LAB # 25: LE CHATELIER’S PRINCIPLE
Accelerated Chemistry 1
Imagine you are starting a terrarium. You add dirt, plants, and water then seal the container. Some of the water
in the terrarium will vaporize. Soon the air in the terrarium will hold as much water as possible. At this point,
some of the vapor will begin to condense. Eventually, as much water is condensing as is vaporizing; the liquid
water and its vapor are in equilibrium. What happens to the equilibrium if the conditions change? A change of
conditions in the system is called a stress. For example, if the terrarium gets colder (undergoes a stress), some of
the water vapor will condense. In response to the stress, a new equilibrium will be established. The new
equilibrium will have less water vapor and more liquid water than the old equilibrium.
Are there any rules to help predict what happens when a stress is applied to physical and chemical systems in
equilibrium? Yes, Le Chatelier's principle applies to systems in equilibrium. It states that, if a stress is placed on
a system at equilibrium, the system will change in a way that relieves the stress. In this experiment, you will
impose stresses on physical and chemical systems at equilibrium to see how the systems change in response to
the stresses.
Materials
8 disposable pipets per lab table
5 small test tubes per pair
saturated potassium nitrate, KNO3
potassium nitrate, KNO3(s)
0.1 M potassium chloride solution, KCl
ice bath
0.1M potassium thiocyanate, KSCN
0.1 M iron(III) chloride, FeCl3
0.1M tincture of cobalt(II) chloride, CoCl2
0.1M silver nitrate, AgNO3
sodium chloride, NaCl
hot water bath
Part A. Effect of Temperature on a Physical Equilibrium
1. Add 2-3 mL of saturated potassium nitrate solution to a clean test tube.
2. Cool the test tube in a 250-mL beaker of ice water for 5 minutes. Record the results.
Part B. Common Ion Effect on a Chemical Equilibrium
1. Use a graduated cylinder to add 50 mL of distilled water to a 100-mLbeaker. Add 1 mL of 0.1M iron (III)
chloride and 1 mL of 0.1M potassium thiocyanate to the water; stir. The total molecular equation for the
reaction is:
FeCl3(aq) + KSCN(aq) → (FeSCN)Cl2(aq) + KCl(aq)
(eq. 1)
The color that appears is due to the presence of ferrothiocyanate ions, FeSCN2+. Record your observations.
2. Label four identical clean, dry test tubes with the numbers 1-4. Pour 5 mL of the mixture from Step 1 into
each. Hold the tubes over a white background and look down into them.
3. Tube 1 will be the control in this experiment. To tube 1, add 20 drops of tap water. To tube 2, add 20 drops
of 0.1M iron(lII) chloride. To tube 3, add 20 drops of 0.lM potassium thiocyanate. To tube 4, add 20 drops
of 0.1M potassium chloride. Flick each tube to mix the solutions. Compare the colors of the solutions in
tubes 2, 3, and 4 with the color of the solution in the control tube (tube 1). Record your observations.
4. Discard the solutions down the drain; return the test tubes to the dishpan in front of the class.
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Part C. Common Ion and Temperature Effects on a Chemical Equilibrium
In this experiment, you will investigate a system containing two complex ions. A complex ion is made up of a
single central atom or ion, usually a metal ion, to which other atoms, molecules, or ions are attached. The
system is described by this equation:
Co(H2O)62+(aq) + 4 Cl (aq)  CoCl42 (aq) + 6 H2O(l)
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−
(eq. 2)
Part of your task will be to determine which complex ion in the equation above-- Co(H2O)62+ or CoCl42 --is blue,
and which is pink.
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1. Prepare a small (100 mL beaker) hot water bath (70-80ºC).
2. Add 20 drops of 0.1 M tincture of cobalt(II) chloride to a clean, dry test tube. [A tincture is an alcoholic
extract of a nonvolatile substance.] Record your observations.
3. Add 7 drops of 0.1M silver nitrate, AgNO3, solution. Gently flick the tube to ensure good mixing. Record
your observations.
4. Heat the tube in a hot water bath for 30 seconds. Record your observations.
5. Remove the tube from the hot water bath. Add approximately 0.3 g sodium chloride, NaCl. Gently flick the
tube to ensure good mixing. Heat the solution for 60 seconds. Record your observations.
