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Le Châtelier’s Principle (7 Favorites)

DEMONSTRATION in Le Châtelier's Principle, Establishing Equilibrium, Equilibrium Constants. Last updated April 26, 2019.


Summary

In this demonstration, students will witness a system at equilibrium and apply different stresses to see how the equilibrium shifts.

Grade Level

High school

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.
  • 6.8 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.

Objectives

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

  • Understand how equilibrium shifts.
  • Recognize that a change in concentration does not change K but a change in temperature does.
  • Know how to use Q to see if reaction conditions have reached equilibrium.

Chemistry Topics

This lesson supports students’ understanding of

  • Equilibrium
  • Le Châtelier’s principle

Time

Teacher Preparation: 45 minutes

Lesson: 1 class period

Materials

For each group:

  • 8 test tubes
  • Test tube rack
  • CoCl2 (s)
  • Water
  • HCl (concentrated)
  • NaCl (s)
  • AgNO3 (s)
  • Hot water bath
  • Ice bath

Safety

  • Always wear safety goggles when handling chemicals in the lab. Students should also wear safety goggles when carrying out the demonstration.
  • Take precaution when working with concentrated acid. If any gets on your skin, rinse immediately with excess water.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.

Teacher Notes

  • Prepare the cobalt equilibrium by mixing 4 g of CoCl2in 20 mL of water (pink) and another 4 g of CoCl in 20 mL concentrated HCl (blue). Mix the two solutions together to achieve an intermediate purple color.
  • Have eight test tubes set up ahead of class time for students to observe (all the same). One control, seven test subjects. The hot bath and ice bath should be ready to go at the beginning of the demonstration. You can put test tubes in the baths ahead of time to allow for the change in equilibrium position.
  • CoCl42- is blue, Co(H2O)62+ is pink. The forward reaction is exothermic (-ΔH).
  • Expected results:

 

Add CoCl2

Add HCl

Add H2O

Add AgNO3

Add NaCl

Heat

Ice bath

Actual

pink

blue

pink

white ppt (AgCl) & pink

blue

blue

pink

For the Student

Lesson

Background

The following reaction describes the process you see in the front of the classroom…

Similar to other transition metals, depending on its charge, cobalt is a different color. When it is a complex ion with water it is _________, but when it is a complex ion with chlorine it is _________. Fill in these colors on the lines under the reactants and products.

Observations

Before your teacher makes these stresses on the equilibrium, predict what color you think will appear. Note what actually happens after the changes are applied to the system.

Add CoCl2

Add HCl

Add H2O

Add AgNO3

Add NaCl

Heat

Ice bath

Predict

Actual

Now that you know how temperature effects the reaction, fill in whether ΔH is + or -.

Analysis

LeChâtelier’s principle says that when conditions of a reaction change, the equilibrium rebalances to make all the concentrations meet the equilibrium constant ratio.

When this solution was initially at equilibrium, what was there more of, reactants or products? Explain.

What happened when you added CoCl2? What does that mean about K? Explain.

What happened when you added HCl? What does that mean about K? Explain.

What happened when you added H2O? What does that mean about K? Explain.

What happened when you added NaCl? What does that mean about K? Explain.

What happened when you added AgNO3? What does that mean about K? Explain.

What happened when you heated up the solution? What does that mean about K? Explain.

What happened when you cooled the solution? What does that mean about K? Explain.

So, if you are ever given concentrations for chemicals in a reaction, you can tell how the reaction will change to reach equilibrium by comparing the Q value to K. Examples…

Are any of the following situations in equilibrium? If yes, explain why. If no, explain what needs to happen to make the solution be in equilibrium.

Conclusion

Explain in your own words how LeChâtelier’s principle works.