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Making Connections in Kinetics, Equilibrium and Thermochemistry (8 Favorites)

LESSON PLAN in Catalysts, Entropy, Equilibrium Constants, Enthalpy, Activation Energy, Energy Diagrams, Reaction Quotient, Spontaneous vs. Non-spontaneous Reactions. Last updated October 19, 2018.


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)

AP Chemistry Curriculum Framework

  • Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matter.
    • 5.18 The student can explain why a thermodynamically favored chemical reaction may not produce large amounts of product (based on consideration of both initial conditions and kinetic effects), or why a thermodynamically unfavored chemical reaction can produce large amounts of product for certain sets of initial conditions.
  • Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
    • 6.25 The student is able to express the equilibrium constant in terms of Δ Go and RT and use this relationship to estimate the magnitude of K and, consequently, the thermodynamic favorability of the process.

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

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

*Graph Created by Christine Taylor

  1. Slowly add a small scoop of MnO2 to the H2O2 in the volumetric flask.
  2. The reaction will quickly occur decomposing the hydrogen peroxide and releasing copious amounts of water vapor, oxygen gas and heat.
  3. Ask the students “Do you think this reaction is under thermodynamic or kinetic control?”
  4. 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.

For the Student

Download all documents for this lesson, including the teacher guide, from the "Downloads box" at the top of the page.