Lab
3: Equilibrium and Le Châtelier’s Principle
Objectives:
To explore the effect of changing the concentrations of reactants and products on the equilibrium
composition of four equilibrium systems.
Introduction:
Most chemical reactions do not result in a 100% yield of products based on the stoichiometry of the
reaction. This is usually due to the equilibrium state that is reached when the forward rate of reaction
equals the rate of the reverse reaction. In this lab, the effect of qualitative changes on a number of
reactions at equilibrium will be studied.
Le Châtelier’s Principle states that
“If a change in conditions is imposed on a system at equilibrium,
the equilibrium position will shift in a direction that
tends to reduce that change in conditions.”
For example, the change in conditions could be either the temperature or concentration and the
effects observed. It should be noted that for a system, there exists many equilibrium positions but
only have one equilibrium constant at a specific temperature.
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Le Châtelier’s Principle
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In this experiment, we will study the equilibrium of four systems and observe the reaction of the
equilibrium systems as predicted by Le Châtelier’s Principle. The four systems are:
Part A - The Equilibrium of Co(II) Complex Ions
The element cobalt can form compounds in two different oxidation states, +2 and +3. The +2 state is
more common. The chloro complex of cobalt (II), CoCl 42-, is tetrahedral while the aquo complex of
cobalt (II), Co(H2O)62+, is octahedral. Both of these complexes exhibit different colours. Cobalt
complexes are used as drying agents with the colour change indicating when the drying agent should
be changed. The equilibrium reaction is:
Co(H2O)62+ (aq) + 4 Cl- (aq)
pink
CoCl42- (aq) + 6 H2O (l)
blue
ΔH = +50 kJ
(3-1)
Part B - The Equilibrium of the thiocyano-iron(III) complex ion
When colourless aqueous solutions of iron (III) ion, Fe 3+, and thiocyanate ion, SCN-, are combined,
the reaction that occurs produces the thiocyanoiron (III) complex ion, FeSCN 2+, which is responsible
for the equilibrium mixture's deep red colour.
Fe3+ (aq) + SCN- (aq)
colourless colourless
FeSCN2+ (aq)
red-brown
(3-2)
The colour of the thiocyanoiron (III) complex ion, FeSCN2+, solution will indicate how the equilibrium
system is being affected.
Part C - The Equilibrium of a Mg+2 precipitate
Reactions which form precipitates are written as an equilibrium reaction using the solubility product.
If there is a precipitate MX, then the Ksp expression is:
MX (s)
M+(aq) + X-(aq)
(3-3)
Part D: The Equilibrium of an Acid-Base Indicator
An acid-base indicator can be used to observe an equilibrium reaction. Indicators are weak acids
which show one colour in the acid form, HInd, and another colour in the basic form, Ind-. At the pKa
of the indicator there is equimolar amounts of the conjugate forms and the observed colour is a
mixture of the two. Bromothymol blue is a yellow-green-blue indicator which has a pKa of 7.0. The
reaction of the indicator bromothymol blue can be illustrated as follows:
HInd (aq)
yellow
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H+ (aq ) + Ind- (aq)
blue
(3-4)
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Apparatus:
1. Large test tubes
2. Hot plate (1 per student)
Solutions:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
0.1 M CoCl2·6H2O (15 mL per student)
concentrated HCl (place in fumehood)
0.1 M AgNO3
0.1 M Mg(NO3)2
6M NH4OH
NH4Cl solid
pH 7 buffer solution (3 mL per student)
Bromothymol blue indicator
0.1 M Fe(NO3)3 in 0.1M HNO3 (2 mL per student)
1 M Fe(NO3)3 in 0.1M HNO3 (0.1 mL per student)
0.1 M KSCN in 0.1M HNO3 (2 mL per student)
1 M KSCN in 0.1M HNO3 (0.1 mL per student)
0.1 M NaCl
6M NaOH
ice
1 M HCl
1 M NaOH
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Procedure:
Part A - The Equilibrium of Co(II) Complex Ions
1. Obtain four clean and dry test tubes and label them Test tube #1, #2, #3 and Control.
2. Record the initial colour of the stock CoCl2· 6H2O solution.
3. Pour 12 mL of 0.1 M CoCl2· 6H2O into a clean, dry 50 mL beaker. In the fume hood, add
concentrated HCl drop wise and mix with a glass rod until a permanent colour change is
observed. Record the observed colour.
4. Divide the solution equally into four test tubes. Use the Control test tube for colour
comparison.
