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Discovering Equilibrium (13 Favorites)

LESSON PLAN in Le Châtelier's Principle, Establishing Equilibrium, Equilibrium Constants, Reaction Quotient. Last updated April 26, 2019.


Summary

In this lesson students manipulate sets of given conditions to discover what equilibrium is, and how the equilibrium is established from different starting conditions. Students then refer back to the activity as the foundation framework for the rest of Essential Knowledge 6.A, 6.B.1 and 6.B.2. This lesson focuses on both a qualitative and quantitative understanding of equilibrium.

Grade Level

High School (AP Chemistry)

AP Chemistry Curriculum Framework

  • Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
    • 6.1 The student is able to, given a set of experimental observations regarding physical, chemical, biological, or environmental processes that are reversible, construct an explanation that connects the observations to the reversibility of the underlying chemical reactions or processes.
    • 6.2 The student can, given a manipulation of a chemical reaction or set of reactions (e.g., reversal of reaction or addition of two reactions), determine the effects of that manipulation on Q or K.
    • 6.4 The student can, given a set of initial conditions (concentrations or partial pressures) and the equilibrium constant, K, use the tendency of Q to approach K to predict and justify the prediction as to whether the reaction will proceed toward products or reactants as equilibrium is approached.
    • 6.5 The student can, given data (tabular, graphical, etc.) from which the state of a system at equilibrium can be obtained, calculate the equilibrium constant, K.
    • 6.6 The student can, given a set of initial conditions (concentrations or partial pressures) and the equilibrium constant, K, use stoichiometric relationships and the law of mass action (Q equals K at equilibrium) to determine qualitatively and/or quantitatively the conditions at equilibrium for a system involving a single reversible reaction.
    • 6.7 The student is able, for a reversible reaction that has a large or small K, to determine which chemical species will have very large versus very small concentrations at equilibrium.
    • 6.8 The student is able to use Le Chatelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium.
    • 6.10 The student is able to connect Le Chatelier’s principle to the comparison of Q to K by explaining the effects of the stress on Q and K.

Objectives

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

  • Understand the concept of dynamic equilibrium.
  • Understand the meaning of K, and what its value signifies.
  • Compare K to Q in order to predict which direction a system must proceed to reach equilibrium.
  • Perform calculations involving K and manipulations of K.
  • Use a graph to recognize the establishment of chemical equilibrium.
  • Predict what will happen if a system at equilibrium is disturbed.

Chemistry Topics

This lesson supports students’ understanding of

  • Equilibrium
  • Reversible Reactions
  • Equilibrium Constant (K)
  • Reaction Quotient (Q)
  • ICE tables
  • Le Chatelier’s Principle

Time

Teacher Preparation: 5 minutes

Lesson:

  • Introductory activity and discussion: 45 minutes
  • Follow-up notes: several days depending on pace of class

Materials

  • Introductory activity:
    • Large piece of paper or white board (to collect class data)
    • Marker (to record class data)
  • Optional:
    • An optional version of this activity is included should you wish students to physically manipulate the reactions.
    • In this case, you will need approximately 300 small manipulatives for a class of 30.
    • These can be anything small: bingo chips, washers, pennies, paperclips, etc.

Safety

  • This activity does not require any special safety considerations.

Teacher Notes

  • The consideration of forward and reverse reactions and the meaning of K and Q serve to introduce the concepts in Big Idea 6. Please see the included document Teacher Background Notes for an overview.
  • In this lesson an activity is provided to introduce the concept of equilibrium. This activity can be referred back to as the unit progresses (examples provided below in lesson outline).
  • Teaching notes are provided which identify the relevant AP Chemistry Essential Knowledge standards (all of EK 6.A) for this topic.
  • An Excel spreadsheet is provided that the teacher can manipulate to make different equilibrium graphs for graphical analysis practice problems (including disturbances to systems at equilibrium, covered subsequently in EK 6.B)
  • A student note taker document and a teacher key for the note taker with teaching hints for a comprehensive coverage of EK 6.A are provided.
  • Lesson Outline:
  • Day 1: Introducing Equilibrium Activity (see #5 for optional version with manipulatives)
  1. Before students come to class, copy the following class data table onto a large piece of paper or a white board.

  1. Pass out the “Establishing Equilibrium” student worksheet. Have students read through the background notes and the sample procedure.Work through one to two rounds of the example activity to ensure students get the gist of what they are to do.
  2. Assign each pair of students one of the groups (A – J) to collect data on.If you have more than 10 pairs, you can make up any reactant and product initial amounts.You can even let an extra group or two select their own initial amounts, so that the students can see that there is nothing “special” about the initial amounts.
  3. As students complete their set of calculations, have them write their data on the class data table (see below for expected results). A copy of this data table has been provided on the student document as well.