6. Place the test tube in a beaker containing ice water for 30 seconds. Record your observations.
7. Reheat the test tube in the hot water bath. Record your observations.
8. Return the test tube WITH ITS CONTENTS to the teacher. Wash your hands thoroughly.
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Name:
Lab Partner(s):
Per:
Date:
LAB: LE CHATELIER’S PRINCIPLE
Table 1. Observations
System
Observations
Part A
KNO3(saturated),
(room temperature)
KNO3(saturated)
(cooled)
Part B. FeCl3(aq) + KSCN(aq) → (FeSCN)Cl2(aq) + KCl(aq)
FeCl3/KSCN reaction +
water
FeCl3/KSCN mixture +
additional FeCl3
FeCl3/KSCN mixture +
additional KSCN
FeCl3/KSCN mixture +
KCl
Part C. Co(H2O)62+(aq) + 4 Cl (aq)  CoCl42 (aq) + 6 H2O(l)
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−
2. CoCl2 solution
3. CoCl2 + AgNO3
4. CoCl2 + AgNO3,
heated
5. CoCl2 + AgNO3 +
NaCl, heated
6. CoCl2 + AgNO3 +
NaCl, cooled
7. CoCl2 + AgNO3 +
NaCl, heated
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Analyses and Conclusions
Part A
1. Write a balanced equation for the physical change equilibrium you observed.
2. Did lowering the temperature (Step 2) affect the equilibrium? Explain your answer.
3. Explain what happened in the potassium nitrate system in terms of Le Chatelier’s principle.
Part B. FeCl3(aq) + KSCN(aq)  (FeSCN)Cl2(aq) + KCl(aq)
(eq. 1)
4. Write a net ionic equation for the equilibrium that existed after the iron(III) chloride and potassium
thiocyanate were combined in the beaker.
5. What are the colors of all the reactants and products?
6. What evidence was there that the equilibrium shifted when additional iron(III) chloride was added? In which
direction did it shift?
7. What evidence was there that the equilibrium shifted when additional potassium thiocyanate was added? In
which direction did it shift?
8. Compare the effect of adding potassium chloride to the system with your other results [hint: check your net
ionic equation]..
9. Explain the changes observed in the ferrothiocyanate ion system in terms of Le Chatelier’s principle.
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Part C. Co(H2O)62+(aq) + 4 Cl (aq)  CoCl42 (aq) + 6 H2O(l)
−
(eq. 2)
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10. From your observations, which side, pink or blue, was favored by cooling the solution?
11. From the balanced equation, which reaction, forward or reverse, must be favored by adding more chloride
ions? Explain.
12. What is the identity of the white precipitate that formed on addition of silver nitrate? Why did adding silver
nitrate solution affect the equilibrium, even though neither Ag+ ion nor NO3 ions appear in the equilibrium
equation?
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13. Why did the color change after heating in Step 5, but not in step 4?
14. Is the reaction, as written in eq. 2, endothermic or exothermic? Explain.
15. Which complex is pink-- Co(H2O)62+ or CoCl42 ? Explain how you decided.
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Synthesis
16. The equilibrium constant, Keq, is 2.4 × 103 at a certain temperature for the reaction
2 NO(g)  N2(g) + O2(g)
For which of the following sets of conditions is the system at equilibrium? For those that are not at
equilibrium, in which direction will the system shift?
a. A 1.0-L flask contains 0.024 mol NO, 2.0 mol N2, and 2.6 mol O2.
b. A 2.0-L flask contains 0.032 mol NO, 0.62 mol N2, and 4.0 mol O2.
c. A 3.0-L flask contains 0.060 mol NO, 2.4 mol N2, and 1.7 mol O2.
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17. Ethyl acetate (used in some nail polish removers) is synthesized in a nonreacting solvent (not water)
according to the following reaction:
CH3CO2H + C2H5OH  CH3CO2C2H5 + H2O
acetic acid
ethanol
ethyl acetate
Keq = 2.2
For the following mixtures, will the concentration of H2O increase, decrease, or remain the same as
equilibrium is established?
a. [CH3CO2C2H5] = 0.22M, [H2O] = 0.10 M, [CH3CO2H] = 0.010 M, [C2H5OH] = 0.010 M
b. [CH3CO2C2H5] = 4.4M, [H2O] = 4.4 M, [CH3CO2H] = 0.99 M, [C2H5OH] =10.0 M
18. An important reaction in the commercial production of hydrogen is
CO(g) + H2O(g)  H2(g) + CO2(g)
ΔH < 0
How will this system at equilibrium shift in each of the following cases?
a. Gaseous carbon dioxide is removed.
b. Water vapor is added.
c. The pressure is increased by adding He gas.
d. The temperature is increased.
e. The pressure is increased by decreasing the volume of the reaction container.
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