LeChatelier Shift Practice

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Objective

SWBAT explain how a reaction at equilibrium will shift in response to a stress in terms of the effect on the rate of forward and reverse reactions.

Big Idea

Equilibrium is disrupted due to changes in the rates of the forward and reverse reactions.

Background

This lesson is designed to be executed in 35 minutes.  We had a shortened schedule due to PARCC testing through this week.

In the student conclusions from their LeChatelier's Principle Lab, it was evident students still weren't connecting how the rates of the reactions were being changed by the stress, and how that resulted in a new equilibrium point.  I modified this activity from the previous year by adding in the space for students to write the reason the equilibrium was shifting to favor one side or the other. 

I was hopeful that students could make the connection better with explicit practice.  What I found is that I had it backwards.  Rather than identify the direction of the shift first, I should have had students identify the change in the rate, and let them determine the shift from there.

Next year I will be teaching it in this order, which I think will be a big change and allow students to process it much easier.  Rather than deal with the effect and then the cause, we will go in order from cause to effect.

Standards addressed in this lesson begin with 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.

Students are using models in the forms of chemical equations (Science and Engineering Practice 2).  Through the various reaction and stress scenarios, students are reinforcing that "systems can be designed to cause a desired effect" (Cause and Effect Cross Cutting Concept) and that "much of science deals with constructing explanations of how things change and how they remain stable" (Stability and Change Cross Cutting Concept) and that "systems can be designed to do specific tasks." (Systems and System Models Cross Cutting Concept)

 

Opener -- Returned Work

7 minutes

As students enter the room, I am in the midst of passing back their LeChatelier Lab from the previous week.  Once everyone is in and the bell has run, I welcome them back and remind them that we have short time in class all week due to the PARCC testing.

I then ask them to look at the conclusions from their lab.  I ask first:

  • How many questions are there to guide your conclusion writing?
  • Three
  • Hold up fingers for how many you addressed in your conclusion.

Most students put up one or two fingers.

  • Which question do you think is most important?
  • The last one
  • How many of you actually wrote what you learned about LeChatelier's Principle FROM THE LAB in your conclusion?  Not just described it, or re-wrote it, or said it was written down on the front page?

About 3 hands per class go up

So today, we are going to dig back into LeChatelier's Principle, partly because it has been three days and prom weekend since we began it, but mostly because I was unable to judge from your work how well you knew it.

LeChatelier Practice -- Whole Class

10 minutes

I have a student pass out the LeChatelier Practice worksheet.  I ask students to take 4 minutes to answer the first three questions on their own.  This provides me a chance to take attendance and pass out copies of the Haber Process Reading to students who were absent on Friday.

After 4 minutes, I ask for a student to explain LeChatelier's Principle.  Some responses through the day include:

  • "Stressing reactions changes their equilibrium"
  • "Reactions shift to get back to equilibrium"
  • "Stress causes shifts"

All of these definitions are a little incomplete, but each has elements of truth.  Depending on what is offered by students, we firm things up a little before moving on.  One area I didn't focus enough on with students is that the reaction is at equilibrium before being stressed.

We go through the remaining review questions:

  • How does shifting to the left affect the amount of reactants?
  • They would increase
  • What about products?
  • Decrease
  • If the equilibrium shifts to the right, what happens to the amount of reactants?
  • They decrease
  • Why?
  • They're being used to to make more products.

Next we look at the example and first problem together, as modeled in this video.

After doing these together, I ask the students to work as a table on problem #5, shifting the ammonia synthesis reaction.  When they finish, we go over it to ensure everyone has it correct.  This student had to fix the direction of the shift on part b, even though they had the reasoning correct.

 

LeChatelier Practice -- Group and Individual

18 minutes

For the remainder of the period, students work on the back side examples of the paper.  When they finish, I have them turn them in at the front of the room.  Some students finish in under 10 minutes, and I set them to answering questions for classmates.

When students are asking for assistance, I walk them through five questions:

  1. Is the part of the system being stressed on the right or left side of the reaction?
  2. Is it part of the forward or reverse reaction?
  3. Is the amount of it being decreased or increased?
  4. How does that affect the rate of the reaction?
  5. So which way does the equilibrium shift?

I model systematic problem solving for students through these questions, for more information on this process, see my reflection below.  As students get used to approaching the problems systematically, they continue to experience more and more success.  Most of the students are doing exceptionally well by the end of the period.