Lab 5: Equilibrium and Le Châtelier’s Principle
Objectives:
To explore the effect of temperature and changing the concentrations of reactants and products
on the equilibrium composition of five 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 of a
reactant or product and the effects observed. It should be noted that for a system, there exists
many equilibrium positions but only one value of the equilibrium constant at a specific
temperature.
Adding more of a reactant or product will increase the concentration of that species (addition of
a common species). By adding a substance that will react with the reactant or product (eg via a
side reaction) the concentration of the reactant or product will decrease.
In this experiment, we will study the equilibrium of five systems and observe the reaction of the
equilibrium systems as predicted by Le Châtelier’s Principle. The five 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()
ΔH = +50 kJ/mol
(1)
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)
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FeSCN2+(aq)
(2)
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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 Mg2+ Precipitate
Reactions which form precipitates are written as an equilibrium using the solubility product. If
there is a precipitate MX, then the Ksp expression is:
MX(s)
Mn+(aq) + Xn-(aq)
(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 pH = pKa of the indicator there are 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)
(4)
Part E: The Equilibrium of a Saturated Solution
A saturated solution is solution in which the dissolved substance is in equilibrium with some of
the undissolved substance. One example is a salt such as in the equilibrium (3). In this case
when one of the ions is added to the solution, the equilibrium will shift to the left and some of the
solid will precipitate out.
Procedure:
During this lab, record all observations such as colours, any liberation or absorption of heat, and
any precipitates formed or dissolved.
Part A - The Equilibrium of Co(II) Complex Ions
1. Number four clean and dry test tubes. Record the initial colour of the stock CoCl26H2O
solution.
2. Pour 12 mL of 0.1 M CoCl26H2O into a clean, dry 50 mL beaker. In the fume hood, add
concentrated HCl (several squirts) with mixing until a colour change is observed. Record the
observed colour.
3. Divide the solution equally into four test tubes. Set aside test-tube #4 to use as a reference.
4. In Test tube #1 add water with mixing until a colour change is produced. Record the
observed colour.
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5. 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.
6. Cool Test tube #2 in an ice water bath and record the observations. Record the observed
colour. Keep your ice bath for Part B.
7. Heat Test tube #3 in a hot water bath and record your observations. Record the observed
colour. Keep your hot water bath for Part B.
8. 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:
1.5 mL of 0.1 M Fe(NO3)3,
1.5 mL of 0.1 M KSCN,
50 mL H2O.
2. Pour 5 mL of the solution into nine numbered test tubes.
3. Add two drops of H2O to Test tube #1, which will serve as reference for colour. Record your
observations.
4. Add two drops of 1 M Fe(NO3)3 to Test tube #2. Record your observations.
5. Add two drops of 1 M KSCN to Test tube #3. Record your observations.
6. Add 8 drops 6 M NaOH to Test tube #4. The precipitate Fe(OH)3 will take a few minutes to
form. Record your observations.
7. Add 4 drops of AgNO3 to Test tube #5. The precipitate is AgSCN. Record your observations.
8. Add 4 drops of 0.1 M NaCl to Test tube #6. Record your observations.
9. Place Test tube #7 in an ice water bath. Record observations.
10. Place Test tube #8 in a boiling water bath. Record observations.
11. Add 1 mL of distilled water to Test tube #9. Record your observations.
12. Now add an additional 4 mL of water and record your observations.
13. Dispose of the solutions into the waste bottle.
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Part C - The Equilibrium of a Mg2+ precipitate
1. Into a test tube add:
1 mL water,
10 drops of 0.1 M Mg(NO3)2,
10 drops of 6 M NH3 (note it maybe labelled as 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 1 M 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.
Part E: The Equilibrium of a Saturated Solution
1. To 3 mL of saturated sodium chloride (5.4 M) add 1-2 mL of concentrated HCl in the
fumehood.. Record your observations.
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Lab 5: Equilibrium and Le Châtelier’s Principle Date: _______________
Name: __________________________
Partner: _____________
Purpose:
Data and Results
Use the following key to answer some of the questions following the observations for each step.
 Equilibrium shifts left (towards reactants)
 Increase in amount
 No noticeable shift in equilibrium
 No change in amount
 Equilibrium shifts right (towards products)
 Decrease in amount
Part A - The Equilibrium of Co(II) Complex Ions
CoCl26H2O solution – Colour ________________________
Reagents
Observations
CoCl26H2O solution + HCl
Equilibrium Reaction:
Reagents
Observations
Test Tube #1 + H2O
The colour of the solution is due to the presence of which ion? ____________
Direction of equilibrium shift (circle answer)



