UNIT 02: Equilibrium
BCLN CHEMISTRY 12 - Rev. July, 2015
Project: Chemical Equilibrium Lab
Potential Credits:
Name: ________________
/20
Goal and Instructions:
To observe the macroscopic properties of chemical systems at equilibrium and to explain the
observations obtained by applying Le Chatelier’s principle.
Begin by watching this video: Chemical Equilibrium.
Thymol Blue Data and Observations:
There are two equilibrium reactions at work here:
1) Thymol blue (blue) + H+
2) Thymol blue (yellow) + H+
Thymol blue (yellow)
(Color change between pH 8.0-9.6)
Thymol blue (red) )
(Color change between pH 1.1-2.8)
1. What is the original color of the Thymol blue in the video?
2. What is the lowest possible pH the original solution could have? Explain how you know
3. Complete table 1 below. For the shift column, use Right if the equilibrium shifts towards the
products and Left if the equilibrium shifts towards the reactants. The stress ion is the component of
the solution added that affects the equilibrium.
Table 1: Equilibrium Stresses
Solution Added
Stress Ion
Color Change
HCl
More HCl
NaOH
More NaOH
Page 1 of 5
Shift
UNIT 02: Equilibrium
BCLN CHEMISTRY 12 - Rev. July, 2015
Thymol Blue Questions:
1. Explain how adding HCl causes the shift it does.
2. Explain how adding NaOH causes the shift it does. Hint: You must mention a reaction that
creates water in your explanation.
3. If you increased the concentration of HCl or NaOH used, how would that affect the number of
drops required before you saw a color change?
Thiocyanatoiron(III) Ion Data and Observations:
In this section we will be looking at changes to the following equilibrium:
Fe3+(aq) (yellow) + SCN-(aq) (colorless)
FeSCN2+(aq) (blood-red)
Chloride ions are colorless. Potassium ions are also colorless. The above equilibrium can be created
by mixing an Iron (III) chloride solution with a potassium thiocyanate solution. Based on this
information and the colors in the equilibrium above answer the first two questions.
1. What color would an FeCl3 solution be?
2. What color would a KSCN solution be?
3. What color do you get when you mix FeCl3 and KSCN as shown in the video for the control test
tube?
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UNIT 02: Equilibrium
BCLN CHEMISTRY 12 - Rev. July, 2015
4. Complete table 2 below. For the shift column, use Right if the equilibrium shifts towards the
products and Left if the equilibrium shifts towards the reactants. The stress ion is the component of
the solution added that affects the equilibrium.
Solution Added
Stress Ion
Spectator Ion
Color Change
Shift
KCl
Test Tube B
Fe(NO3)3
Test Tube C
KSCN
Test Tube D
NaOH
Test Tube E
Thiocyanatoiron(III) Ion Questions:
1. What is a spectator ion?
2. Using Le Chatelier's principle, explain why the reaction shifts the direction it does when Fe3+ is
added.
2. When NaOH is added, there is a color change to light yellow and a precipitate forms. Using Le
Chatelier's principle, explain why the reaction shifts the direction it does when NaOH is added.
Page 3 of 5
UNIT 02: Equilibrium
BCLN CHEMISTRY 12 - Rev. July, 2015
Copper(II) Complexes Data and Observations:
In this section there will be three equilibriums to consider:
When a small amount of NH3 is first added, it reacts with water to form ammonium (NH4+) ions and
hydroxide ions:
Equilibrium 1: NH3(aq) + H2O(l)
NH4+(aq) + OH–(aq)
Copper(II) ions can then form a precipitate with the hydroxide ions that are formed:
Equilibrium 2: Cu2+(aq) (light blue) + 2OH–(aq)
Cu(OH)2(s)
Copper(II) ions in water are known to exist as tetraaquocopper(II) ions (Cu(H2O)42+), which give a
solution a pale blue color. When more ammonia (NH3) is added to this solution, a new equilibrium
develops:
Equilibrium 3: Cu(H2O)42+(aq) (light blue) + 4NH3(aq)
Cu(NH3)42+(aq) (dark blue) + 4H2O(l)
The complex ion Cu(NH3)42+, called the tetraamminecopper(II) ion, is a deep blue color.
It is also known that H+ ions from acid will react with NH3 and decrease the [NH3].
1. Complete Table 3 below.
CuSO4
+3 drops of NH3
(ammonia)
Color observed
Page 4 of 5
More NH3
HCl added
UNIT 02: Equilibrium
BCLN CHEMISTRY 12 - Rev. July, 2015
Copper(II) Complexes Questions:
1. Explain using the equilibrium from the previous page why a precipitate forms when a few drops of
ammonia are added to the copper (II) sulphate solution.
2. Using equilibrium concepts, explain the color change observed when HCl is added to the
Cu(NH3)42+ ion.
Concluding Questions:
1. State Le Chatelier's Principle.
2. If the concentration of a reactant is increased, what is the effect on the equilibrium and why does it
occur?
3. If the concentration of a reactant is decreased, what is the effect on the equilibrium and why does
it occur?
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