5. In Test tube #1 add water with mixing until a colour change is produced. Record the
observed colour.
6. Heat Test tube #1 in a hot water bath (add boiling chips) and you should see a colour
change. (If you don’t then you have likely added too much water. Try again with another
sample.) Record the observed colour.
7. Cool Test tube #2 in an ice water bath and record the observations. Record the observed
colour. Keep your ice bath for Part B.
8. Heat Test tube #3 in a hot water bath and record your observations. Record the observed
colour. Keep your water bath for Part B.
9. Dispose of the cobalt solutions in the waste bottle.
Part B - The Equilibrium of the thiocyano-iron(III) complex ion
You should compare the colour of each tube with the reference Test tube #1. Note the colour of the
0.1 M Fe(NO3)3.
1. In a 100 mL beaker, combine:
o 1.5 mL of 0.1 M Fe(NO3)3, ** USE THE CORRECT CONCENTRATION**
o 1.5 mL of 0.1 M KSCN,
** USE THE CORRECT CONCENTRATION**
o 50 mL H2O.
Pour 5 mL of the solution into nine numbered test tubes.
2. Add two drops of H2O to Test tube #1, which will serve as reference for colour. Record your
observations.
3. Add two drops of 1M Fe(NO3)3 to Test tube #2. Record your observations.
4. Add two drops of 1M KSCN to Test tube #3. Record your observations.
5. Add 8 drops 6M NaOH to Test tube #4. The precipitate Fe(OH)3 will take a few minutes to
form.
Record your observations.
6. Add 4 drops of AgNO3 to Test tube #5. The precipitate is AgSCN. Record your observations.
7. Add 4 drops of 0.1 M NaCl to Test tube #6. Record your observations.
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8. Place Test tube #7 in an ice water bath. Record observations.
9. Place Test tube #8 in a boiling water bath. Record observations.
10. Add 1 mL of distilled water to Test tube #9. Record your observations.
11. Now add an additional 4 mL of water and record your observations.
12. Dispose of the reagents as instructed.
Part C - The Equilibrium of a Mg2+ precipitate
1. Into a test tube add:
o 1 mL water,
o 2 drops of 0.1 M Mg(NO3)2, and
o 3 drops of 6M NH4OH.
Record your observations.
2. Add a small amount (1/4 spatula) of solid NH4Cl to the test tube and mix to dissolve. Record
your observations. What is the product? Consult a solubility table.
Part D: The Equilibrium of an Acid-Base Indicator
1. Obtain a pH 7 buffer solution and pour 3 mL into a 50 mL beaker. Add 5 drops of
bromothymol blue indicator. Record your observations.
2. Add 1 M HCl drop wise with mixing until the solution is acidic and the indicator shows a
colour change. Record your observations.
3. Add 1M NaOH drop wise to return to the original colour and continue until the solution is
basic and a new colour is reached. Record your observations.
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Datasheet:
Part A - The Equilibrium of Co(II) Complex Ions
Solutions
Colour
CoCl2· 6H2O (step 2)
CoCl2· 6H2O + HCl (step 3)
Test tube #1 + H2O (step 5)
Test tube #1 + H2O + heat (step 6)
Test tube #2 + ice water bath (step 7)
Test tube #3 + hot water bath (step 8)
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Datasheet:
Part B - The Equilibrium of the thiocyano-iron(III) complex ion
Test tubes
Observations
Test tube #1 - Fe(NO3)3 + KSCN + H2O(step 2)
Test tube #2 + 1 M Fe(NO3)3 (step 3)
Test tube #3 + 1M KSCN (step 4)
Test tube #4 + 6 M NaOH (step 5)
Test tube #5 + 4 drops AgNO3 (step 6)
Test tube #6 + 0.1 M NaCl (step 7)
Test tube #7 + ice (step 8)
Test tube #8 + boiling water bath (step 9)
Test tube #9 + 1 mL distilled water (step 10)
Test tube #9 + 5 mL distilled water (step 11)
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Datasheet:
Part C - The Equilibrium of a Mg2+ precipitate
Test tubes
Observations
Mg(NO3)2 + NH4OH (step 1)
Test tube + NH4Cl (step 2)
Part D: The Equilibrium of an Acid-Base Indicator
Solutions
Observations
pH 7 Buffer + bromothymol blue
(HInd) (step 1)
Solution + HCl (step 2)
Solution + NaOH (step 3)
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Postlab Questions:
Part A - The Equilibrium of Co(II) Complex Ions
1. Write the equilibrium equation for step 3 when concentrated HCl was added to CoCl 2· 6H2O.
Equilibrium reaction:
2 .In Step 5, circle the shift in equilibrium observed (if any) when water is added to Test tube #1.
