Summary

In this lesson students will make observations of a colorful homogenous catalyst and intermediate in a reaction demonstration that will spark their interests. They will then work in teams to analyze graphs and data sets in order to make a real-world connection to AP topics in kinetics such as catalysts, intermediates and reaction mechanisms by exploring how CFCs work to break down the ozone layer. Students will also investigate and discuss this environmental issue.

Grade Level

High School (AP Chemistry)

AP Chemistry Curriculum Framework

This lesson supports the following units, topics and learning objectives:

• Unit 2: Compound Structure and Properties
  • Topic 2.5: Lewis Diagrams
    • 2.5.A: Represent a molecule with a Lewis diagram.
  • Topic 2.7: VSEPR and Hybridization
    • 2.7.A: Based on the relationship between Lewis diagrams, VSEPR theory, bond orders, and bond polarities: a. Explain structural properties of molecules. b. Explain electron properties of molecules.
• Unit 3: Properties of Substances and Mixtures
  • Topic 3.12: Properties of Photons 
    • 3.12.A: Explain the properties of an absorbed or emitted photon in relationship to an electronic transition in an atom or molecule.
• Unit 5: Kinetics
  • Topic 5.1: Reaction Rates
    • 5.1.A: Explain the relationship between the rate of a chemical reaction and experimental parameters.
  • Topic 5.3: Concentration Changes Over Time
    • 5.3.A: Identify the rate law expression of a chemical reaction using data that show how the concentrations of reaction species change over time.
  • Topic 5.6: Reaction Energy Profile
    • 5.6.A: Represent the activation energy and overall energy change in an elementary reaction using a reaction energy profile.
  • Topic 5.7: Introduction to Reaction Mechanisms
    • 5.7.A: Identify the components of a reaction mechanism.
  • Topic 5.8: Reaction Mechanism and Rate Law
    • 5.8.A: Identify the rate law for a reaction from a mechanism in which the first step is rate limiting.
  • Topic 5.9: Pre-Equilibrium Approximation
    • 5.9.A: Identify the rate law for a reaction from a mechanism in which the first step is not rate limiting.
  • Topic 5.10: Multistep Reaction Energy Profile
    • 5.10.A: Represent the activation energy and overall energy change in a multistep reaction with a reaction energy profile.
  • Topic 5.11: Catalysis
    • 5.11.A: Explain the relationship between the effect of a catalyst on a reaction and changes in the reaction mechanism.

Objectives

By the end of this lesson, students should be able to:

• Write an overall reaction from a mechanism.
• Identify catalysts and intermediates from a mechanism.
• Analyze graphs and data sets in order to evaluate the effects of a catalyst on an environmentally important reaction.
• Draw a Lewis Dot Diagram and connect to molecular geometries, bond angles, resonance and bond order.
• Calculate the energy associated with the energy of a specific wavelength of light and determine if it can break selected intramolecular bonds.

Chemistry Topics

This lesson supports students’ understanding of:

• Reaction mechanisms
• Catalysts and intermediates
• Light calculations
• Lewis Dot Diagrams

Time

Teacher Preparation: 30 minutes

Lesson: 100 minutes

Materials

• Demo (Amounts Per Class):
  • 80 mL of 6% hydrogen peroxide
  • 15 mL of 0.1M Cobalt Chloride
  • 200 mL of 0.2M Potassium sodium tartrate
  • 1 L beaker
  • 25 mL graduated cylinder
  • 50 mL graduated cylinder
  • 100 mL graduated cylinder
  • Hot plate/Stirrer
  • Magnetic Stir Bar
  • Thermometer
  • Thermometer clamp (optional) on ring stand
• Activity:
  • Copies of Student Handount
  • Access to the internet and speakers

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Follow appropriate disposal procedures.

