Summary

In this lesson students practice applying Q vs K as an explanatory tool in a simulation and demonstration. In both activities, students will consider how a change in concentration of one species subsequently effects all the species as equilibrium is reestablished.

Grade Level

High School (AP Chemistry)

NGSS Alignment

This lesson 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:
  • Using Mathematics and Computational Thinking
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

AP Chemistry Curriculum Framework

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

• Unit 7: Equilibrium
  • Topic 7.1: Introduction to Equilibrium
    • 7.1.A: Explain the relationship between the occurrence of a reversible chemical or physical process, and the establishment of equilibrium, to experimental observations.
  • 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.
• Unit 9: Thermodynamics and Electronegativity
  • Topic 9.3:Gibbs Free Energy and Thermodynamic Favorability
    • 9.3.A: Explain whether a physical or chemical process is thermodynamically favored based on an evaluation of ∆Go.

Objectives

By the end of this lesson, students should be able to

• Qualitatively assess how a stress on a system changes Q.
• Qualitatively compare Q vs K to determine how a system will respond to a stress.
• When appropriate, use Le Chatelier’s principle to explain how a system will respond to a stress.

Chemistry Topics

This lesson supports students’ understanding of

• Equilibrium
• Equilibrium Constant
• Reaction Quotient
• Le Chatelier’s Principle

Time

Teacher Preparation: 30 minutes

Lesson: 100 minutes (2 parts, 50 minutes each)

Materials

• Part 1: “Q, K, and Le Chatelier Part 1” handout
  • Computers with internet access for all students
  • If access is an issue, the simulation can be projected for use as class discussion
• Part 2: “Q, K, and Le Chatelier Part 2” handout
• Demonstration:
  • Milk of magnesia, 20 mL
  • 3 M HCl, about 20 mL
  • 4-5mL of universal indicator (could use a pH meter but it’s not as fun to watch)
  • Universal indicator color chart (if not available, write the colors on the board as a point of reference for students)
  • Water, up to 800 mL (tap is acceptable)
  • 1L beaker (or other suitably large, clear vessel)
  • Graduated cylinders
  • Ice (a 250 mL beaker filled with crushed ice is sufficient)
  • Magnetic stirrer and stir bar
  • Pipets

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Wash hands thoroughly before leaving the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Resources

• AACT Simulation: Predicting Shifts in Equilibrium Quiz Q vs K
  • This would be a great informal assessment before starting these activities to ensure students understand the basic relationship between Q and K
• Flinn version of the Milk of Magnesia demonstration
  • This is the procedure I generally follow for the demonstration
• AACT version of the Milk of Magnesia demonstration
  • This procedure might be better if 3M HCl is not conveniently available
• Part 1 Simulation: Disturbing a Chemical Equilibrium (flash required)

Teacher Notes

• I use the textbook Chemistry: The Central Science, 12^th edition, by Brown et al. I like to do this activity as the opener for Chapter 16, Acid-Base Equilibrium. That said, we cover K_sp (Ch17) and free energy (Ch19) in earlier chapters so my students are already familiar with those topics at the start of Ch16.
• To make the activity more broadly applicable, I have used just K instead of K_sp and K_A, please substitute those if it would be appropriate for your students.
• I tell my students that Q is always an acceptable way to examine the direction a reaction will move while Le Chatelier’s principle is only applicable to a system disturbed at equilibrium. I then tell them that if they are not sure, Q vs K is always a good choice. Recent examples where students have had to discuss Q vs K are listed below.
  • 2016 Operational Exam, FR#6
  • 2016 Secure Exam, FR#1
    • only accessible in your CB Audit page
  • 2018 Operational Exam, FR#5
• This activity briefly incorporates free energy and rate constants during the Part 1 activity. If appropriate for your pacing, those discussions could be continued through the Part 2 demonstration.
• Students will need to know the equations . If you have not covered these yet, you could easily omit those questions from the activities. If they were not covered recently, you could provide the equations on the board for students to reference.
• If you are short on time, the Part 1 activity could be completed independently for homework. You could open class with a quick discussion of the answers before jumping into the demo.

Lesson Outline:

• Part 1: “Q, K, and Le Chatelier Part 1”
• There is a student handout for this activity and an answer key document for teacher reference.
• Students will use a simulation in the activity. Ideally, they will work in small groups while you walk the room and facilitate. It might be helpful, especially if this is a review activity, to go over the Pre-Activity Questions as a whole class to make sure everyone has the basics before exploring further.
• Additional (optional) resource: This is another great equilibrium simulation that neatly illustrates K, k, and Q. The simulation requires a little more thinking so it might be a useful homework assignment as part of this activity.
• Part 2: “Q, K, and Le Chatelier Part 2”
• There is a student handout for Part 2 and an answer key document for teacher reference.
• You will lead a demonstration while facilitating a discussion with the students. The procedure is below with notes about when to pause for students to answer questions from the Student Handout.
• When you reach a pause point, let students write answers, then you will lead a discussion addressing the questions. Students often need time to revise, or add to, their written answers. Then continue and repeat.
• A suggested amount of time to pause at each point has been included. This will vary based on the dynamics of your students and their preparedness.
• There are additional details about running this demo and discussion in the Answer Key.
• The ice is only needed to slow down the reaction which makes the colors more fun to watch. If a student has not already asked by the end of the demo, then I often ask them why the ice is in the beaker. It is a great kinetics refresher if you have completed that unit.
• The demo can be scaled down if that would better suit your supplies.
• There is usually enough magnesium hydroxide left such that the system will reach a pH of around 7 if you let it sit for a few hours. This would be a great follow-up discussion if you take a picture and show it to your students the next day. Conveniently, it also means you can just pour the solution down the drain at that point.
• Procedure: This is what the teacher will do while students answer questions from the Student Handout.

1. Gather supplies.
   1. 20 mL of milk of magnesia in a clear container so students can observe it
   2. ~250 mL of ice, crushed is great, cubed will also work
   3. ~700 mL of water, tap is fine
   4. ~20 mL of 3M HCl
   5. Pipet for adding the HCl to the beaker
   6. 1 L beaker or other large, clear vessel for the reaction
   7. ~4 mL of universal indicator and a color chart for the kids to see
   8. Magnetic stir plate and stir bar
2. Pause and let students observe the milk of magnesia and answer questions 1-5 (~10 minutes). Add a few drops of the universal indicator.
   1. Ideally, kids will discuss amongst themselves and then you will go over the answers with them before moving on to the next step.
3. Add the milk of magnesia to the beaker and fill with water and ice to about 800 mL.
4. Add the stir bar and start stirring. Add the rest of the universal indicator. Pause again as kids will take more observations and answer question 6 (~10 minutes).
   1. Again, let them answer and then go over answers together to make sure they understand before moving on.
   2. Universal indicator does not distinguish between pH values above 10. Depending on where you are in the curriculum, it might be useful to discuss that the only real conclusion from the indicator is that the concentration of hydroxide ion stays above 10^-4 M.
5. Add one pipet full of HCl. Pause to let students record observations and answer question 7 (~10 minutes).
   1. You may need access to a whiteboard to write out equations to make sure they understand the effect of hydrogen ions.
6. Repeat the HCl addition two more times. Pause and let students record observations and answer question 8 (~5 minutes).
   1. Question 8 should go quickly.
7. Repeat the HCl additions until the solution turns clear. It is usually in the acidic range at this point. Students will record final observations and answer questions 9-11.
   1. This question could take a long time as kids try to apply what they know and come up with a procedure. It may be helpful for you to go over the answers to questions 9 & 10 first and then give the students plenty of time to brainstorm and compose an answer for question 10.