Summary

In this activity, students will gain a better understanding of how applying a stress to a reaction system will shift the equilibrium. The students will be able to predict the direction a reversible reaction will shift based of the value of the reaction quotient (Q) and the equilibrium constant (K). This activity should be completed after students have completed the activity “Equilibrium Particulate View.”

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
    • TRA-6.B: 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
    • TRA-7.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 7.8: Representations of Equilibrium
    • TRA-7.F: Represent a system undergoing a reversible reaction with a particulate model.
  • Topic 7.9:Introduction to Le Châtelier’s Principle
    • TRA-8.A: Identify the response of a system at equilibrium to an external stress, using Le Châtelier's principle.
  • Topic 7.10: Reaction Quotient and Le Châtelier’s Principle
    • TRA-8.B: 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 reversible reaction as various conditions change.
• Determine how changes in various conditions affect the value of Q for a reversible reaction.
• Determine the direction a reaction will shift based on the value of Q in comparison to K.
• List what stressors on a reaction do and do not cause a change in K.
• Explain why certain stressors cause the changes they do in a reversible reaction.

Chemistry Topics

This project supports students’ understanding of:

• Reaction quotient
• Equilibrium constant
• Le Châtelier’s Principle

Time

Teacher Preparation: 10 minutes
Lesson: 80-120 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, reaction quotients, and Le Châtelier’s principle.
• “Equilibrium Particulate View” is a precursor to this activity. The particulate model at equilibrium from that activity is the starting point for this activity, and the concentrations are included in the student instructions for this activity if you do not have them complete the previous activity first.
• As the students finish each model 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 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.
• My suggestion would be to complete Diagrams 1 – 4 in one 40- or 60-minute period and Diagrams 5 – 8 in another 40- or 60-minute period. The students may really struggle with Diagrams 5 – 8. The group discussions will be very important for student understanding.
• 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:
  • In Diagram 2, I noticed your group found that Q > K_c. Can you explain how came to the conclusion?
  • I can see that in Diagram 5 your group stated the value of Q will increase due to a decreasing pressure. Can you tell me a little bit more about that?
• Common Misconceptions & Issues:
• On Diagram 5 & 6 the student may struggle to show the concept that the reaction vessel is getting larger or smaller because the box is already pre-drawn. Just tell them they need to find a way to represent the change in the reaction vessel size (by drawing a larger or smaller box over part of that diagram’s table, for example, or drawing only half of the particles inside of the box and the other half outside of the box to show the reduced concentration). The key is to recognize that by changing the volume of the container, you are also changing the concentrations (and partial pressures), which changes Q.
• When pressure changes it is important that students understand that equilibrium will only be affected if the pressure of one or more of the reactants/products changes. After Diagram 5 & 6 you may want to ask the students what would happen if Argon gas was accidentally inserted into the reaction vessel. They should come to the conclusion that the concentrations/partial pressures of the reactants and products will not change if another gas is added to the container, so neither will Q or K_c.
• It is important that students understand the reason that a reaction shifts when temperature is changed is because the K_c value changes. You may instruct them to read a section in the textbook if they are struggling with Diagrams 7 & 8.
• After Diagram 7 & 8 I would recommend having a discussion about what would happen is the reaction was actually endothermic. Would the increase/decrease in temperature have the same effect on the system? They should come to the conclusion that the effect on the system would be opposite what occurred in the exothermic reaction.
• 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.
• If students are still struggling with these concepts, especially the more challenging Diagrams 5-8, it could be helpful to review with this (or a similar) video. However, as mentioned before, it is important that students engage in that productive struggle first, so give them plenty of time to work through it on their own before showing a video.
• Following the activity the student should complete this quiz to test their knowledge.