Shifting Equilibrium

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Objective

SWBAT apply LeChatelier's Principle to identify specific parts of a chemical reaction to stress in order to create a desired shift.

Big Idea

Chemical systems at equilibrium can be deliberately altered to produce desired results.

Background

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

The previous day, students did well in explaining the reasons why reactions at equilibrium shift in response to a stress on the system.  Today I wanted to revisit the same topic from a slightly different angle.  Instead of working from a stress to how the equilibrium would respond, I wanted them to think of ways to stress a system to force the shift in equilibrium in a specific direction.

Additionally, I wanted students to think of how to achieve the desired shift by both adding something to the system, and removing something from the system.  I believe that if students internalized how stresses cause shifts in the equilibrium position of a system, they should be able to identify ways to stress a reaction to achieve a desired shift.  Students who could show this transference of the concept of equilibrium shifts from stress to shift and in reverse should be fully in control of the topic.

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

4 minutes

When students are entering, I am passing back their practice from the previous day.  Once the bell rings, I explain that I was very happy with the results of the homework, and that they are making good progress on understanding how chemical equilibrium works.

I tell the class that today we are building upon yesterday's work, and extending it a little bit.  Yesterday, they were provided the stress and had to predict how the reaction system would shift in response.  Today we are going the opposite direction: starting with a desired shift and identifying how we can force that shift to occur.

I refer back to the Haber-Bosch process, and remind students that they began with a specific need: produce more ammonia.  It was from that need that they identified all the conditions that would shift the equilibrium to the right and boost the ammonia production.

Individual Practice

25 minutes

I pass out the LeChatelier Principle Extra Practice paper and orient students to how it looks different to our prior practice.  I project a copy using our document camera and ask students to do the first one with me.

We look at the reaction between methane and hydrogen sulfide to create carbon disulfide and hydrogen gas.

  • Part A wants us to shift the equilibrium left by increasing something.  What would we have to increase to push the reaction left?
  • A product?
  • So what chemicals?
  • CS2 or H2
  • Perfect, now why would having more of those cause it to shift left?
  • They would speed up the reverse reaction
  • So now part B, if we still want it go left, what would we have to decrease?
  • CH4 or H2S
  • Why?
  • Losing them would slow down the forward reaction, so it would shift left.

We continue the process through parts C and D of the first reaction.

The first common question is whether they have to write ALL the possible ways to shift it, or just one.  I tell them they can provide just one.

Some students pick this up very quickly, so I transition this to individual practice after the first example and begin to circulate the room.  I tell the students that they must get checked off by me when they are done, and then can check their answers on the two answer keys provided at the front of the room.  When they finish, they need to pick up the unit test review and work on that.

Students do very well on this.  Many pick up on the fact that since I didn't vary the order of desired shifts, the reasons repeat throughout the whole page.  Most students list all the possible stresses to cause the shifts, as in this example.

Closing -- Test review

6 minutes

When students complete the extra practice, they drop it off on the front table and pick up the Equilibrium Test Review.  Whatever time remains is for students to begin working on the review.  I explain that they will have approximately half the period the following day to finish, and then we will check in and go over the review.

I continue to encourage students to try the review without their notes, and use the notes to fill in the gaps that they need to study later.  This student clearly filled in from their notes, if seen by nothing more than the definition of LeChatelier's Principle ripped straight from the PowerPoint.

The last section of the review on LeChatelier shifts went well once students got started.  I like how this particular students color coded each reactant and product to track it through their thinking and identify if it was a participant in the forward or reverse reaction.

Two days later the students took the second half of the Kinetics and Equilibrium test.  I chunked this test into two parts due to the size of the unit, as elaborated upon here.  Students had difficulty with the kinetics section more so than the the equilibrium objectives.  However, the wordiness of the LeChatelier questions caused some students to make errors in answering them.  Students did better on the LeChatelier portion of the review than they did on the exam.  Conversely, they did better on the exam in identifying definitions than they did on writing them out in the review.

Up to this year, our district has been requiring forced choice district wide exams.  In order to assess if students were able to provide both the direction and reason for the shifts observed, the questions got very long.  I believe the length of the answers directly impacted student performance as they likely did not read each answer choice fully to make sure they had both the direction and reason identified and matched up in their answer choice.  In the review, students could give concise, open responses which they tend to do better at.

Due to the low scores, I decided to do whole class remediation, reteaching and reassessing.