no shift
in equilibrium
shifts to the right
shifts to the left
3. In Step 5, the colour of the solution is due to the presence of which ion?
4. In Step 6, circle the shift in equilibrium observed (if any) when Test tube #1 is heated.
no shift
in equilibrium
shifts to the right
shifts to the left
5. Circle the correct response. Which statement is true?
heating favours
an exothermic process
heating favours
an endothermic process
6. In Step 6, the colour of the solution is due to the presence of which ion?
7. In Step 7, circle the shift in equilibrium observed (if any) when Test tube #2 is in the ice bath.
no shift
in equilibrium
shifts to the right
shifts to the left
8. Circle the correct response. Which statement is true?
cooling favours
an exothermic process
cooling favours
an endothermic process
9. In Step 7, the colour of the solution is due to the presence of which ion?
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10. In Step 8, circle the shift in equilibrium observed (if any) when Test tube #3 is in the hot water
bath.
no shift
in equilibrium
shifts to the right
shifts to the left
11. Circle the correct response. Which statement is true?
heating favours
the equilibrium products
heating favours
the equilibrium reactants
12. In Step 8, the colour of the solution is due to the presence of which ion?
Part B - The Equilibrium of the thiocyano-iron(III) complex ion
1. Write the equilibrium equation for step 1 when Fe(NO3)3, KSCN and water was combined.
Equilibrium reaction:
2. In Step 2, circle the shift in equilibrium observed (if any) in Test tube #1 when 2 drops of water
is added.
no shift
in equilibrium
shifts to the right
shifts to the left
3. In Step 3, circle the shift in equilibrium observed (if any) when Test tube #2 when 2 drops of
1 M Fe(NO3)3 is added.
no shift
in equilibrium
shifts to the right
shifts to the left
4. In Step 4, circle the shift in equilibrium observed (if any) when Test tube #3 when 2 drops of
1 M KSCN is added.
no shift
in equilibrium
shifts to the right
shifts to the left
5. In Step 5, circle the shift in equilibrium observed (if any) when Test tube #4 when 8 drops of 6
M NaOH is added.
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no shift
in equilibrium
shifts to the right
shifts to the left
6. In Step 6, circle the shift in equilibrium observed (if any) when Test tube #5 when 4 drops of
AgNO3 is added.
no shift
in equilibrium
shifts to the right
shifts to the left
7. In Step 7, circle the shift in equilibrium observed (if any) when Test tube #6 when 4 drops of
0.1 M NaCl is added.
no shift
in equilibrium
shifts to the right
shifts to the left
8. In Step 8, circle the shift in equilibrium observed (if any) when Test tube #7 when it is placed in
ice water bath.
no shift
in equilibrium
shifts to the right
shifts to the left
9. In Step 9, circle the shift in equilibrium observed (if any) when Test tube #8 when it is placed in
hot water bath.
no shift
in equilibrium
shifts to the right
shifts to the left
10. In Step 10, circle the shift in equilibrium observed (if any) when Test tube #9 when 1 mL of
water is added.
no shift
in equilibrium
shifts to the right
shifts to the left
11. In Step 11, circle the shift in equilibrium observed (if any) when Test tube #9 when 5 mL of
water is added.
no shift
in equilibrium
shifts to the right
shifts to the left
12. Write the equilibrium constant expression and explain the observation when water is added.
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Part C -The Equilibrium of a Mg2+ precipitate
1. Write the equilibrium equation (written as Ksp) for step 1 when Mg(NO3)2 and NH4OH was
combined.
Equilibrium reaction:
2 .In Step 2, circle the shift in equilibrium observed (if any) in the test tube when NH 4Cl is is added.
no shift
in equilibrium
shifts to the right
shifts to the left
Part D -The Equilibrium of an Acid-Base Indicator
1. Write the equilibrium equation (written as Ka) for step 1 when the pH 7 buffer and 5 drops of
bromothymol blue (HInd) is combined.
Equilibrium reaction:
2 .In Step 2, circle the shift in equilibrium observed (if any) in the test tube when 1 M HCl is added
to the solution.
no shift
in equilibrium
shifts to the right
shifts to the left
3 .In Step 3, circle the shift in equilibrium observed (if any) in the test tube when 1 M NaOH is
added to the solution.
no shift
in equilibrium
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shifts to the right
shifts to the left
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