Le Chatelier’s Principle
Objective : Study equilibrium and Le Chatelier’s Principle.
Prelab Questions :
(PL1) In step (1), what is the stress and which direction should the equilibrium
shift?
(PL2) In step (5), what is the stress and which direction should the equilibrium
shift?
(PL3) In step (7), what is the stress and which direction should the equilibrium
shift?
Procedure :
There will be 6 stations for 6 reactions. You will go around each station and
the reagents will remain at one place.
NaCl (s)  Na+ (aq) + Cl- (aq)
(1) *Add few drops of conc. HCl to 2 to 3 mL of saturated NaCl solution.
NH4Cl(s)  NH4+ (aq) + Cl-(aq)
(2) *Add few drops of conc. HCl to 2 to 3 mL of saturated NH4Cl solution.
Fe+3(aq) + SCN-(aq)  Fe(SCN)+2(aq)
(3) Prepare a stock solution by mixing 2mL each of 0.1M FeCl3 and 0.1M KSCN
until 100mL. Note the color. (use 100mL graduated cylinder) – If you are the
first one to use this station, make this solution and save it for the rest of the
class.
(4) *Add about 1mL of 0.1M FeCl3 to about 5mL of stock solution.
(5) *Add about 1mL of 0.1M KSCN to about 5mL of stock solution.
(6) *Add 0.1M AgNO3 DROPWISE (less than 1mL) to about 5mL of stock solution
until almost all the color is discharges. (The white ppt is a mixture of AgCl and
AgSCN.) Split the solution into two test tubes including the precipitate.
(6a) Add 0.1M KSCN dropwise (about 2mL) to one tube
(6b) Add 0.1M FeCl3 dropwise (about 2mL) to the other tube.
2CrO4 –2(aq)
+ 2H+ (aq)  Cr2O7 –2(aq)
+
H2O(l)
(7) *Add 2 drops of 6M HNO3 to about 3mL of 0.1M K2CrO4.
(8) Add 10% NaOH to (7) dropwise until the original color of K2CrO4 is restored.
(9) *Add 2 drops of 6M H2SO4 to about 3mL of 0.1M K2CrO4.
(10) Add 10% NaOH to (9) dropwise until the original color of K2CrO4 is restored.
Co(H2O)6 +2(aq) + 4Cl-(aq)
 CoCl4 –2(aq) + 6 H2O
(11) *Add about 3mL of Conc. HCl solution dropwise to about 2mL (no more than) of
0.1M CoCl2.
(12) Add water to (11) until the reverse reaction is evident.
NH3(g) + H2O(l)  NH4+ (aq) + OH-(aq)
(13) Prepare a stock solution by mixing 4 drops of conc. NH4OH and 3 drops of
phenolphthalein until 100mL. (use 100mL graduated cylinder) – If you are the
first one to use this station, make this solution and save it for the rest of the
class.
(14) *Dissolove a very small amount of solid NH4Cl in about 5mL of stock solution.
(15) *Add a few drops of 6M HCl to about 5mL stock solution.
Postlab Questions :
(Q1) In step (1), which direction DID the equilibrium shift? What was the visual
indication of the shift?
(Q2) In step (5), which direction DID the equilibrium shift? What was the visual
indication of the shift?
(Q3) In step (8), which direction DID the equilibrium shift? What was the visual
indication of the shift?
(Q4) Write a balanced net ionic equation showing what happened in step (6).
(Q5) In step (6a), what is the stress and which direction DID the equilibrium shift?
What was the visual indication of the shift? Use the equation from Q4.
(Q6) In step (10) , which direction DID the equilibrium shift? Using balanced
equations, explain the direction of this shift.
Summary
Rewrite 6 chemical equations. Under each component, write its characteristic such
as color. Make sure to include HEAT as a reactant or product if now known.