LAB – Using Le Chatelier’s Principle
INTRODUCTION:
Many chemical reactions do not go to completion. In these reactions there will be measurable amounts of reactants and
products even after a considerable period of time. When equilibrium is established the ratio of products to reactants will remain
constant if temperature is kept constant. It is possible to shift the equilibrium in a desired direction by applying a stress to the system.
This process is explained by Le Chatelier's Principle which states that, "When a system at equilibrium is subjected to a stress, the
system will react so as to relieve the stress." Some examples of stresses that can be applied to a system are changes in concentration
(both increasing and decreasing), pressure (for systems involving gases), and temperature. In this experiment the effects of changes in
concentration will be observed.
The system studied is :
Pink
Blue
Co(H2O)62+(aq) + 4Cl-(aq) ↔ CoCl42-(aq) + 6H2O(l)
With this system, it is easy to observe the shifts in equilibrium because the cobalt compounds involved have two distinct
colors. When a stress is applied to this system, the direction in which the equilibrium was shifted may be readily observed.
MATERIALS:
3 test tubes
Test tube tongs
0.2M CoCl2
test tube rack
stirring rod
6MHCl
hot plate
250 mL beaker
0.1M AgNO3
plastic pipette (4)
50 mL beaker
Acetone
SAFETY:
• Goggles and aprons must be worn during all experiments.
• The HCl used in this experiment is very corrosive. Handle with CAUTION. The vapors are irritating.
• The AgNO3 used in this experiment can stain skin. Handle carefully.
• Acetone is flammable. There should be no open flames during this experiment
PROCEDURE:
1. Complete the Pre-Lab data table by predicting the color change that will occur when stresses are applied to the equilibrium system.
2. Note the original color of the CoCl2 solution.
3. Using a plastic pipette, add1.0 mL of the CoCl2 solution to both test tubes #1 and #2. (The mark on the pipette bulb is
approximately 1 ml.)
4. Fill the 50 mL beaker with approximately 10.0 mL of tap water.
Part 1: To test tube # 1 (Record all observations on the data table)
8. Place test tube #1 in the test tube rack and add 1 ml acetone slowly along the glass side of test tube #1, WITHOUT stirring.
Observe and record the color at the TOP of the liquid.
9. Add 1 ml water slowly along the glass side of the test tube. Observe and record the color of the liquid.
10. Add 6M HCl dropwise, while stirring, until you observe a distinct color change. (You may observe a color that is between blue
and pink, i.e. purple).
11. Add 0.10M AgNO3dropwise, while stirring, until you observe a distinct color change.
Part 2: To test tube #2
12. Fill the 250 mL beaker halfway and begin to heat on the hot plate.
13. Carefully add 6M HCl dropwise, while stirring, until the color appears to have about equal amounts of blue and pink (purple).
14. Pour approximately half of the purple solution into test tube #3 and immerse test tube #3 in hot water. Be careful not to get water
into the sample.
15. Answer the post-lab questions while you are waiting for the sample to get hot.
16. Compare the color of test tube #3 to test tube #2 once a distinct color change has occurred.
17. Pour all the contents from the test tubes down then sink and run water for approximately 30 seconds.
18. Thoroughly rinse all of the test tubes, drain them, and place them upside down in the test tube rack.
Name ___________________________________________ Date ______________________ Period ________
Group Member Names _______________________________________________________________________
LAB – Using Le Chatelier’s Principle
PRE-LAB:
Refer to the following equation to predict the effect each of the following changes will have on the equilibrium system.
Pink
Blue
Co(H2O)62+(aq) + 4Cl-(aq) ↔ CoCl42-(aq) + 6H2O(l)
#
1
1.
Adding acetone (Acetone is often used to remove water)
2.
Adding H2O
3.
Adding HCl
4.
Adding AgNO3 (silver ions and chloride ions react to form an
insoluble solid)
5.
Increasing the temp, if reaction is endothermic.
6.
Increasing the temp, if reaction is exothermic.
2
3
4
5
6
DATA: Record all observations from steps in the table below.
Step
Stress
2
Original
8
Acetone added
(Color at the
TOP)
9
H2O added
10
HCl added
11
AgNO3 added
13
Test Tube #2
15
Heat Added
Color Observation
Color Prediction (Pink or Blue)
POST LAB:
Co(H2O)62+(aq) + 4Cl-(aq) ↔ CoCl42-(aq) + 6H2O(l)
1.
When acetone is added to the equation above, identify if the stress caused a shift to the right or left.
#1 Answer:
2.
When water is added to the equation above, identify if the stress caused a shift to the right or left.
#2 Answer:
3.
When hydrochloric acid is added to the equation above, identify if the stress caused a shift to the right or left.
#3 Answer:
4.
When silver nitrate is added to the equation above, identify if the stress caused a shift to the right or left.
#4 Answer:
5.
Using the data gathered from the lab, identify if the reaction is exothermic or endothermic.
#5 Answer:
6.
Fill in the table below using the following equation.
H2(g) + I2(g) + energy
Stress
Direction of Shift (Left, Right, No Change)
Add H2
I2 is removed
Pressure is increased
Temperature is decreased
7.
↔2HI(g)
Write the equilibrium constant for the equation in #6.