  1. Optional Manipulative Version: Students can be given physical “reactants” and “products” to manipulate instead of performing all of the work in their calculators.
    1. Use the same group assignments (A – J starting conditions).
    2. Use groups of three students instead of two (one to ‘work’ the reactants, one to ‘work’ the products, and one to record results).If this is done, be sure that groups H-J are also covered for discussion purposes.
    3. Distribute small items to use for the reactants and products (examples given in material list).
    4. To differentiate between “reactants” and “products”, have students use two pieces of paper, one labeled “reactants” and the other labeled “products”.Have students physically move manipulatives from one “reaction vessel” to the other.
    5. Have students round to nearest whole number (since they cannot move a fraction of a chip).
    6. The results will be very similar:

  1. Once their data is recorded, have students begin working on the Analysis section of the worksheet. They can complete questions 1 – 5 using their data only, therefore do not need to wait for their classmates to finish.
  2. Once all groups have recorded their data, review the Analysis questions before proceeding to the Class Discussion. Students should have recognized that when equilibrium is reached, no observable changes occur in the system, that reactant and product molecules are present, that the concentration of all species remains constant, and that a graph of concentration versus time reveals when equilibrium is established (EK 6.A.a and f).
  3. The Class Discussion questions allow students to refine their understanding of the characteristics of a system at equilibrium, and how that equilibrium was achieved.This would also be an excellent time to introduce the concept of “equilibrium positions” (the actual concentrations of reactants and products at equilibrium, of which there are infinite possibilities) versus “equilibrium constant” (one value).
  • Day 2 and additional class periods:
  • The activity from Day 1 provides a foundation for the remainder of EK 6A, 6B.1 and 6B.2.Three documents have been provided for teacher use in covering these topics:
  • Topics covered include:
    • reversible reactions
    • characteristics of equilibrium
    • equilibrium constant/expression/calculations
    • graphical analysis
    • particulate diagrams
    • the response of K to manipulating equations
    • calculating Q
    • predicting if a reaction system will proceed to the left or right to reach equilibrium
    • Le Chatelier’s Principle
  • Assessment/practice. The following FRQs provide excellent opportunities for student practice:

For the Student

Lesson

Background

Many chemical and physical processes are reversible. One physical process that you are familiar with is the melting of ice. Ice melts when it is left in a glass on your kitchen counter. If you return the ice to the freezer, the process reverses, and the ice freezes. Many chemical reactions are also reversible. One example that you are sure to encounter in this course is the industrial production of ammonia via the Haber process:

N2 + 3 H2 ⇌ 2 NH3

An important biological example is the binding and releasing of oxygen gas by the hemoglobin molecule (Hb):

Hb(aq) + 4 O2(g) ⇌ Hb(O2)4(aq)

Objectives

  • Develop a definition for the term “dynamic equilibrium.”
  • Recognize how to identify when a system which has reached equilibrium.
  • Understand how equilibrium is established.

Procedure

You will be simulating a reaction system which is proceeding towards equilibrium. The following is provided as an example of what you will be doing:

Example

Take the reaction system R ⇌ P to equilibrium. The given set of conditions are:

Initial Amount of R = 30 moles
Initial Amount of P = 0 moles
Forward reaction: Each round, 30% of R reacts (R → P)
Reverse reaction: Each round, 25% of P reacts (P → R)

To calculate the final amount for each round:

Initial Amount - Amount Reacted + Amount Formed = Final Amount

Consider the reversible reaction R ⇌ P.

  1. Obtain a set of starting conditions from your teacher. Record your set of conditions below:
  • Initial amount R _______
  • Initial amount P _______
  • Forward reaction _______
  • Reverse reaction _______
  1. Perform several rounds of reactions. Round to two decimal places. Continue until the system reaches equilibrium. You will know when the system arrives at equilibrium.
  2. Once equilibrium is reached, perform two more rounds (this will be helpful later when graphing your results).
  3. Once you are done, record your data on the Class Data Table provided by your teacher.

Analysis

Use your data to answer the following questions:

  1. Describe the changes in both the amounts of P and R from time “0” until equilibrium was reached. How did you know when you reached equilibrium?
  2. Do the forward and reverse reactions actually cease? Why is equilibrium called dynamic equilibrium? What is happening on the molecular level?
  3. Graph your data below.

  1. How does the graph indicate when equilibrium has been reached?
  2. For each round, the forward rate is the “amount of R reacted”, the reverse rate is the “amount of P reacted”.
    1. Select several rounds for your reaction series calculate both the forward rate and the reverse rate at each of these time intervals.
      Time Forward Rate Reverse Rate
    2. What is happening to the rates? What happens to the rates at equilibrium?

Class Discussion

Use the data from the Class Data Table to answer the following questions:

  1. Consider the data from Groups A – G. At equilibrium, are the concentrations of R and P always the same? Provide an example to support your answer.
  2. Consider the data from Groups A – G. Does the P/R ratio depend on the initial amounts of P and R? Provide an example to support your answer.
  3. What happens to the P/R ratio if the system begins with only products instead of only reactants?
  4. The P/R ratio can be called the “equilibrium constant.” Considering your answers to the questions above, why is this a valid term?
  5. Consider the data from Groups A – J. Do the forward and reverse reaction percentages affect the P/R equilibrium ratio? Explain the mathematical relationship.
  6. Consider the data from Group A and Group H.
    1. Compare the P/R ratios. What do you observe?
    2. The reaction from Group A is said to “favor the products”, while that from Group H is said to “favor the reactants.” What do you think this means?
    3. Make an observation about the P/R ratio and whether the reactants or products are favored.
    4. If a reaction system has a very large P/R ratio (or a very small one), what would this indicate?
    5. Compare the graphs for each. What do you observe?

Class Data Table

Use this table to copy down the results from each student group.