Schedule

a

WWW #11

What is the Effect of Stress?
(Le Châtelier's Principle)

This assignment is past due and can no longer be submitted.

Le Châtelier's principle states that when stress is applied to a system at equilibrium, the equilibrium shifts to relieve the stress. This assignment examines a chemical reaction at equilibrium where the reactants and products have different colors. You will predict the result when various stresses are applied to this system.

In a chemical reaction "stress" could be:
  • adding or removing products
  • adding or removing reactants
  • changing the temperature
  • changing the pressure
Although more complex than presented below, we can represent the behavior of cobalt chloride in aqueous solution with the following net ionic equation. If the equilibrium constant, K, was near 1, we might expect a purple solution, since similar amounts of blue and pink cobalt species would be present.

CoCl42– has a blue color and Co(H2O)62+ has a pink color.

A stress causing a shift to the right (toward products) would result in the solution becoming more pink.

A stress causing a shift to the left (toward reactants) would result in the solution becoming more blue.

1. When chloride ion is added to the system at equilibrium, what direction will the equilibrium shift (toward reactants or toward products)?
 
2.

What color will the reaction mixture become (blue or pink)?
 
3. When water is added to the system at equilibrium, what direction will the equilibrium shift (toward reactants or toward products)?
 
4. What color will the reaction mixture become (blue or pink)?
 
This reaction is exothermic, so heat is written as a product of this reaction.

5. What is the sign of DH (negative or positive)?
 
6.

If the reaction mixture is heated, what direction will the equilibrium shift (toward reactants or toward products)?
 
7. What color will the reaction mixture become (blue or pink)?
 
8. If heat is removed, what direction will the equilibrium shift (toward reactants or toward products)?
 
9. What color will the reaction mixture become (blue or pink)?
 

The web site for your textbook has a short QuickTime movie that illustrates the result when an aqueous cobalt chloride mixture at equilibrium is subjected to the stresses that you predicted above: addition of chloride ion, addition of water, addition of heat and removal of heat. Watch the video to see if you made correct predictions. I recommend watching the movie twice. First, view the whole thing. Next, stop and start the video after each stress to the system. Click here to view the situations above. This video clip is from the McMurry textbook companion web site (http://cwx.prenhall.com/bookbind/pubbooks/mcmurrygob/).

10. How did you do? Were your predictions correct? Did the movie help you understand the principles?
 

 

This assignment is past due and can no longer be submitted.

This site is made possible by funding from the National Science Foundation (DUE-9981111).
©2004, 2003, S. Holladay & K. Anliker/ IUPUI, all rights reserved.