Summary

In this activity students observe various chemical and physical processes to qualitatively predict and explain the signs of ∆S and ∆H. Based on their observations, they will predict the sign of ∆G and will determine the driving force of the process. Students will then calculate ∆S, ∆H and ∆G. This lesson focuses on thermochemical predictions, calculations and explanations.

Grade Level

High School (AP Chemistry)

AP Chemistry Curriculum Framework

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

• Unit 6: Thermodynamics
  • Topic 6.1: Endothermic and Exothermic Processes
    • ENE-2.A:Explain the relationship between experimental observations and energy changes associated with a chemical or physical transformation.
• Unit 9: Applications of Thermodynamics
  • Topic 9.1: Introduction to Entropy
    • ENE-4.A: Identify the sign and relative magnitude of the entropy change associated with chemical or physical processes.
  • 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.

Objectives

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

• Observe a chemical or physical process, and predict the sign of the entropy change.
• Observe a chemical or physical process, and predict the sign of the enthalpy change.
• Given the signs of both ΔH° and ΔS°, determine if a process is thermodynamically favorable, and if that favorability occurs at higher or lower temperatures.
• Calculate ΔH°, ΔS°, ΔG°.
• Relate the significance of the signs of ΔH° and ΔS° to thermodynamic favorability.
• Predict what will happen in terms of thermodynamic favorability if temperature of system is increased or decreased (how the value of ΔG is affected).

Chemistry Topics

This lesson supports students’ understanding of:

• Enthalpy
• Entropy
• Gibb’s free energy
• Thermodynamic favorability

Time

Teacher Preparation: 15 minutes for demo; 30 minutes for activity

Lesson:

• Introductory demonstration/discussion: 15 minutes
• Culminating activity: 45 minutes for student observations; 60 - 90 minutes to perform calculations and answer questions.

Materials

Demo (for each class):

• Aluminum foil, 6” x 6”, 2 pieces, loosely crumpled
• 0.5 M sodium chloride solution, 100 mL
• 0.5M copper (II) chloride, 100 mL
• Glass Stirring rod
• 250-mL beakers, Pyrex, 2 OR 100-mL graduated cylinders, Pyrex, 2

Activity (amounts estimated for one class of 30 students working in 10 groups of 3 students each):

Station 1:

• Vinegar, 400-mL
• Baking soda, approx. ½ of 14-ounce box, or 200 g
• Paper cups, 10 (Note: paper cups, such as bathroom cups, must be used for students to feel the solution getting colder. Do not use Styrofoam cups. An alternative would be to provide a thermometer as well.)
• Scoopula or plastic spoon
• 1, 50-mL graduated cylinder

Station 2:

• Steel wool, fine (10 small clumps)
• 1, 9V battery

Station 3:

• 6 Ice cubes (keep some in a Styrofoam cup to set out for students who get to this station later)

Station 4:

• Candle
• Matches

Optional: “Heat Solution” hand warmer (or any supersaturated sodium acetate-based hand warmer)

• Available from Flinn
• Available from Amazon

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Always use caution around open flames. Keep flames away from flammable substances.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
• Open flames can cause burns. Liquid wax is hot and can burn the skin.
• When lighting the match and wooden splint, be cautious with the flame.
• An operational fire extinguisher should be in the classroom.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.

Teacher Notes

• The consideration of thermodynamic driving forces serves to unify the concepts in Big Idea 5. Please see the provided supplemental document for an overview of these concepts.
• This resource:
  • provides a demo to introduce the unit. The demo can be referred back to as the unit progresses.
  • provides teaching notes which identify the relevant AP Chemistry Essential Knowledge standards (EK 5.E. 1 – 5).
  • provides, as the primary focus of the resource, a culminating activity which enables students to make connections between something they observe happening (common occurrences that they are familiar with) to calculations they have been performing (∆H, ∆S, ∆G) in this unit.
  • provides students with valuable practice (in terms of the AP exam) with making predictions, providing explanations, and justifying answers.
• Lesson Outline:
• Engagement/demo:
  Use the following demonstration to introduce the unit on thermodynamics (Big Idea 5). Note: This is a shortened version of the Flinn demo “Foiled Again” (Parts 1 and 3 only). If desired, free background information for this demo can be obtained here from Flinn.

• Demonstration:
  System 1:
  1. Measure approximately 90-mL 0.5 M copper (II) chloride in either a 250-mL Pyrex beaker or a 100-mL graduated cylinder. (Note: if a larger set up is desired, use 300-mL solution in 600-mL beaker.)
  2. Place crumpled aluminum foil into solution, push down with stirring rod.
  3. Observe signs of chemical reaction taking place.
• System 2:
  1. Repeat using sodium chloride solution instead of copper (II) chloride.
  2. No reaction will ensue.

• Discuss/Elicit: What just happened? Why did one of these systems result in a reaction and one did not? Chemical and physical processes don’t simply “occur”. There must be some sort of driving force.
• Conclude: Thermodynamic favorability is a component of thermodynamics. The laws of thermodynamics describe the essential role of energy and explain and predict the direction of change in matter (Big Idea 5). For a chemical or physical process to occur, a change in enthalpy or entropy (or both) must drive the process.

• Throughout this unit, refer back to this demo.
  • For example, when discussing enthalpy change and system/surroundings:
    • What must the sign of ∆H have been? What evidence is there for this conclusion?
    • What would we expect to observe for a system with a (+)∆H? Explain.
    • Not all reactions are exothermic. If endothermic reactions occur, what does this tell us? (there must be another driving force)
  • When discussing entropy:
    • Can we make any conclusions about the entropy change of this system simply from making observations? Explain.
• Instruction: Please see the provided supplemental document Teacher Background Notes for an overview of Big Idea 5.E.1 – 5.
• Culminating Activity: Set up the following stations for students to rotate through. For a class of 30 students divided into groups of 3, you will need 3 of each station.
  • Station 1: Baking soda and vinegar (also place graduated cylinder, cups and scoopula here)
  • Station 2: Steel wool and battery
  • Station 3: Ice cube
  • Station 4: Candle (and matches)

Students will make observations, and then predict (based on their observations only) the signs of ∆H, ∆S, and ∆G. They will then be asked to identify what the driving force is behind the reaction or process. They will also draw energy diagrams to enable them to make connections with the content they have learned in this unit. An answer key has been provided for teacher reference.

• Optional Wrap up: Question #1 on student post-lab question section asks students to predict ∆H and ∆S for the crystallization of sodium acetate from a supersaturated solution.
  • A hand warmer demo could be performed as a closure to the activity. Follow this link for a hand warmer lesson that could easily be modified to a demo.
  • An alternative would be to make this a 5^th station if enough hand warmers are available.

• Assessment: These concepts presented in this resource appear frequently on the AP Chemistry Exam. The following FRQs provide excellent opportunities for student practice.
  • 2017 #2 (b – d)
  • 2009B #5 (c-e)
  • 2005B#7 (d-e)
  • 2002#8 (a-c)
  • 1993#8 (all)

Credit: National Math and Science Initiative for the original idea for creating this classroom resource.