Teacher Notes

• Introduction/Engagement: As the students to walk into class, ask them to jot down what they know about the terms: homogeneous, heterogeneous, catalyst and enzyme in their table groups. Discuss these terms in order to activate prior knowledge. (5 minutes)
• Points to emphasize:
  • Homogeneous: mixture of two or more components in the same state of matter uniformly combined (such as salt water or air).
  • Heterogeneous: mixture of two or more components either in two states of matter (such as sand and water) or mixture in the same state of matter which is not uniformly combined (oil and water or granite countertops).
  • Catalyst: one of several factors which can speed up a reaction by lowering the activation energy of the reaction. Catalysts often create different pathways (different mechanisms) in order to lower the activation energy. These are not used up in the chemical reaction (if they are altered during the reaction, they are reformed at the end of the reaction).
  • Enzyme: biochemical catalysts which lowers the activation energy for complex biochemical reactions. These often bind to a target molecule (substrate) often changing the shape of the molecule in order for the active site to be properly aligned for a chemical reaction. A cool example of an enzyme is lipase in saliva which begins to digest fats in your food even before the food hits your stomach! Lipases are also added to laundry detergents to help to break down grease and oil stains in clothing, carpet, or other fibers.

• Demo: This is a modified version of a demo called Pink Catalyst.
  1. Write the reaction on the board:
     C₄H₄O₆^2-+ 5H₂O₂ → 4CO₂ +6H₂O + 2OH^-
     (Tartrate ion)
     (Na+ an K+ are spectator ions.)
  2. Ask the students to predict what would be evidence of the reaction occurring (Carbon dioxide gas bubbles is the answer you are looking for. Some students might say the production of base, but remind them that you are not using an indicator to observe this change. All of the reactants and products are clear and colorless).
  3. Measure 100mL of 0.2M potassium sodium tartrate. Add to 1L beaker.
  4. Add magnetic stir bar to beaker.
  5. Place on the hot plate. Set the stirrer to medium speed.
  6. Place thermometer in the beaker and set to medium-high heat (6-7 out of 10 scale).
  7. Wait until the temperature of the solution reaches approximately 70^oC.
  8. Measure 40mL of 6% hydrogen peroxide. Add it to the 1L beaker.
  9. Wait for 30 seconds. Ask the students if they see evidence of the reaction occurring. (There should be no evidence of the reaction occurring).
  10. Measure 15mL of 0.1M cobalt chloride.
  11. Ask the students to note the color of the cobalt ions in the solution. Also ask them if this appears in the balanced chemical equation. (Cobalt 2+ ions are pink. Cobalt 2+ is not part of the balanced chemical equation. Cobalt 2+ is a catalyst. Catalysts are not included in the products or the reactants of the balanced chemical equation. They are sometimes written above the reaction arrow).
  12. The solution in the beaker should still be around 70^oC. Add the cobalt chloride to the mixture in the 1L beaker.
  13. In approximately 20-30seconds, the solution will turn green. The solution will then begin to bubble vigorously.
  14. The bubbles stop and the solution turns back to pink.
  15. Discuss with the class what occurred (see discussion section below).
  16. Ask the students if the catalyst can be reused. Yes, but the challenge is that the catalyst is “trapped” in the solution since it is homogeneous and cannot easily be separated out. Ask the students to problem solve how to reuse it. (Answer: It can be reused by adding more of the reactants into the beaker to restart the reaction).
  17. Make sure the solution is still around 70^oC.
  18. Measure 100mL of 0.2M potassium tartrate. Measure 40mL of 6% hydrogen peroxide. Add them at the same time.
  19. The reaction will repeat. The pink and green colors of the cobalt will be less intense since it is more dilute. It is important to have the students note this. It also may take slightly longer for the reaction to occur though this is not usually a noticeable time difference.
  20. Ask the students if more reactants could continuously be added in order to reuse this catalyst. Realistically, the catalyst would become so dilute that it would not be effective over time and the reaction vessel would need to get larger and larger. See demo discussion section for more discussion points.
  21. Turn off the hot plate. When the solution is cool, follow local and state regulation for disposal methods.
• Demo Discussion:
  • Effects of a Catalyst on the Speed of a Reaction: This reaction will occur without the addition of the catalyst (pink Co ²⁺). It just proceeds very, very slowly. When the catalyst is added, the activation energy is lowered (and may also take a different reaction pathway with lower activation energy) and the reaction therefore happens at a faster rate. Illustrations are below:

