Using Le Chatelier’s Principle
Introduction:
Many chemical reactions do not go to completion. In these reactions there will
always be measurable amounts of reactants and products. Equilibrium is established, and
the ratio of concentrations of products to reactants will remain constant if the 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 LeChatelier’s Principle that states
“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, in pressure for systems involving gases, and in temperature.
The system described by the reaction
Co(H2O)62+(aq) + 4 Cl-(aq)  CoCl42-(aq) + 6 H2O(l)
pink
blue
makes it easy to observe the effects of changes in concentration and temperature on
equilibrium concentration of the cobalt containing ions because of the two distinct colors.
When a stress is applied to this system the direction in which the equilibrium was shifted
may be readily observed.
Purpose:
The purpose of this laboratory activity is to
1. predict the changes observed in the equilibrium system involving Co2+ ions.
2. explain the changes in terms of LeChatelier’s Principle.
Equipment/Materials:
3 test tubes
Test tube rack (well plate)
Stirring rod
0.2 M CoCl2 in rubbing alcohol
0.1 M AgNO3
6.0 M HCl
water
acetone
Safety:
 Always wear safety glasses and an apron in the lab.
 The HCl used in this experiment is very corrosive. HANDLE WITH CAUTION.
The vapors are irritating as well
 The AgNO3 used in this experiment can stain skin. Handle carefully.
 Acetone is flammable. There should be no open flames during this experiment.
ASIM: LeChatlier's Principle
Revised 10/06
p. 1
Procedure:
1. Complete Section I of the data table by predicting the changes that will occur when
stresses are applied to the equilibrium system.
2. Note the color of the CoCl2 solution.
3. Transfer about 2 mL of the CoCl2 solution to test tube #1 and #2.
Part 1: To test tube # 1
4. Add acetone dropwise with stirring to test tube #1 until you observe a distinct color
change. Record the observations in the data table.
5. To the same test tube, add water dropwise with stirring just until you observe a
distinct color change. Record the observations in the data table.
6. To the same test tube (#1), add 6 M HCl dropwise with stirring until you observe a
distinct color change. Record the observations in the data table.
7. To test tube #1, add 0.10 M AgNO3 dropwise with stirring until you observe a distinct
color change. Record the observations in the data table.
Part 2: To test tube #2
8. To test tube #2 carefully add 6 M HCl dropwise with stirring until the color appears to
have about equal amounts of blue and pink. Pour half of the purple solution into test
tube #3. Immerse test tube #3 in hot water. Be careful not to get water into the
sample.
9. Answer the questions while you are waiting for the sample to get hot.
10. Compare the color of test tube # 3 to test tube # 2.
11. Dispose of the contents of all test tubes as directed by your teacher.
12. Thoroughly rinse all of the test tubes, drain them and return the well plate and test
tubes to the bag.
ASIM: LeChatlier's Principle
Revised 10/06
p. 2
Name _________________________
Partner’s Name(s) _________________________
Period ______Date_______________
LeChatelier’s Principle
Data Table Section I:
Refer to the following equation to predict the effect each of the following changes will
have on the equilibrium system.
Co(H2O)62+(aq) + 4 Cl-(aq)  CoCl42-(aq) + 6 H2O(l)
pink
blue
1. Adding acetone (Acetone is often used by chemists to remove the last traces of water
from glassware.)
2. Adding H2O
3. Adding HCl (a source of Cl- ions)
4. Adding AgNO3 (silver ions and chloride ions react to form an insoluble solid)
5. Increasing the temperature, if heat were a reactant.
6. Increasing the temperature, if heat were a product.
ASIM: LeChatlier's Principle
Revised 10/06
p. 3
Name _________________________
Partner’s Name(s) _________________________
Period ______Date_______________
Data Table Section II:
Procedure
Observation
Acetone added
H2O added
HCl added
AgNO3 added
Heat added
Questions:
1. Acetone has a strong attraction for water molecules. Use this fact and LeChatelier’s
Principle to explain the observations made when acetone was added
2. Explain in terms of LeChatelier’s Principle the observations made when H2O was
added.
3. Explain in terms of LeChatelier’s Principle the observations made when HCl was
added as a source of Cl- ions.
4. Silver ions from AgNO3 react with Cl- ions to produce an insoluble solid. Use this
information and LeChatelier’s Principle to explain the observations made when
AgNO3 was added.
5. Explain in terms of LeChatelier’s Principle and your observations upon the addition
of heat why you believe the reaction to be endothermic (heat is a reactant) or
exothermic (heat is a product).
ASIM: LeChatlier's Principle
Revised 10/06
p. 4