Summary

In this activity, students will gain a better understanding of what it means for a reaction to be in a state of equilibrium and how a reaction progresses over time to establish equilibrium. Students will also relate the equilibrium constant to the amount of products and reactants present at equilibrium.

Grade Level

High School (AP Chemistry)

NGSS Alignment

This project will help prepare your students to meet 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:
  • Using Mathematics and Computational Thinking
  • Developing and Using Models

AP Chemistry Curriculum Framework

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

• Unit 7: Equilibrium
  • Topic 7.2: Direction of Reversible Reactions
    • 7.2.A: Explain the relationship between the direction in which a reversible reaction proceeds and the relative rates of the forward and reverse reactions.
  • Topic 7.3: Reaction Quotient and Equilibrium Constant
    • 7.3.A: Represent the reaction quotient Q_c or Q_p, for a reversible reaction, and the corresponding equilibrium expressions K_c=Q_c or K_p=Q_p.
  • Topic 7.4: Calculating the Equilibrium Constant
    • 7.4.A: Calculate K_c or K_p based on experimental observations of concentrations or pressures at equilibrium.
  • Topic 7.5: Magnitude of the Equilibrium Constant
    • 7.5.A: Explain the relationship between very large or very small values of K and the relative concentrations of chemical species at equilibrium.
  • Topic 7.8: Representations of Equilibrium
    • 7.8.A: Represent a system undergoing a reversible reaction with a particulate model.
  • Topic 7.10: Reaction Quotient and Le Châtelier’s Principle
    • 7.10.A: Explain the relationships between Q, K, and the direction in which a reversible reaction will proceed to reach equilibrium.

Objectives

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

• Draw particle-level diagrams modeling a reaction as it approaches equilibrium.
• Calculate Q and K values for a reversible reaction as it approaches equilibrium.
• Explain what happens to the forward and reverse reaction rates once a reaction reaches equilibrium.
• Explain how K relates to the amount of products and reactants present at equilibrium.

Chemistry Topics

This project supports students’ understanding of:

• Equilibrium constant
• Reaction quotient

Time

Teacher Preparation: 10 minutes
Lesson: 40-60 minutes

Materials

• Student Handout
• One 2-foot by 2-foot whiteboard per group (or 1 sheet of regular paper per diagram can be used in place of whiteboards)
• Dry-erase markers and eraser or rag

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• Students should be working in groups of three or four.
• Prior to this activity, students should have been introduced to the concepts of equilibrium, equilibrium constants, and reaction quotients. Question 6 would require students to have had at least a basic introduction to Le Châtelier’s principle as well.
  • The follow-up activity, “Le Châtelier’s Principle Particulate View,” goes into more detail on the effects of stressors on a reversible reaction and how the system will need to adjust to return to equilibrium.
• As the students finish each of the four models you may find it useful to do the following:
  • Have the students do a gallery walk to observe what the other groups have created. (You may want to have one of the student in each group stay with the diagram to answer questions from other class members.)
  • Give the groups time to modify the models.
  • Create a consensus model with the class.
• While the students are working on their models, you should be walking around the room and engaging in conversations about the students’ models. It is good to ask the students questions about both things they understand as well as areas where they may have some misconceptions. Example questions include:
  • I noticed at equilibrium (20 seconds) you chose to draw an equal amount of reactants and products. Can you tell me a little more about why you chose to do that?
  • Why did you chose to make your drawings at 20 seconds the same as your drawing at 30 seconds?
• Be careful not to give away too much to the students. The goal is for them to engage in some productive struggle in order to develop and master the concepts. Each consensus model should be student-developed.