Summary

In this lab, students will explore Le Châtelier’s Principle using non-toxic materials, while still visualizing the equilibrium shifts through color changes. Traditionally, equilibrium experiments and Le Châtelier’s Principle are illustrated using chemicals that undergo color changes as the equilibrium position shifts such as cobalt (IV) chloride and iron (III) thiocyanate. While these reactions effectively demonstrate Le Châtelier’s Principle, they utilize reagents that are toxic.

Grade Level

High School

NGSS Alignment

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

• 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:
  • Analyzing and Interpreting Data
  • Constructing Explanations and Designing Solutions
  • Engaging in Argument from Evidence

AP Chemistry Curriculum Framework

This lab supports the following units, topics and learning objective:

• Unit 7: Equilibrium
  • Topic 7.9: Introduction to Le Châtelier’s Principle
    • 7.9.A: Identify the response of a system at equilibrium to an external stress, using Le Châtelier’s principle.

Objectives

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

• Explain the concept of chemical equilibrium.
• Analyze how changing temperature can affect the equilibrium shift of a chemical reaction.
• Interpret how changing concentration can affect the equilibrium shift of a chemical reaction.
• Explain the “greenness” of the chemical reactions using the 12 principles of green chemistry.

Chemistry Topics

This lab supports students' understanding of:

• Equilibrium
• Le Châtelier’s Principle
• Chemical Reactions
• Reversible Reactions
• Temperature
• Concentration

Time

Teacher Preparation: 30 minutes

Lesson: 45 minutes

Materials

Per group:

• 6 test tubes
• Test tube rack
• 30 mL of butterfly pea tea
• 15 drops of vinegar or lemon juice
• 1 g of baking soda
• 3 drops of tincture of iodine
• Spray starch
• Ice
• Hotplate/electric kettle
• 1 small (100-mL) beaker
• 2 larger (~400 mL) beakers (for hot and cold water baths)
• Stirring rod
• Thermometer
• 2 plastic pipettes
• 100 mL graduated cylinder
• 1 scoopula
• Balance

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.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
• Iodine is a minor eye irritant and vinegar can cause skin irritation.

Teacher Notes

• To learn even more about using this lab in your classroom, read the article, Designing a Greener Le Châtelier’s Principle Lab , published in the March 2020 issue of Chemistry Solutions.
• This lab is written for a general chemistry level but can be updated for AP Chemistry by including discussion of Q vs. K for each of the activities.
• Starch solution: Spray starch into a 300 mL beaker (cover the bottom of the beaker with spray or use 5-10 drops of liquid starch). Fill the remainder of the beaker with water and stir. Be sure to check the ingredients list when purchasing spray starch to ensure starch is one of the primary ingredients.
• Butterfly Pea tea: Prepare 1000 mL of butterfly pea tea—the darker, the better. Prepare with electric kettle and allow it to cool to room temperature.
• Additional reagents that may be used for student inquiry include black tea (substitute for the butterfly pea tea) and lemon juice (substitute for the vinegar).
• Activity 1 uses an iodine-starch complex to demonstrate the effect of temperature on equilibrium. The formation of the complex is an exothermic reaction and results in a deep purple color.

Iodine (aq) + Starch (aq) ↔ Starch-Iodine Complex (aq)

• The Iodine-starch complex is colorless, and the starch-iodine complex is blue-black.

• The soluble starch acts as an indicator of molecular iodine. In the above reaction, the “shifts” in equilibrium position produced by temperature changes are in accordance with Le Chatelier’s principle. Enthalpy, or ΔH, is negative (exothermic) in the forward direction as written above because heat is being released. Cooling the system, or taking away heat, causes a “shift” to the right, resulting in the formation of the blue-black starch-iodine complex. Adding heat to the system causes a “shift” in the reverse direction (ΔH is positive) and the starch-iodine complex dissociates in an endothermic reaction, causing the system again to become clear and colorless.
• In Activity 2, students evaluate the effect of pH on equilibrium using butterfly pea tea, baking soda, and vinegar. When vinegar is added to the tea, the color of the solution lightens and changes from blue to purple. When baking soda is added to the tea, the solution darkens and turns a blue/green color. This can be reversed, and students can change the tea solution in color to observe the shift in equilibrium. The lab presents this reaction in the format of:

Tea (aq) + H⁺ (aq) ↔ TeaH⁺(aq)

• The Tea (aq) + H⁺ (aq) is blue/green, and the TeaH⁺(aq) is purple.
• Butterfly Pea Tea can act as a base and react with an acidic solution (H ⁺) to form the conjugate acid, a complex with the tea and hydrogen ion. In this lab, vinegar will be the source of the hydrogen ions. In addition you will observe how the color changes when a base, baking soda, NaHCO ₃ is added. When NaHCO₃ is added, it can accept a hydrogen ion, decreasing its concentration in solution.
• Depending on the level of your students, you may prefer to share more information with them. Supplemental information is available for teacher download and use.
• An Answer Key has been provided for teacher reference.
• Special Acknowledgement: Thank you to a collaboration from a 2008 Beyond Benign Green Chemistry workshop for the original creation of this lab and all of the many edits made over the years from curriculum specialists, lead teachers, and workshop participants.