Thymol Blue has 2 color changes:
#1 - as pH increases the change is
red to yellow between pH 1.2
Chemical Equilibrium Lab
and 2.8
so equation should be
Purpose: To observe macroscopic properties of chemical systems at
equilibrium and to explain the observations obtained by applying
Le Chatelier’s principle.
Thymol Blue (red) <--> Thymol
Blue (yellow) + H+
#2 - as pH increases the change is
yellow to blue between pH 8.0
Procedure: Watch the video Chemical Equilibrium carefully
and 9.6 so equation should be
Thymol Blue (yellow) <--->
Observation:
Thymol Blue (blue) + H+
Part 1—Equilibrium Involving Thymol Blue
What is the initial color of Thymol Blue? _______________________________
The equilibrium reactions for Thymol Blue are:
Equilibrium 1: Thymol blue (red)
Thymol blue (yellow) + H +
Equilibrium 2: Thymol blue (yellow)
Thymol blue (blue) + H +
Table 1
Solution added
Stress ion
Color change
Direction of
equilibrium shift
HCl
HCl
NaOH
NaOH
Questions Part 1
1. Complete table 1. For direction of equilibrium shift use right for shift to products
and left for shift to reactants.
2. Explain how adding HCl and NaOH changes (shifts ) the above equilibrium?
________________________________________________________________________
________________________________________________________________________
3. How would increasing the concentration of HCl and NaOH affect the number of
drops required for the observed color changes?
________________________________________________________________________
________________________________________________________________________
Part II—Equilibrium Involving the Thiocyanatoiron(III) Ion
This part involves the light yellow iron(III) ion, Fe3+, and the colorless thiocyanate ion,
SCN–. They react together to form the complex ion FeSCN2+ which is a blood-red color:
What is color of the FeCl3 solution? ___________________
What is the color of the KSCN solution? _______________
What is the color of the resultant FeSCN2+ solution? ___________________
What is the function of Test tube A? _________________________________
Table 2
Solution Added
Stress ion
added
Spectator ion
Color observed
Direction of
equilibrium
shift
KCl
Test tube B
Fe(NO3)3
Test tube C
KSCN
Test tube D
NaOH
Test tube E
Questions Part 2
1. Complete the above table.
2. What is a spectator ion? _________________________________________
_____________________________________________________________
3. Apply Le Chatelier’s principle to explain the results obtained when NaOH was
added to the thiocyanatoiron(III) ion equilibrium? ______________________
_______________________________________________________________
_______________________________________________________________
Part III—Equilibrium Involving Copper(II) complexes
The color of solutions with copper(II) ions is blue.
What is the initial color of the CuSO4 in the test tube? ____________________________
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) + 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 colour. When more ammonia (NH3) is added to this
solution, a new equilibrium develops:
Equilibrium 3: Cu(H2O)42+(aq) + 4NH3(aq)
Cu(NH3)42+(aq) + 4H2O(l)
2+
The complex ion Cu(NH3)4 , called the tetraamminecopper(II) ion, is a deep blue colour.
It is also known that H+ ions from acid will react with NH3 and decrease the [NH3].
Table 3
CuSO4
+3 drops of
NH3(ammonia)
More NH3
ammonia
HCl added
Color observed
Part 3 Questions
1. Complete table 3 by watching the video.
2. Explain why a precipitate could form when a few drops of ammonia are first
added to the copper(II) sulphate solution.
________________________________________________________________
3. Using equilibrium concepts, explain the color change observed when HCl is
added to the Cu(NH3)42+ ion.
________________________________________________________________
Conclusion
State the effect on the position of equilibrium if a change is made in the concentration of
a reactant or product. State Le Chatelier’s principle.