• Intermediates, Activated Complexes and Transition State : An activated complex occurs as the bonds are breaking and forming as the reaction progresses. This is where the molecules/atoms are colliding and briefly (very, very, very briefly) forming highly unstable transition species which are called the activated complex. The absolute peak of the energy diagram with the highest potential energy is known as the transition state. In an elementary step reaction (one step mechanism), there is still an activated complex and transition state even though there are no intermediates. An example of this is the reaction between H ₂ and I₂. This is a single elementary step mechanism. The activated complex would occur as the hydrogen and iodine molecules collide and the H-H bond and I-I break and the H-I bonds are formed.
• Intermediates only occur in multistep reactions. Intermediates occur in a valley of an energy diagram (see below). In a two-step reaction, in addition to intermediate(s), there would be two activated complexes – one for each step. Intermediates are also short-lived (relatively speaking). They are sometimes isolated and analyzed, but often they are detected through instrumentation such as a spectrophotometer. Activated complexes and intermediates are frequently confused because both are short-lived, relatively unstable, and not easily isolated during the chemical reaction. In reactions in which the mechanism is not fully understood or known, it can be difficult to tell whether a species is an activated complex or an intermediate.

• The green color which forms is the result of an intermediate which includes Co ³⁺ (some sources call the green color an activated complex. It lasts long enough to appear to be an intermediate. Since it is complex reaction, it may be that the green color is part of an intermediate and activated complex forming. There are some proposed mechanisms; however, none seem conclusive or fully empirically proven true). When the reaction is proceeding, the green color is present. Bubbles form which is carbon dioxide gas which is a product. Shortly after the gas stops forming, the solution changes from green back to pink representing the reformation of the Co ²⁺ ions illustrating how a catalyst can be used DURING the reaction, but not consumed in the reaction. It also shows the consumption of the green intermediate.
• Homogeneous vs. Heterogeneous Catalyst : When the reaction occurs a second time, the green color appears after more reactants are added. The reaction will again turn back to the pink color. With each reaction, the color of the pink will become lighter and lighter in color illustrating reduced concentration. Because this is a homogenous catalyst and dissolved into the solution, there are no simple methods of removing the ions from the resulting solution. This is a great chance to talk about industrial uses of catalyst and environmental impacts of catalysts. Catalyst in theory can be reused infinite number of times; however, homogenous catalysts can become so dilute over time that they become ineffective if reactants in solution are added over and over again. This would be problematic for industry because catalysts can be an expensive component to a process and can also present challenges in dealing with the waste solution. Cobalt ions for example cannot be rinsed down the drain (in most states and areas) so the school or industry has to pay for the chemical disposal fees which can really add up. On the other hand, a heterogeneous catalyst can be filtered from a solution, collected, rinsed and reused over and over and over again. For example, platinum (really expensive) is used in industrial applications as a catalyst in many organic reactions. It can be filtered and reused. Research scientists have been designing polymer beads to attached to catalysts which have been traditionally homogeneous catalysts. The catalyst molecule is chemically bonded to the polymer bead so that at the end of the reaction the polymer beads can be filtered, rinsed, and reused.
• Activity: This is designed for students work in groups of 2-3, and should take 50-60 minutes.
• Pass out one student activity sheet per group, titled Team Activity: Analyzing the Effects of CFC’s on Ozone (or one per student if you prefer to have each record their answers).
• There are sections in boxes on the student handout. Those are not questions, but instead sections for the groups to read to gather information. It may be beneficial to ask your students to read these out loud to the group. It is also helpful for students to have access to a digital copy to see the graphs/images in color and have access to the links for the videos. Some questions on the first 3 pages are review on light calculations and Lewis dot diagrams. These are great for the students to review and also to show how the concepts in chemistry connect throughout the year. You can cut these questions out if you have not taught these topics yet or if you are in a rush for time.
• The first video included for the students is from ChemMatters magazine and explains the role of ozone throughout the layers of the atmosphere: The students determine the overall rate law for the decomposition of ozone from the mechanism and identify the catalyst and intermediates. They discuss in their groups how a catalyst is reformed and can therefor continue to decompose the ozone over and over and over. Students are provided data of the levels of CFCs over time to see how the Montreal Protocol was able to curb the addition of new CFCs into the air but not totally remove those already released. The students analyze graphs to see how the ozone hole continued to increase in size even after the Montreal Protocol (since the CFCs were still there and were acting as catalysts able to react over and over and over again). It would be easy to end the activity at question #10 if you are limited on time. After question 10, there are some graphs about the occurrence of skin cancer around the globe, but it is important to note that this is not purely due to a depletion of ozone (it’s a factor but not the only factor).
• The second video is an ad from the Australian government for sun safety with a catchy cartoon and song. The last few questions provide information on how the scientists were able to figure out that CFCs that were destroying the ozone and the propaganda that pushed against regulating CFCs. You could make these questions optional so that all of the groups could finish at the same time. It’s really important during this activity to walk around and check in on groups.
• Discussion: At the end of the activity, the whole class can meet for 15-20 minutes to discuss what students learned from the activity. It is a great time to discuss some challenges with catalysts and compare similarities and differences between CFCs and the pink catalyst. You can use this time to talk about a heterogeneous catalyst which is environmentally beneficial: catalytic converter in cars. This video is a great resource. You can also expand to talk about other current environmental issues which may or may not relate to catalysts.
• Assessment: It’s a great idea to check student understanding throughout the demo with the series of verbal questions as the demo proceeds. Walking through and checking on students’ progress and understanding throughout the ozone activity is also important. It is possible to fix misunderstandings during the activity. The sheet from each group can be collected at the end of the activity; an answer key has been provided. This aligns with AP Free Response Question #3 parts d-e from the 2009 exam.

