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Ch.18 - Chemistry of the Environment
All textbooksBrown 14th EditionCh.18 - Chemistry of the EnvironmentProblem 63
Chapter 18, Problem 63

If an average O₃ molecule “lives” only 100–200 seconds in the stratosphere before undergoing dissociation, how can O₃ offer any protection from ultraviolet radiation?

Verified step by step guidance
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Step 1: Understand the role of ozone (O₃) in the stratosphere. Ozone absorbs harmful ultraviolet (UV) radiation from the sun, preventing it from reaching the Earth's surface.
Step 2: Recognize that the ozone layer is a dynamic system. Although individual O₃ molecules have a short lifespan, the continuous formation and destruction of ozone maintain a stable concentration in the stratosphere.
Step 3: Learn about the ozone-oxygen cycle. Ozone is constantly being formed and destroyed in the stratosphere through a series of reactions involving oxygen molecules (O₂) and oxygen atoms (O).
Step 4: Consider the balance between ozone formation and destruction. The rate of ozone formation is approximately equal to the rate of its destruction, ensuring a steady state concentration of ozone that can effectively absorb UV radiation.
Step 5: Reflect on the importance of this balance. Despite the short lifespan of individual ozone molecules, the overall concentration of ozone in the stratosphere is sufficient to provide significant protection from UV radiation.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ozone Layer and Its Function

The ozone layer, located in the stratosphere, contains a high concentration of ozone (O₃) molecules that absorb the majority of the sun's harmful ultraviolet (UV) radiation. This protective layer is crucial for life on Earth, as it prevents excessive UV radiation from reaching the surface, which can cause skin cancer and other health issues.
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Ozone Formation and Decomposition

Ozone is continuously formed and decomposed in the stratosphere through photochemical reactions. UV radiation splits oxygen molecules (O₂) into individual oxygen atoms, which can then react with other O₂ molecules to form ozone. Although individual O₃ molecules have a short lifespan, the dynamic balance between formation and decomposition maintains a stable concentration of ozone in the stratosphere.
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Time Scale of Atmospheric Processes

The time scale of atmospheric processes, such as the formation and dissociation of ozone, is critical for understanding its protective role. Even though an O₃ molecule may only exist for 100–200 seconds, the continuous regeneration of ozone ensures that there is always a sufficient amount present to absorb UV radiation, providing ongoing protection against its harmful effects.
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Related Practice
Textbook Question

In the following three instances, which choice is greener in a chemical process? Explain. (a) A reaction that can be run at 350 K for 12 h without a catalyst or one that can be run at 300 K for 1 h with a reusable catalyst.

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Textbook Question

Suppose that on another planet the atmosphere consists of 17% Kr, 38% CH4, and 45% O2. What is the average molar mass at the surface? What is the average molar mass at an altitude at which all the O2 is photodissociated?

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Textbook Question

Show how Equations 18.7 and 18.9 can be added to give Equation 18.10.

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