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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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), CoCl42-, 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) CoCl42- (aq) + 6 H2O (l) ΔH = +50 kJ(3-1)

pink blue

Part B - The Equilibrium of the thiocyano-iron(III) complex ion

When colourless aqueous solutions of iron (III) ion, Fe3+, and thiocyanate ion, SCN-, are combined, the reaction that occurs produces the thiocyanoiron (III) complex ion, FeSCN2+, which is responsible for the equilibrium mixture's deep red colour.

Fe3+ (aq) + SCN- (aq) FeSCN2+ (aq) (3-2)
colourless / colourless / red-brown

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) H+ (aq ) + Ind- (aq)(3-4)
yellow / blue

Apparatus:

  1. Large test tubes
  2. Hot plate (1 per student)

Solutions:

  1. 0.1 M CoCl2·6H2O (15 mL per student)
  2. concentrated HCl (place in fumehood)
  3. 0.1 M AgNO3
  4. 0.1 M Mg(NO3)2
  5. 6M NH4OH
  6. NH4Cl solid
  7. pH 7 buffer solution (3 mL per student)
  8. Bromothymol blue indicator
  9. 0.1 M Fe(NO3)3 in 0.1M HNO3 (2 mL per student)
  10. 1 M Fe(NO3)3 in 0.1M HNO3 (0.1 mL per student)
  11. 0.1 M KSCN in 0.1M HNO3 (2 mL per student)
  12. 1 M KSCN in 0.1M HNO3 (0.1 mL per student)
  13. 0.1 M NaCl
  14. 6M NaOH
  15. ice
  16. 1 M HCl
  17. 1 M NaOH

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. InTest 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:
  2. 1.5 mL of 0.1 M Fe(NO3)3, ** USE THE CORRECT CONCENTRATION**
  3. 1.5 mL of 0.1 M KSCN, ** USE THE CORRECT CONCENTRATION**
  4. 50 mL H2O.

Pour 5 mL of the solution into nine numbered test tubes.

  1. Add two drops of H2O to Testtube #1, which will serve as reference for colour. Record your observations.
  2. Add two drops of 1M Fe(NO3)3 to Test tube #2. Record your observations.
  3. Add two drops of 1M KSCN to Test tube #3. Record your observations.
  4. Add 8 drops 6M NaOH to Test tube #4. The precipitate Fe(OH)3 will take a few minutes to form.
    Record your observations.
  5. Add 4 drops of AgNO3 to Testtube #5. The precipitate is AgSCN. Record your observations.
  6. Add 4 drops of 0.1 M NaCl to Test tube #6. Record your observations.
  7. PlaceTesttube #7in an ice water bath. Record observations.
  8. Place Testtube #8 in a boiling water bath. Record observations.
  9. Add 1 mL of distilled water toTest tube #9. Record your observations.
  10. Now add an additional 4 mL of water and record your observations.
  11. Dispose of the reagents as instructed.

Part C - The Equilibrium of a Mg2+ precipitate

  1. Into a test tube add:
  2. 1 mL water,
  3. 2 drops of 0.1 M Mg(NO3)2, and
  4. 3 drops of 6M NH4OH.

Record your observations.

  1. 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.

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)

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)

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)

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 CoCl2· 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?

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.

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.

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 NH4Cl 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 /
shifts to the right /
shifts to the left

1

Last updated 12/6/2009 9:27 PM