LAB 17: LE CHATELIER’S PRINCIPLE
 This lab does NOT require a lab report.
 You will work with a partner for this lab.
I Introduction
LeChatelier’s principle states that: If an equilibrium system is subjected to a stress, the
system will react to remove the stress. To remove a stress, a system can only do one of two
things: form more products and use up reactants (shift right), OR reverse the reaction and
form more reactants, using up products (shift left). In this experiment, you will form several
equilibrium systems at different stations. Then, by putting different stresses on the systems,
you will observe how equilibrium systems react to a stress.
! Warnings!


This lab uses concentrated (12 M) HCl, hydrochloric acid. This is EXTREMELY
dangerous. It is extremely corrosive to skin and eyes, so where your safety goggles and
apron. Its fumes are choking and poisonous, so use it only in the fume hood.
This lab uses cobalt chloride dissolved in ethanol. Cobalt chloride is mildly toxic; do
not ingest and clean up spills immediately. Wear your safety goggles. Ethanol is
flammable, so keep away from open flames.
Procedure
Experiment 1: Equilibrium in a Saturated Solution.
You will investigate the equilibrium in a saturated sodium chloride solution:
NaCl(s)  Na+(aq) + Cl-(aq)
Directions
1) Set up your lab notebook.
2) Fill a test tube ¾ full of the saturated NaCl solution (you can see the excess
NaCl(s) lying on the beaker bottom to verify its saturation). Be careful not to
transfer any solid NaCl into the test tube. This is your initial equilibrium state.
3) To this saturated (and solid-free) solution of NaCl, add some Cl- ions in the
form of concentrated HCl.
Experiment 1
Initial
Equilibrium
Condition
Stress
Saturated
NaCl(aq)
solution
Add
concentrated
HCl(aq)
Predicted
Shift
Observations
Observed
Shift
Experiments 2 & 3: An Acid-Base Indicator Equilibrium
Acid-base indicators are large organic molecules that can gain and lose hydrogen ions
to form substances that have different colors. The reaction of the indicator
bromthymol blue (HIn in the equation below) can be illustrated as follows:
HIn(aq)  H+(aq) + In-(aq)
yellow
blue
In this reaction, HIn is the neutral indicator molecule and In- is the indicator ion after
the molecule has been stripped of a hydrogen ion.
Directions
1) Set up your lab notebook.
2) Fill a small test tube about ½-full of distilled water. Add many (15 – 20) drops of
bromthymol blue indicator solution. This is your initial equilibrium state.
3) Add 5 drops of 0.1 M HCl and stir. This will increase the amount of H+ in solution.
Note the color of the indicator.
4) Next add 0.1 M NaOH drop by drop with stirring until no further color change
occurs. Adding OH- ions causes the H+ ion concentration to decrease as the ions
combine to form water molecules (H+ + OH- → H2O). Again, note the color.
5) See if you can add the right amount of acid to this test tube to cause the solution
to be green in color after it is stirred (half of the indicator is blue and half is
yellow).
Initial
Equilibrium
Condition
Stress
Experiment
2
HIn in distilled
water
Add HCl(aq)
Experiment
3
Acidified
solution formed
in Demo 2
Add
NaOH(aq)
Predicted
Shift
Observations
Observed
Shift
Experiment 4: An Equilibrium with Cobalt Complex Ions
In this section we will investigate the equilibrium between two different complex ions of
cobalt. The reaction is endothermic:
Co(H2O)62+(aq) + 4 Cl-(aq)

CoCl42-(aq) + 6 H2O(l)
pink
H = + 50 kJ/mol
blue
Directions
1) Setup your lab notebook.
2) Fill a test tube about ½-full of a solution of cobalt(II) chloride, CoCl2, dissolved in
ethanol (already prepared for you). The solution should be purple. This is your
initial equilibrium state.
3) Add 3 drops of distilled water, one drop at a time with stirring, noting what
happens with each drop.
4) Keep your resulting solution for Experiment 5.
Experiment 4
Initial
Equilibrium
Condition
Stress
CoCl2 dissolved
in anhydrous
ethanol
Add H2O
Predicted
Shift
Observations
Observed
Shift
Experiment 5: An Equilibrium with Cobalt Complex Ions
In this section we will investigate the equilibrium between two different complex ions of
cobalt. The reaction is endothermic:
Co(H2O)62+(aq) + 4 Cl-(aq)

CoCl42-(aq) + 6 H2O(l)
pink
H = + 50 kJ/mol
blue
Directions
1) Setup your lab notebook.
2) Your initial equilibrium state is the test tube contents from Experiment 4, the
hydrated cobalt ion. Note that your solution should be pink.
3) Add Cl- ions in the form of 5 drops of concentrated (12 M) HCl. Add the acid one
drop at a time, with stirring. Note the results.
Initial
Equilibrium
Condition
Aqueous (pink)
solution of
Experiment 5
Co(H2O)62+
formed in Demo
4
Stress
Add
concentrated
HCl(aq)
Predicted
Shift
Observations
Observed
Shift
Experiment 6: An Equilibrium with Cobalt Complex Ions
In this section we will investigate the equilibrium between two different complex ions of
cobalt. The reaction is endothermic:
Co(H2O)62+(aq) + 4 Cl-(aq)
pink

CoCl42-(aq) + 6 H2O(l)
blue
H = + 50 kJ/mol
Directions
1) Setup your lab notebook.
2) Fill a test tube about ½-full of a solution of cobalt(II) chloride, CoCl2, dissolved in
ethanol (already prepared for you). The solution should be purple. This is your
initial equilibrium state.
3) Add 10 drops of 0.1 M silver nitrate (AgNO3) solution, one drop at a time with
stirring. Silver and chloride ions combine to form a precipitate of AgCl. Note the
results. You may wish to let the precipitate settle to observe the solution color
more easily or you may centrifuge the test tube and its contents.
Experiment 6
Initial
Equilibrium
Condition
Stress
CoCl2 dissolved
in anhydrous
ethanol
Add AgNO3(aq)
Predicted
Shift
Observations
Observed
Shift
Experiments 7 & 8: An Equilibrium with Cobalt Complex Ions
In this section we will investigate the equilibrium between two different complex ions of
cobalt. The reaction is endothermic:
Co(H2O)62+(aq) + 4 Cl-(aq)
pink

CoCl42-(aq) + 6 H2O(l)
blue
H = + 50 kJ/mol
Directions
1) Setup your lab notebook.
2) Obtain a sealed pipet containing some of the alcoholic cobalt chloride-water
system. Note its color.
3) Immerse the large end of the pipet in some hot water (about 60o) and see if there
is a color change.
4) Now chill the pipet in an ice bath to see if the color change in the previous step is
reversible. Note the results.
Initial
Equilibrium
Condition
Stress
Experiment 7
CoCl2 dissolved
in anhydrous
ethanol
Heat in hot
water bath
Experiment 8
Heated CoCl2
dissolved in
anhydrous ethanol,
from Demo 7
Cool in ice bath
Predicted
Shift
Observations
Observed
Shift