LAB 24
LeCHATLIER’S PRINCIPLE
BACKGROUND: You now know many reactions
QUESTIONS PART I
reach something called equilibrium. This means they
can be reversed. In most of the previous labs it has
1. A. How does the chromate, dichromate
appeared that the reactants have all been converted
equilibrium depend on the [H+]? You may use up and
into products, this lab will show appreciable
down arrows in your answer which will represent the
reversibility. You will be able to observe the change
increase or decrease in concentration.
since the products and reactants are different colors.
One of the jobs of a chemist is to control the
B. Balance the equation for the equilibrium
reversibility of a reaction; you will get some
between the chromate-dichromate ions by placing the
experience with that now.
H+(aq) and H2O(l) on the proper sides of the equation.
In this lab an aqueous solution of the chromate
ion CrO4-2(aq), can be converted into the dichromate
C. Use LeChatlier’s principle to explain the color
ion Cr2O7-2(aq). The opposite is also true, Cr2O7-2 (aq) changes noted in steps 3 & 5.
can be changed into CrO4-2 (aq). The extent of the
change, or relative concentrations, depends on the
2. A. How does the chromate-dichromate
[H+]. You will increase this concentration by adding
equilibrium depend on the [OH-]? You may use up
HCl, hydrochloric acid and decrease it by adding
and down arrows in your answer which will represent
NaOH, sodium hydroxide. NaOH is a source of OHthe increase or decrease in [ ].
+
(aq). When H (aq) are present with OH (aq) a
reaction takes place that forms HOH, or water.
B. Balance the equation for the equilibrium
between the chromate-dichromate ions by placing the
PROCEDURE PART I:
OH-(aq) and H2O(l) on the proper sides of the
equation.
CrO4-2 (aq)-- Cr2O7-2 (aq) equilibrium
C. Use LeChatlier’s principle to explain the color
1. Obtain a bottle of K2CrO4, potassium chromate and changes noted in steps 2 & 4.
a bottle of K2Cr2O7, potassium dichromate. Record
the color of each. These are both ionic solutions and
PART II: BaCrO4(s) <===> Ba+2(aq) + CrO4-2 (aq)
will therefore supply the chromate and dichromate
ions you are testing.
1. Place 10 drops of .1 M K2CrO4 in a 13x100 mm tt.
Add 2 drops of 1 M NaOH. Now add .1 M Ba(NO3)2
2. In a 13x100 mm test tube place 10 drops of
a drop at a time until a change is noted. Record this
K2CrO4. In a different 13x100 mm test tube place 10
change in the data table. Save this tt for step 4.
drops of K2Cr2O7. Add 1 M NaOH a drop at a time to
each test tube until a change is noted in one of the test 2. Place 10 drops of .1 M K2Cr2O7in a 13x100 mm tt.
tubes. Record the color changes. Keep these test
Add 2 drops of 1 M HCl. Now add 10 drops of
tubes they will be used in step 5.
.1 M Ba(NO3)2. Record the result and save this tt for
step 5.
3. Repeat step 2 with new test tubes and new
solutions, but use HCl instead of the NaOH. Record
3. You have just prepared solutions which contain
the change in color. Save these tt for step 4.
either BaCrO4 or BaCr2O7. Which is more soluble in
water? Hint, which mix showed a ppt. ?
4. Now add 1 M NaOH to the tt’s from step 3 a drop
at a time. When a color change is noted record in the
4. To the tt from step 1 add 1 M HCl a drop at a time
data table.
until a change is noted. Record in data table.
5. Repeat step 4 using the tt’s from step 2 and
replacing the NaOH with 1 M HCl. Add a drop at a
time until a change in color is noted. Record in data
table.
5. To the tt from step 2 add 1 M NaOH a drop at a
time until a change is noted. Record in data table.
LAB 24
LeCHATLIER’S PRINCIPLE
6. Predict how you could change, or reverse, the
QUESTIONS PART III:
reactions observed in steps 4 & 5. Do these
experiments and record both your prediction and
1. A. List the substances that changed the color of the
results.
solution to that of the CrO4-2 (aq).
7. In a clean tt add 10 drops of .1 M K2CrO4. Now
add a few drops of .1 M Ba(NO3)2. Record your
observations.
8. In a clean tt add 10 drops of .1 M K2Cr2O7. Now
add a few drops of .1 M Ba(NO3)2. Record your
observations.
QUESTIONS PART II:
1. A. Write the equation for the equilibrium reaction
between solid barium chromate and its ions.
B. Using this balanced equation, what you learned
in part I and LeChatlier’s principle explain the results
obtained in steps 3, 4 and 5 of part II.
2. From your observations in steps 7-8, what can be
concluded about the relative equilibrium concentration
of CrO4-2 (aq) in the K2CrO4 and K2Cr2O7 before the
Ba(NO3)2 was added?
PART III:
Do other substances act like HCl or NaOH with
respect to the CrO4-2 (aq)-- Cr2O7-2 (aq) equilibrium
1. Place 10 drops of K2CrO4 in a clean 13 x 100 mm
tt and 10 drops of K2Cr2O7 in a different clean 13 x
100 mm tt. Add 4-6 drops of 1 M HCH3COO, acetic
acid, to each tt. Record any changes.
2. Repeat step one with fresh tt testing 1 M solutions
of each of the following:
HNO3, nitric acid
KOH, potassium hydroxide
LiOH, lithium hydroxide
C2H5OH, ethyl alcohol
NH3, ammonia
B. List the substances that changed the color of
the solution to that of the Cr2O7-2 (aq).
2. A. What ions do the substances in 1 A have in
common?
B. What ions do the substances in 1 B have in
common?
3. Explain the results noted when each of the
following was added:
A. C2H5OH
B. NH3
4. In general, what types of substances would change
the chromate-dichromate equilibrium?