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Enthalpy and Entropy as Driving Forces (33 Favorites)

LESSON PLAN in Entropy, Gibb's Free Energy , Enthalpy. Last updated February 1, 2021.


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

  • 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 9.2: Absolute Entropy and Entropy Change
      • ENE-4.B. Calculate the entropy change for a chemical or physical process based on the absolute entropies of the species involved in the process.
    • 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 ∆Go.
    • 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.

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 
  • Stirring rod, glass
  • 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:

  • Citric acid (approx. 10 rounded tsp). (Note: citric acid can be obtained in the canning section of a grocery store. You can also use lemon juice (400 mL) – in that case you would not need to add water.
  • Baking soda (approx. ½ of 14-ounce box, or 200 g)
  • Ziploc sandwich baggies, 10
  • Scoopula or plastic spoon
  • 50-mL graduated cylinder (1 per station)
  • NOTE: An alternative is to use vinegar, however using the baggie and citric acid makes it easier to feel the temperature change. (See “Teacher Document (Original Version)” in the Downloads section for this option).

Station 2:

  • Steel wool, fine (5 full pieces cut in half)
  • thermometer
  • Vinegar, 600 mL
  • Cups, 2
  • Saran wrap, enough to cover cup with thermometer
  • Note: an alternative version calls for touching a 9-V battery to the steel wool instead of immersing in vinegar. This does work very well, but extreme caution must be used as the battery tends to get significantly hot if used too often (if class is large). (See “Teacher Document (Original Version)” in the Downloads section for this option).

Station 3:

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

Station 4:

  • Hydrogen peroxide, 200 mL
  • Potato slices, 20 (or process potato in food processor and use ‘mash’)
  • Test tubes, 1 per station (a small plastic cup could be used as well)
  • Plastic spoon if using processed potato
  • Note: The alternative version calls for burning a candle, but using the potato (catalase) allows for the question on kinetic control. (See “Teacher Document (Original Version)” in the Downloads section for this option).

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

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.
  • 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

  • Note that this lesson plan was updated by the author in January 2020. Changes have been made to the materials used at each station. If needed, you can find the original version of the lesson plan in the “Download” section of the sidebar.
  • The consideration of thermodynamic driving forces serves to unify the concepts in Units 5 and 9. Please see the document Supplemental Teacher Notes available in the sidebar 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 (Topics 9.1 – 9.4 in the 2020 Course Exam and Description).
    • 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 Applications of Thermodynamics (Unit 9). 
  • 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 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 2nd Law of Thermodynamics deals with the tendency towards increasing entropy. Entropy increases when matter becomes more dispersed (ENE-4. A.1). Entropy increases when energy is dispersed (ENE-4. A.2).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 Supplemental Teacher Notes for an overview of these topics.
  • 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 citric acid (and baggies, scoopulas, and graduated cylinder)
    • Station 2: Steel wool, cups, vinegar, thermometer, Saran wrap
    • Station 3: Ice cube
    • Station 4: Hydrogen peroxide, potato, test tube or small cup
  • 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.  
  • 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 5th 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. 

For the Student

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