• Sample MC Test Questions:
  1. The catalytic converter in your car (expensive!) takes breaks down CO gas and NO gas which are formed as a byproduct of combustion of impure fuel. The catalytic converter in your car is made of solid platinum and rhodium. Which of the following statements is true about catalytic converters?
     • It is an example of a heterogeneous catalyst and the metals need to be replenished often because they are used up.
     • It is an example of a homogenous catalyst and the metals need to be replenished often because they are used up.
     • It is an example of a heterogeneous catalyst and it does not need to be replaced because catalysts are not used up the chemical reaction
     • It is an example of a homogenous catalyst and it does not need to be replaced because catalysts are not used up the chemical reaction
     • 
• 2. NO is produce in the incomplete combustion from gasoline in cars. If the NO goes into the atmosphere because the catalytic converter fails to destroy it, NO can react with ozone O₃ in the series of steps above. How would NO be classified based on the reaction above?
  • Catalyst
  • Reactant
  • Product
  • Intermediate
• 
• 3. The products for the overall reaction are:
  • Ce⁴⁺ and Tl³⁺
  • Ce³⁺ and Tl³⁺
  • Mn³⁺, Mn⁴⁺, Ce³⁺ and Tl³⁺
  • Mn²⁺, Ce³⁺ and Tl³⁺
• 
• 4. Based on the graphs above, what is the overall order of the reaction?
  • Zero
  • First
  • Second
  • Cannot be determined
• 5. Which of the following could potentially be mechanisms for the reaction above taking into account the graphs provided and the overall reaction written above?
  • A + B → E (slow)
    A + E → C + D (fast)
  • A + B → E (slow)
    E → C + D (fast)
  • A + A → E (fast)
    E + B → C +2D(slow)
  • B + B → E (slow)
    E + A → C + D(fast)

• Answers for Sample Questions:
  1. C: because the metals are solid while the reactants and products are gases. This is a heterogeneous catalyst. Catalysts are NOT used up in the reaction and can be reused.
  2. A: because NO is added in the mechanism in step 1 as a “reactant,” but is reformed in step 2 on the “product” side. This is how catalysts will often appear in kinetics mechanisms.
  3. B: because Ce ⁴⁺ and Tl⁺ are reactants (added and never reformed). Mn ²⁺ is a catalyst because it is added in step 1 but reformed in step 3. Mn ³⁺ and Mn⁴⁺ are intermediates because they are formed and then used in later steps. Ce ³⁺ and Tl³⁺ are both formed as products.
  4. C: because the graph of 1/(molarity) gives a straight best fit line. This aligns to another objective in kinetics. Good to show connection between objectives.
  5. A: because it is second order according to the slow step in the mechanism. When the two steps are added together, it equals the overall reaction which is also a requirement for a mechanism to be acceptable for a reaction.

• Resources: Free ChemMatters article on Ozone. This would be a more effective tool as a follow up after the demo, lesson, and activity. This way, students can gather the data and analyze the graphs without already knowing some of the answers. The article is nice to reinforce that students have learned from the activity. The Teacher’s Guide includes resources and questions for the article. Additionally, this National Geographic article discusses how the ozone hole has finally starting to reduce in the size over the last few years.

• Demo Resources:
  The demo included is a modified version of Flinn Scientific’s Pink Catalyst. A video for teachers about the demo is also available from Flinn.