Ozone Depletion

Contextualize

The phenomenon of ozone depletion in our planet's stratosphere offers the opportunity to introduce several central concepts in chemistry related to chemical kinetics like reaction mechanisms and effects of temperature and catalysts on reaction rate, as well as reinforce ideas on chemical bonding. It is also an excellent example how understanding this phenomenon has led to actions that are helping to address the issue.

Focus

The following infographic depicts the systems in interaction analyzed during the lesson. Interactions between chemical substances in the stratosphere and sunlight induce chemical processes that absorb radiation dangerous for living organisms. Human activities emit pollutants that interfere with these processes, creating environmental and health problems:

Define

Central Ideas

The nature and rate of the chemical transformation of atmospheric components support but also may endanger life on our planet.
Chemical reactions are processes in which atoms that make up the molecules in a system rearrange. Bond breaking and forming lead to particles with different compositions and structures.
During a chemical reaction, energy is absorbed when bonds break and released when new bonds form. The net result determines whether the reaction absorbs or releases energy.
Determining the mechanism of a reaction is of central importance to understanding the process and predicting and controlling its rate (how fast it occurs).
The rate of chemical processes is affected by different factors such as the concentration of different species, temperature, pressure, and the presence of catalysts that speed up the process. These effects can be understood and predicted using a simple collision model.

Core Practices

Analyze and use data to estimate energy exchanged during chemical reactions.
Build models to represent matter and energy transformations during each step in reaction mechanisms.
Use data and apply chemical models to explain and construct arguments about photochemical processes occurring in the atmosphere.

Systems Thinking Skills

System Composition: Identify and characterize the properties of entities participating in photochemical reactions in the atmosphere.
System Structure: Explore light-matter and matter-matter interactions that lead to chemical transformations of atmospheric substances.
System Behavior: Infer the behaviors that emerge from atmospheric interactions resulting in matter and energy transformations in the atmosphere.
System Effects: Analyze the effects of photochemical reactions and their products on the environment and human health.

Socio-environmental Competencies

Analyze, discuss, and evaluate the socioenvironmental impacts of the emission of substances that lead to ozone depletion in the stratosphere and the formation of photochemical smog in the troposphere.

Design

The following presentation includes a sequence of content and activities for a proposed two-week lesson (approximately six 50-minute sessions) that engages students in the development and application of chemical systems thinking to the understanding of ocean acidification. The lesson is designed for an introductory general chemistry lecture course at the university level.

Ozone Depletion.pptx

This example lesson introduces and applies the following chemistry concepts: Reaction mechanism, factors that affect reaction rate, bond energy and heat of reaction.
This example lesson assumes students already have a basic understanding of the particulate model of matter, chemical bonding, and matter and energy transformations during chemical reactions.
The following diagram depicts a suggested schedule for the implementation of this example lesson:

The example lesson includes a set of interspersed activities (labeled "Let's Think") that students are expected to complete in small collaborative groups and then share their ideas in whole class discussions. These activities ask students to share and explore chemical concepts, actively engage in core science practices like analyzing data, making predictions, applying models, and generating explanations, and practice systems thinking skills. The specific system thinking skills that each activity may help foster are highlighted using representative icons. They also create opportunities for the instructor to formatively assess student learning and provide specific feedback to advance their understanding.

Map Out

During the "map out" phases of a lesson, students are introduced to the socioenvironmental problem under analysis to identify the systems in interaction. This phase should allow them to develop an overall view of the nature and complexity of the problem or phenomenon to be analyzed. These activities create a need to know and opportunities for students to activate and share prior knowledge and experiences related to the phenomenon. As illustrated below, this is accomplished in the example lesson on ozone depletion by having students watch a video about the issue and then asking them to build an initial system map based on what they heard and their prior knowledge:

Zoom In

During the "Zoom In" phases of a lesson, students engage in activities that help them identify the main components in the systems of interest, analyze their properties, and characterize their interactions at levels of granularity that are productive in making sense of the problem or phenomenon under consideration. In the example lesson on ozone depletion, students first explore the reaction mechanisms for the formation of ozone at the molecular level, as illustrated by the following "Let's Think" activity:

In the second part of the lesson, students explore that affect the rate of chemical reactions at the particulate level as illustrated below:

Zoom Out

Once students model and understand the phenomena of interest at submicroscopic levels of granularity, it is important to "zoom out" using activities that help them recognize system-level properties and behaviors that emerge from the interactions between components. For example, in the first part of the proposed lesson on ozone depletion, students are asked to apply what they have learned to explore the process of ozone destruction in the stratosphere:

While in the second part of the example lesson, they apply their understanding to compare the rates of ozone destruction in the stratosphere and troposphere attending to relevant factors:

Connect

In the "Connect" phases of the lesson, students engage in activities that allow them to explore the effects of interactions between relevant subsystems, as illustrated in the following activity where students analyze the implications for life in our planet of ozone formation and destruction in the stratosphere:

As well as apply their knowledge in analyzing data to compare the environmental impacts of different substances used as refrigerants:

Evaluate

The "Let's Think" activities interspersed in the example lesson create diverse opportunities to formatively assess student learning and provide specific feedback to advance their understanding to meet the lesson's learning objectives. These activities also help students evaluate strengths and areas needing improvement in their learning. As part of the summative assessment, we suggest implementing an activity that requires students to apply their understanding to analyze a different system of interest. This summative assessment could be completed individually or in small groups, inside or outside the classroom. An example of this type of summative assessment is included at the end of the example lesson as a "Let's Apply" (LA) activity focused on the formation of photochemical smog.

Reflect

During the implementation of the lesson, it is important to systematically gather information about student learning and performance that can help us critically reflect on aspects of the lesson that need to be modified to support student learning of the central ideas, core practices, and socio-environmental competencies targeted by the lesson.