Le Chatelier and Equilibrium Systems
Purpose:
To explore how to cause SHIFT in an equilibrium system.
Materials:
0.10 M FeCl3, 0.10 M KSCN, 3.0 M NaOH, 0.10 M AgNO3, solid NH4Cl, phenolphthalein, 0.10 M K2CrO4,
saturated NH4Cl, Concentrated 12 M HCl, Concentrated ammonia.
CAUTION: 12 M HCl and Concentrated ammonia are DANGEROUS. DO NOT INHALE, or TOUCH!
A. Complex Ion Equilibria
Fe+3
+
SCN-
<->
Amber
FeSCN+2
Red
1. Prepare a stock sample of the the bright red complex ion FeNCS2+ by mixing 2 ml of 0.10 M iron(III)
chloride and 2 ml of 0.10 M KSCN solutions. DO THIS IN A BEAKER. The color of this mixture
is too intense to use as it is, so dilute this mixture with 100 ml of water.
2. Pour about 5 ml of the diluted red stock solution into each of five test tubes. Label the tubes as
1,2,3,4 and 5.
3. Using test tube one as a reference, determine what equilibrium shifts occur upon the addition of:
1) iron ions
2) thiocyanate ions
3) hydroxide ions
4) silver ions
Use a different tube for each test.
(one of the 5 tubes is simply for comparison purposes)
B. Acid/Base Equilibria
NH3
+
H2O
<->
NH4+
Clear (in indicator)
+
OHpink (in indicator)
1. In a fume hood, prepare a dilute ammonia solution by adding 2 drops of concentrated ammonia to 25
ml of water.
2. Add 2 drops of phenolphthalein to the dilute ammonia solution.
3. Place about 5 ml of the pink ammonia solution in a test tube.
4. Determine what equilibrium shifts occur upon the addition of:
1) ammonium ions
2) concentrated ammonia
Use the same tube for each test.
Avoid inhaling the concentrated ammonia!
C. Saturated Solution Equilibrium
NH4Cl(s) <-> NH4+(aq) + Cl-(aq)
1. Starting with 1-3 mL of a saturated solution of NH4Cl, determine the effect of the addition of:
1) chloride ions (from concentrated 12 M HCl) LET IT SIT IF NECESSARY!
2) water to tube used for test #1
Use the same tube for each test
BE CAREFUL WITH THE 12 M HCl. Get it in a dropper and place the dropper in a beaker to
carry it back to your station.
D. Another Saturated Solution Equilibrium
NH4Cl(s) <-> NH4+(aq) + Cl-(aq)
Note: This dissolving process is highly endothermic
1. Starting with 3-5 mL of a saturated solution of NH4Cl, determine the effect of:
1) Cooling the solution by placing the test tube containing the solution in an ice water bath for a
few minutes.
2) Heating the solution by carefully and gently heating the tube in a Bunsen Burner flame.
Do not allow the solution to boil.
Use the same tube for each test.
E. Chromate/Dichromate Equilibrium
2 CrO42-(aq) + 2 H+(aq) <-> Cr2O72- (aq) + H2O (l)
Yellow
Orange
1. Place 2 ml of 0.10 M potassium chromate in a test tube.
Determine the effect on the equilibrum by:
1) Adding 12 M HCl until a color change occurs.
2) Adding hydroxide ions to the solution you just made in number 1.
Use the same tube for each test.
Data / Analysis:
For each test, include a system of lines and arrows showing initial stress, impact on individual
concentrations, and direction of shift. Also include a section for specific observations.
Conclusion:
Write a conclusion in which you summarize how an equilibrium system responds to disruptions from the
outside. Give multiple specific examples from the actual lab exercise.