Unit: Reaction Rates
This Unit Activity will help you meet these educational goals: • Science Inquiry—You will explore the ozone layer by doing online research and collecting information. You will then communicate your results in written form. • STEM—You will use your knowledge of kinetics to analyze scientific investigations. • 21st Century Skills—You will employ online tools for research and analysis.

Introduction
You have probably heard about the ozone layer and/or the hole in the ozone layer. Ozone is an allotrope of oxygen (O3) which is a very important part of our lives. In this activity, you will learn more about ozone and how it is formed. You will also investigate what causes the ozone layer to be depleted over time in the upper atmosphere.
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Directions and Analysis

Task 1: The Ozone Layer

Read about the ozone layer and answer the following questions. You can use the key word “ozone” or “ozone layer” in a search engine to learn more about it.

Key search terms: ozone, ozone layer, benefits of ozone

1. Describe a few of the properties of ozone and compare these to the properties of diatomic oxygen.

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2. Describe the process by which ozone can be formed in large cities.

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3. Even though ozone may be dangerous to our health here on Earth, how can it be beneficial in the upper atmosphere?

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4. How is ozone formed in the upper atmosphere?

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5. How does the ozone protect us from harmful radiation?

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Task 2: Kinetics of ozone

As you have seen, ozone can be a very beneficial part of our world. It is required to shield us from the harmful rays that are given off by the sun. Let’s take a look at some kinetics involved with ozone.

1. As you have seen, ozone is formed in the upper atmosphere through the reaction: O(g) + O2 (g) ( O3(g) Data has shown that this is first order with respect to oxygen atoms and first order with respect to diatomic oxygen. Write the rate law equation for this reaction.

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2. The rate constant is found to be 3.9 x 10-5 M-1 s-1. Calculate the rate of formation of ozone when the concentration of oxygen atoms is 3.0 x 10-14 molar and the concentration of diatomic oxygen is 1.3 x 10-14 molar.

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3. In your reading, perhaps you saw that in 1995, three chemists were awarded the Nobel Prize for their studies involving the ozone layer. First, in 1970, scientist, Paul Cruzen, hypothesized that nitrogen oxides could participate in the destruction of ozone. He was concerned that high-flying jet aircraft, such as the Concorde, could be the culprit that created the nitrogen oxide. He proposed the following mechanism to show how these compounds were involved in the destruction of ozone:

Step 1 - NO(g) + O3(g) ( NO2(g) + O2(g) Step 2 - NO2(g) + O(g) ( NO(g) + O2(g)

The rate equation for this reaction was found to be: Rate of reaction = k[NO][O3] Looking at this rate law, which of the above steps would be the rate determining step and why?

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4. What is the overall reaction for the above mechanism?

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5. Is the NO shown acting as a catalyst or an intermediate in the reaction? Why?

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6. It was found that the effect of jet aircraft on the ozone layer was too insignificant to be the cause of ozone depletion. There were simply not enough aircraft to inflict lasting harm on the ozone layer. So, in 1973, Professor Sherry Rowland and his colleague Dr. Mario Molina began to investigate chlorofluorocarbons, or CFCs. Because they are such stable compounds, CFCs were used extensively in industry as aerosol propellants, refrigerants and in insulation foams.

So Rowland and Molina had two questions – first, how persistent are these stable CFCs, meaning what is the amount of time CFC molecules could remain in the lower atmosphere without reacting? Second, the researchers also wanted to find out what might happen if these CFCs made their way into the upper atmosphere and got subjected to high-energy ultraviolet radiation there. Their investigation found that, because of their stability, CFCs could remain in the air for many centuries without reacting. It was the answer to their second question that really got their interest.

They found that the breakdown of CFCs by ultraviolet radiation produces chlorine atoms:

CF2Cl2(g) ( CF2Cl(g) + Cl(g)

Their next question was what would happen to the chlorine atoms that would be floating around in the atmosphere. They concluded that the chlorine could potentially react with ozone using the following reaction mechanism:

Step 1 – Cl(g) + O3(g) ( ClO(g) + O2(g) (slow step) Step 2 – ClO(g) + O(g) ( Cl(g) + O2(g) (fast step)

Predict the rate equation for the mechanism for the reaction between chlorine atoms and ozone molecules and write the overall reaction.

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7. So, it is seen that the destruction of ozone can be catalyzed by NO or by Cl. It has been found that the activation energy for the reaction catalyzed by Cl is 2 kJ/mol and for the reaction catalyzed by NO is 12 kJ/mol. Which is the more effective catalyst? Why?

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8. Which of the reactions will have the greater rate constant? Why?

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9. On average, one chlorine atom can destroy 100,000 ozone molecules. Such destructive power is tremendous when it is related to the millions of tons of CFCs that had been produced up until that time.

When Rowland and Molina calculated the likely effect on the ozone layer, they could not believe their figures. They consulted another scientist Hal Johnson who confirmed their results. In 1974, Rowland and Molina published their predictions about ozone depletion in the scientific journal Nature. Although the United States was quick to ban the production of CFCs, many other countries including Britain, China, France and Japan did not. Look at the ozone depletion images from 1979-1994. These images show the amounts of ozone in the atmosphere above Antarctica in October. The colors indicate the relative amounts of ozone present. The “red” color indicates the highest amounts and the “purple” indicates the smallest amounts. In 1994, the “ozone hole” showed to have a loss of 65% of its original ozone amounts.

Using the images collected on the site, describe the trends you see over time in the strength of the ozone coverage.

Type your response here:

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Resources • ozone layer • ozone depletion

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Evaluation
This project will be evaluated on a rubric that is based on the completeness, clarity, and thinking you exhibit in the Directions and Analysis section above.

10 points possible: • The ozone layer: 5 points maximum • Kinetics of ozone: 5 points maximum

|the ozone layer |kinetics of ozone |
|5 points |5 points |
|Properties of ozone (1 point) |Rate law and reaction rate of ozone (1 point) |
|Process by which ozone can be formed in large cities (1 point) |NO catalyzed ozone destruction (1 point) |
|Benefits of ozone in the upper atmosphere (1 point) |Cl catalyzed destruction (1 point) |
|Process by which ozone is formed in the upper atmosphere (2 points) |Conclusions drawn from the two different reactions |
| |(2 points) |

Self-Checked Activity: For Advanced Understanding
Note: This section may be optional; follow your instructor’s directions.

The free response test items accessed below are related to this unit’s concepts and content, but will draw from your prior chemistry knowledge as well. They will challenge your knowledge of the unit concepts, help you tie different concepts together, and help you gain confidence in addressing written response test items.

Download: Free Response Test

• Respond to question 3. • Read the directions section carefully. • Answer all parts of the question thoughtfully and concisely, to the extent of your ability. • Unless directed otherwise, answer the question in essay form; do not use bullets, outline format, etc.

For Scoring: Scoring Guidelines

• See Question 3

Download: Free Response Test

• Respond to question 2. • Read the directions section carefully. • Answer all parts of the question thoughtfully and concisely, to the extent of your ability. • Unless directed otherwise, answer the question in essay form; do not use bullets, outline format, etc.

For Scoring: Scoring Guidelines

• See Question 2

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Chemistry

Unit Activity