This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
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Reagents
Observations
Test Tube #1 + H2O + hot water bath
The colour of the solution is due to the presence of which ion? ____________
Direction of equilibrium shift (circle answer)



Which statement is true? (circle response)
a) heating favours an exothermic process
b) heating favours an endothermic process
Reagents
Observations
Test Tube #2 + ice water bath
The colour of the solution is due to the presence of which ion? ____________
Direction of equilibrium shift (circle answer)



Which statement is true? (circle response)
a) cooling favours an exothermic process
b) cooling favours an endothermic process
Reagents
Observations
Test Tube #3 + hot water bath
The colour of the solution is due to the presence of which ion? ____________
Direction of equilibrium shift (circle answer)



Heating favours the equilibrium __________? (circle response)
a) reactants
b) products
Write the equilibrium reaction with heat as a product or reactant
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Part B - The Equilibrium of the thiocyano-iron(III) complex ion
Reagents
Observations
Test Tube #1 - Fe(NO3)3 + KSCN + H2O
Equilibrium Reaction:
The colour of the solution is due to the presence of which ion? ____________
Reagents
Observations
Test Tube #2 + Fe(NO3)3
Direction of equilibrium shift (circle answer)



This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
Reagents
Observations
Test Tube #3 + KSCN
Direction of equilibrium shift (circle answer)



This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
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Reagents
Observations
Test Tube #4 + NaOH
Direction of equilibrium shift (circle answer)



This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
Reagents
Observations
Test Tube #5 + AgNO3
Direction of equilibrium shift (circle answer)



This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
Reagents
Observations
Test Tube #6 + NaCl
Direction of equilibrium shift (circle answer)



Provide an explanation of your observations
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Reagents
Observations
Test Tube #7 + ice water bath
Direction of equilibrium shift (circle answer)



Provide an explanation of your observations
Reagents
Observations
Test Tube #8 + hot water bath
Direction of equilibrium shift (circle answer)



Provide an explanation of your observations
Write the equilibrium reaction with heat as a product or reactant.
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Reagents
Observations
Test Tube #9 + 1 mL water
Test Tube #9 + 5 mL water
Direction of equilibrium shift (circle answer)



Write the equilibrium constant expression.
The colour of the solution is due to the presence of which ion? ____________
Using the equilibrium constant expression, explain the observations when water is added.
Part C - The Equilibrium of a Mg2+ precipitate
Reagents
Observations
Mg(NO3)2 + NH3
Equilibrium reaction (written as a Ksp):
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Reagents
Observations
Tube + NH4Cl
Direction of equilibrium shift (circle answer)



This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
Part D: The Equilibrium of an Acid-Base Indicator
Reagents
Observations
Buffer + bromothymol blue (Hind)
Equilibrium reaction (written as a Ka):
Reagents
Observations
Solution + HCl

On addition of this reagent the [H+] has (circle the answer)
Direction of equilibrium shift (circle answer)





This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
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Reagents
Observations
Solution + NaOH

On addition of this reagent the [H+] has (circle the answer)
Direction of equilibrium shift (circle answer)





This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
Part : The Equilibrium of a Saturated Solution
Reagents
Observations
Solution + HCl
Equilibrum Equation (written as Ksp)
Direction of equilibrium shift (circle answer)



This shift (if any) is due to (circle response)
a) common species (name it) ___________________
b) side reaction (show below)
What is the identity of any precipitate formed? ____________________
Conclusions:
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