful ultraviolet (UV) light energy from the sun in
the following reactions:
(3) breakdown of molecular oxygen to 2 oxygen atoms using UV energy from the sun
O2 + UV Æ O + O,
(4) binding of the highly reactive oxygen atom to molecular oxygen to form ozone
O+O2Æ O3,
(5) breakdown of ozone to molecular and atomic oxygen using UV energy from the sun
O3 + UV Æ O2 + O
This final reaction is the most vital in shielding the earth from the sun's UV radiation.
Question C: How do stratospheric ozone levels remain constant
even when ozone is broken down by sunlight?
Removing the Ozone Layer
Under normal conditions, the formation and breakdown of ozone in the stratosphere are balanced
so there is no net loss of ozone from the ozone layer. However, humans have introduced a new
factor into the equation that causes the breakdown of ozone to exceed its formation. This factor is
a group of chemicals called chlorofluorocarbons, or CFCs. CFCs are commonly used for
refrigerants, solvents, and foam blowing agents and are very stable in the troposphere. When
extra CFCs escape to the stratosphere, they are broken down by the ultraviolet (UV) light energy of
the Sun. The release of highly reactive atoms, such as chlorine (Cl), from the CFCs leads to the
following reaction:
(6) breakdown of ozone molecules and depletion of the protective ozone layer.
Cl+O3 Æ O2+ClO
© 2001 The Homeschool Learning Network, all rights reserved. The Homeschool Learning Network permits teachers
and parents to reproduce this page for non-profit and educational purposes only.
http://www.homeschoollearning.com
Homeschool Learning Network
Name ___________________________
Date ___________________________
The rate of this reaction in the stratosphere is very fast and the formation of ozone by the synthesis
reaction shown in equation (4) cannot keep up. The final result is the depletion of the protective
layer of ozone in the stratosphere and the increase in the amount of UV energy that reaches the
Earth’s surface (and YOU!)
Question D: What are the different roles that ozone plays in the overall health of our
environment?
© 2001 The Homeschool Learning Network, all rights reserved. The Homeschool Learning Network permits teachers
and parents to reproduce this page for non-profit and educational purposes only.
http://www.homeschoollearning.com