# Le Chatelier's Lab and Unit 9 Review

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## Objective

Students will be able to explain what factors will make an equilibrium system shift towards the products through performing a lab.

#### Big Idea

Equilibrium systems will shift to counteract stresses.

## Introduction

In this lesson students are practicing what they have learned about Le Chatelier's Principle through performing a lab activity.

• This lesson covers the Next Generation Science and Engineering Performance Expectation 1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amount of products at equilibrium.  It does so because students are learning about equilibrium and Le Chatelier's Principle.
• This lesson aligns with several of the Next Generation Science and Engineering Practices including:
• 2: Developing and Using Models.  It does so because students are using models of chemical equations to determine the effect of changes on equilibrium.
• 3: Designing and Carrying out Experiments. It does so because students are performing an experiment.
• 4: Analyzing and Interpreting Data: It does so because students are analyzing the data from their experiment to determine the direction of shift from each stressor.
• 6: Constructing explanations: It does so because students are making a claim as to which factors will cause the equilibrium to shift towards the products using evidence from an article and from their lab.
• This lesson aligns with several of the Next Generation Science and Engineering Crosscutting Concepts including:
•  4: Systems and System Models.  It does so because students are able to see how Le Chatelier's Principle can be used as a model to see the effect on an equilibrium system.

For this lesson each group of students needs the following resources for their lab:

• Hot plate with 250mL beaker (for warm-water bath)
• 100mL Beaker half-filled with ice (for ice bath)
• 6 mini test tubes
• test tube tong
• scoopula
• toothpicks
• 1 plastic pipette with marker for 1mL
• H2O in dropper bottle
• 1.5 M copper (II) chloride (CuCl2) in dropper bottle
• solid sodium chloride (NaCl)
• 0.05M silver nitrate (AgNO3) in dropper bottle

## Explain

15 minutes

To begin this lesson I introduce students to the lab where they will be determining what stressors can be used to make the equilibrium reaction involving the formation of copper (II) complexes shift to give additional product.  I then help students with the prelab questions.

• I start by passing out the le chatelier's lab paper.
• I then have students spend a few minutes looking over the lab to themselves.
• After students have looked over the lab I ask for volunteers to help with explaining what the goal of the lab is.  I make sure that they reflect the idea that it is to figure out how to make the equilibrium shift towards the products side.
• I then have students do the prelab questions.  I give them about 5 minutes to try them on their own.  I stress that they should look back at their unit 9 notes if they are not sure and that they should include the justification of how they know which direction the equilibrium will shift.
• After most students have completed the prelab questions I go over the answers with students.  I do this by calling on students for their responses.  If students give incorrect answers then I help with what the correct answer/justification is.  This is a copy of one of the sets of answers for the prelab.
• I then go over the setup and materials that students will be using.  This is a video of me doing this.
• While going over the materials I show students how to set up their warm water bath and how to place the test tube inside the bath.  This is a video of me doing this.
• Finally I break students into cooperative groups and have them start the lab.  For more details on cooperative groups see my reflection.

## Elaborate

60 minutes

In this section of the lesson students are actually performing their lab with their cooperative groups.

• As students are working on their lab I walk around to ensure that they are on task and to answer any questions.
• This is a picture of what the final test tubes looked like for one group.
• One of the parts of the lab that students have difficulty with is heating up the test tube for the warm water bath.  This is a picture of what it looks like when they balance the test tube in the bath with the test tube tongs.
• I also make sure that students understand the two different charts on the second page of the lab.  I make sure that they are writing their observations in the first chart for data, and then which direction the equilibrium shifts in the analysis chart.
• Finally, as students are working on their conclusion I make sure that they understand what they are supposed to be writing.  If they are confused I have them look back at their analysis and ask them, "Which ones shifted to the products?" and explain that this is their claim, and then ask them "how did you know which ones shifted?" and I tell them that this is their evidence.  If students are not sure how to explain why with their rationale I tell them to look back at the justification that they did in the prelab.  This movie shows me doing this with one group.
• When students are done with their lab I have them turn them in and start with the unit 9 review.

## Evaluate

After students have turned in their labs I grade them using the Le Chatelier's lab rubric.

As I graded the labs most students get their points for the prelab as that we reviewed it as a class.

Where I found that students got the most confused was with dealing with the control in the data and analysis sections.  Also, not all students wrote complete conclusions.

These are several examples of students labs which demonstrate the variety which I received in my classes:

• In this first example, the student had a complete lab and earned all of their points.
• In this second example, the student only discussed one factor that could shift the equilibrium in their conclusion, when there should have been two.
• In this third example, the student was confused with the purpose of the control as that they showed that it shifted when they should have said that there was not shift because there was no stress applied.
• In this fourth example, the student did not give any evidence as to how they know that the equilibrium shifted towards the products.

Based on these labs I realize that for next time I do this lab I will need to make sure to really explain the purpose of the control to students as a means to compare the results of the 5 tests.  In the future I think that I will put an X in the column for control so that students realize that they should not be writing anything in that column.  This is a revised version of the lab.

## Exam Review

30 minutes

For the final part of this lesson students are working on their unit 9 review.  Rather than giving an exam at the end of this unit, I have students take a big quiz, and then move onto Unit 10.  They are then tested on both Unit 9 and Unit 10 at the same time.

The review is due at the next class where I stamp for completion and then review the answers with students.  This is an example of one student's completed review.

For the most part students did well on the review.  The biggest confusions were where to place the activated complex (we did not discuss it that much in class), how to determine which of the lines are reactants/products on the graph at the bottom of the first page, and how to figure out what happens to the concentration of a reactant/product with equilibrium shifts on the last part of the second page.