Summary

In this lesson students will understand the connections between the equilibrium constant (K) and the reaction quotient (Q) as well as how they determine the favorability of a reaction. Additionally students will be able to determine if a reaction is kinetically favored or thermodynamically favored.

Grade Level

High School (AP Chemistry)

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
• HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

AP Chemistry Curriculum Framework

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

• Unit 5: Kinetics
  • Topic 5.6: Reaction Energy Profile
    • TRA-4.C: Represent the activation energy and overall energy change in an elementary reaction using a reaction energy profile.
• Unit 7: Equilibrium
  • Topic 7.4: Calculating the Equilibrium Constant
    • TRA-7.B: Calculate Kc or Kp based on experimental observations of concentrations or pressures at equilibrium.
• Unit 9: Applications of Thermodynamics
  • Topic 9.3: Gibbs Free Energy and Thermodynamic Favorability
    • ENE-4.C: Explain whether a physical or chemical process is thermodynamically favored based on an evaluation of ∆G^o.
  • Topic 9.4: Thermodynamic and Kinetic Control
    • ENE-4.D: Explain, in terms of kinetics, why a thermodynamically favored reaction might not occur at a measurable rate.
  • Topic 9.5: Free Energy and Equilibrium
    • ENE-5.A: Explain whether a process is thermodynamically favored using the relationships between K, ΔG^o, and T.

Objectives

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

• Use K vs. Q to determine the direction a reaction proceeds.
• Understand how equilibrium relates to favorability of a reaction.
• Determine the favorability of a reaction based upon kinetics and thermochemistry.

Chemistry Topics

This lesson supports students’ understanding of

• Equilibrium
• Equilibrium Constant
• Reaction Quotient
• Kinetics
• Activation Energy
• Catalysts
• Thermodynamic Favorability
• Energy Diagrams
• Enthalpy
• Entropy
• Spontaneity
• Free Energy

Time

Teacher Preparation: 30 minutes (Preparing and practicing the demonstration)

Lesson: Activity: 60-90 minutes

Demo: 30 minutes including discussion

Materials

• Demo Materials
• 30% H ₂O₂ (50 mL per demo)
• MnO₂ solid (a few grams per demo–exact amount not required)
• 250 mL volumetric flask
• 50 or 100 mL graduated cylinder (to measure peroxide solution)
• Scoopula or spoon to obtain solid catalyst
• Flinn Disposal #26 is a disposal option. (*If you do not have a Flinn Reference Catalog please order a free copy.)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• 30% Hydrogen Peroxide is a strong oxidizer, please read the safety sheet prior to handling this chemical.
• Wear gloves when using 30% Hydrogen Peroxide.

Teacher Notes

• This lesson is intended to help students make connections between the following concepts: equilibrium, thermochemistry and kinetics. The students should have knowledge of general equilibrium, kinetics, catalysts, enthalpy and entropy before completing the activity.

Lesson Outline:

• The demonstration should follow the Guided Inquiry Activity as a formative assessment of the students understanding of the lesson.
• Guided Inquiry Activity: Determining Reaction Favorability (student handout provided). Randomly divide students into teams of 2-4. Students should work through this activity together and divide the work among the members as noted in the activity.
• An Answer Key document has been provided for teacher reference.
• After the groups have finished, discuss the answers and be certain all questions have been addressed.
• Demo: Catalyzed decomposition of hydrogen peroxide using manganese dioxide.
• *Note: ALWAYS practice a demonstration prior to attempting in front of students.
• Video of Demonstration for reference.
• Procedure:

1. Introduce the demo by drawing the potential energy diagram for H ₂O₂ without a catalyst on the board.

*Graph Created by author

2. Ask the students “What is the sign of ∆H? Why?”
3. Confirm the student’s choice. Answer: the sign of ∆H is negative, the products are lower in energy than the reactants so energy is released.
4. Pour some (50 mL) hydrogen peroxide (30% preferred) in a 250 mL volumetric flask.
5. Ask the students “What do you observe?” Answer: students may observe some small bubbles, the solution is clear.
6. Ask the students “What is the sign of ∆S? Why?
7. Confirm the student’s choice. Answer: the sign of ∆S is positive, the reactants include one aqueous solution and the products contain liquid water and oxygen gas, disorder increases.
8. Ask the students “Considering the sign of ∆H and of ∆S, predict the sign of ∆G, why?”
9. Confirm the student’s choice. Answer: the sign of ∆H is negative and the sign of ∆S is positive which means the sign of ∆G is negative at all temperatures.
10. Ask the students “Considering the sign of ∆G is negative, what does this indicate about the thermodynamic favorability of this reaction?”
11. Students should say “thermodynamically favorable”
12. Ask the students “If this reaction is thermodynamically favorable, why do we not see a reaction taking place?”
13. Students should say things like “it is slow, has a high activation energy, needs a catalyst, etc.
14. Introduce the Manganese Dioxide catalyst to the students and ask “what will adding the catalyst do to the graph on the board?
15. Confirm the student’s choice by adding the catalyst line on the graph lowering the activation energy.

*Graph Created by Christine Taylor

16. Slowly add a small scoop of MnO₂ to the H₂O₂ in the volumetric flask.
17. The reaction will quickly occur decomposing the hydrogen peroxide and releasing copious amounts of water vapor, oxygen gas and heat.
18. Ask the students “Do you think this reaction is under thermodynamic or kinetic control?”
19. Confirm the student’s choice. Answer: Due to the high activation energy and the fact that the reaction is spontaneous but does not visibly occur without a catalyst, the reaction is under kinetic control.

• Additional Resource Suggestions:
  • Try this Predicting Shifts in Equilibrium Quiz with students from AACT.
  • Use this Free Energy and Temperature simulation to reinforce concepts (flash needed).