LeChatelier Lab

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Objective

SWBAT test predictions regarding stresses on a reaction system at equilibrium.

Big Idea

Reaction systems at equilibrium can be manipulated toward favoring either the reactants or products.

Background

The previous day students learned how to shift the equilibrium position of a reaction by stressing it.  However, notes and worksheets are poor substitutes for real experience, so today students will be predicting the results of four stresses, and then testing them in lab.  Students will be using the dissociation of copper II chloride to test their understanding.

When dissolved but still together, copper (II) chloride has a green color in an aqueous solution.  However, as it dissociates into copper and chloride ions, it takes on a blue color.  The dissociation is also exothermic, so heat can be used to stress the system.

The materials needed per student group are:

  • 1.5M copper (II) chloride solution
  • 0.1M silver nitrate solution in dropper bottle
  • 4.0M sodium chloride solution (I have seen variations where solid NaCl is added, but it didn't work as fast or as well as this set up)
  • Deionized water
  • Test Tube rack
  • 5 test tubes (size doesn't matter so long as they can hold 5mL of total solution)
  • 10mL graduated cylinder
  • Hot water bath (I have it boiling so we know the water temperature)
  • Ice water bath

This lab setup is fairly simple, so I set it up independently in both of my classrooms to avoid having to switch rooms back and forth.

This is a standards-heavy lesson.  The core content is present in 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.

We are also carrying out an investigation (Science and Engineering Practice 3), analyzing and interpreting data (Science and Engineering Practice 4) and constructing explanations (Science and Engineering Practice 6) regarding the changes in the system.  From this lab, students are learning 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)

 

Opener -- Pre-Lab

8 minutes

When students enter the room, I hand them the LeChatelier Lab.  When the period begins, I ask them to read the Problem, Materials and Pre-Lab information.  While students are reading, I am making sure the hot water and ice baths are set up and at the correct temperature (with permission from the teachers I share rooms with, I began these mid-way through the previous period).

As students finish up, I circle back up to the board and ask what the reaction is we are exploring today.  When a student says "CuCl2" I ask

  • "What color will it be?
  • "Green"

I write CuCl2 in green on the board.

  • "What will this (pointing to CuCl2 on board) become?
  • "A blue copper ion and a colorless chlorine ion" (we did not teach nomenclature, so I don't expect the -ide ending)

I then write the products on the board in blue.

  • "So how will we know which way the equilibrium is shifted?"
  • "By the color?"

I ask them to think about their practice yesterday and predict how raising and lowering the temperature will affect the equilibrium on their paper.

Next I explain the effect of adding silver nitrate, that it will remove the chlorine from the solution.  I ask them to predict the shift.  Then I ask

  • "How will adding table salt affect the solution?"
  • "It will add in extra chlorine?
  • "Yes, now predict on your paper how it will shift the equilibrium."

Here is a student set of predictions, along with his lab data.  You can see the "Reason for Shift" column has been deleted from the uploaded version.  It was too much for students to process on only the second day of studying LeChatelier.  The pictured procedure and data table listed are the originals, which were changed before uploading.

When all students are done writing, I move to the lab space and demonstrate how each chemical is stored, where the waste beaker is located, and that we will wear aprons and goggles due to the silver nitrate's ability to stain clothing and skin.  Students should handle the silver nitrate with care, and report any spills.  Students are warned that this high of a concentration of copper (II) chloride is toxic, and to avoid having it near their mouth at any time.  When students are ready, I invite them back to the lab.

Carrying Out Investigation

27 minutes

A pair of lab partners that work efficiently can complete this investigation in about 10 minutes.

Students come back and put on their safety gear.  They find the green copper (II) chloride solution at their table and put 3 mL into each test tube.  Students have difficulty interpreting the instructions in steps two and three, so I have edited them in the attached version to make them more understandable to students.  Additionally, I labeled the rows of the data table to their corresponding procedure step to further reduce confusion.

Students then record the color of their control, and stress the four remaining test tubes and record the color of each and the direction of shift indicated by the color.  This video shows the control, the tube with the silver nitrate, the tube with the sodium chloride, the hot bath and cold bath.

 

Some students want to know if they will shift back and forth if they change the temperatures.  So I allow them to explore if they have the time.

 

When students have completed the investigation, I ask them to clean up, shut off the hot plates, and return to the front of the room to work on their conclusion questions and conclusion.

Drawing Conclusions

15 minutes

The timing of this transition is fluid, as some groups will finish earlier than others.  As students transition back to the front of the classroom, I touch base with them to explain that they should be answering the three questions and then writing the conclusion.

This student example is common to what I saw.  Students were able to identify the shifts based on the color changes, but were unable to explain why having more or less of something caused the resultant shift.

The conclusion here only addresses the first two parts of the guiding questions.  I have found throughout the year that students have difficulty summarizing what they learn about a concept from an activity.  This was one of the best efforts I received.  As a result, I knew that in the coming days we would need to spend time focused on defining LeChatelier's principle, and why the shifts occur in terms of the affect the stress causes on the rates of the forward and reverse reactions.