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.
High School (AP Chemistry)
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 Qc or Qp, for a reversible reaction, and the corresponding equilibrium expressions Kc=Qc or Kp=Qp.
- Topic 7.4: Calculating the Equilibrium Constant
- TRA-7.B: Calculate Kc or Kp based on experimental observations of concentrations or pressures at equilibrium.
- Topic 7.5: Magnitude of the Equilibrium Constant
- TRA-7.C: 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
- TRA-7.F: Represent a system undergoing a reversible reaction with a particulate model.
- 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.
- Topic 7.2: Direction of Reversible Reactions
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.
This project supports students’ understanding of:
- Equilibrium constant
- Reaction quotient
Teacher Preparation: 10 minutes
Lesson: 40-60 minutes
- 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
- No specific safety precautions need to be observed for this activity.
- 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.
For the Student
You work for a chemical company where the following exothermic chemical reaction is taking place: N2(g) + 3H2(g) ↔ 2NH3(g), ∆H∘=–92 kJ/mol. The products of the reaction will eventually be cooled into a liquid and used in a cleaning product.
Your team has been asked to do a short presentation on the contents of the reaction vessel at four different times during the reaction.
- 0 seconds
- 10 seconds
- 20 seconds
- 30 seconds
Your boss has asked your team to use the next hour to create a rough draft (using a white board separated into four sections as seen below) of a particulate-level diagram of the reaction vessel at each point in time. She has given you some conditions to help you in your work:
- The reaction vessel should start with four particles of N2 and ten particles of H2.
- The reaction takes place at a temperature where there are two particles of N2 remaining at equilibrium.
- The reaction is known to reach equilibrium after 20 seconds.
Please work out your particulate-level diagrams on your whiteboards and once your group has agreed on a consensus model for each diagram, copy them into the boxes below.
A. 0 seconds
B. 10 seconds
C. 20 seconds
D. 30 seconds
- Calculate the Kc value for this reaction.
- What does this Kc value tell you about the relative amounts of products and reactants at equilibrium?
- What can you say about the relationship between the forward rate and reverse rate for this equilibrium reaction at each of the four times?
- 0 seconds:
- 10 seconds:
- 20 seconds:
- 30 seconds:
- Explain what Q means in relation to Kc. What does it mean for the reaction when Q > Kc? When Q < Kc? When Q = Kc?
- Which diagram(s) would represent a Q value and which diagram(s) would represent a Kc value? Calculate the Q values for the diagram(s) representing Q values.
- What happens to the value of K when the temperature changes?
- What are two potential strategies you would recommend to increase production of the NH3?