Horace Mann School
Chemistry
Delanty/Dilley/Hatch
Mitchell/Rosenthal
2006-07
Le Chatelier’s Principle
Introduction
A reversible reaction at equilibrium proceeds in both directions; both the forward and reverse reaction occur simultaneously at the same rate. However, when conditions change, this equilibrium may be disturbed, and the reactions will momentarily proceed at different rates. Chemists are interested in predicting how equilibria are affected when reaction conditions change.
Le Chatelier’s Principle states simply that if a change or stress is placed on an equilibrium, it will respond to counteract that stress. The system doesn’t actually
“respond”; rather, one or both of the reactions speed up or slow down. To describe these changes, we say that whichever reaction will have a faster rate is “favored”.
Alternatively, we can say that “products are favored” if the forward reaction is favored, or “reactants are favored” if the reverse reaction is favored.
In this experiment, you will work with solutions of potassium chromate (K
2
CrO
4
) and potassium dichromate (K
2
Cr
2
O
7
). Since chromate and dichromate ions can be converted into one another in aqueous solution, both ions are always present in either solution. However, you can tell which ion is in greater concentration (and which reaction is favored) by the color: chromate forms an yellow solution, while dichromate forms a orange solution. Thus, by adding substances to either solution, you’re creating a stress on the equilibrium. By observing whether the color changes or remains the same, you can decide which direction of the reaction is favored.
Procedure:
Part 1: Adding acids and bases to the chromate/dichromate equilibrium.
1.
Record the color of chromate and dichromate solutions.
2.
Put 10 drops of K
2
CrO
4
(aq) into a small test tube. Put 10 drops of
K
2
Cr
2
O
7
(aq) into a different test tube. Add drops of 1M NaOH, one at a time, alternately to each of the test tubes until you observe a color change in either or both of the tubes. Set these tubes aside for later tests.

3.
Put 10 drops of K
2
CrO
4
(aq) into another small test tube. Put 10 drops of
K
2
Cr
2
O
7
(aq) into a different test tube. Add drops of 1M HCl, one at a time, alternately to each of the test tubes until you observe a color change in either or both of the tubes.
4.
Add 1M NaOH dropwise to the tubes from step 3 until you observe a change.
5.
Add 1M HCl dropwise to the tubes from step 2 until you observe a change.
Analysis
1.
Balance the equation for the chromate-dichromate equilibrium by adding the proper number of OH
-
ions and H
2
O molecules to each side of the equation as needed. Based on this balanced equation, use LeChatelier’s principle to explain the color changes observed in steps 2 and 4.
2.
Balance the equation for the chromate-dichromate equilibrium by adding the proper number of H
+
ions and H
2
O molecules to each side of the equation as needed. Based on this balanced equation, use LeChatelier’s principle to explain the color changes observed in steps 3 and 5.
Part II: Adding barium ions to the equilibrium.
1.
Place 10 drops of K
2
CrO
4
(aq) in another test tube. Add 2 drops of 1M NaOH.
Then, add drops of barium nitrate, Ba(NO
3
)
2
(aq) until a change is noted.
Record your observations and save the test tube for later tests.
2.
Place 10 drops of K
2
Cr
2
O
7
(aq) in another test tube. Add 2 drops of 1M HCl.
Then, add drops of barium nitrate, Ba(NO
3
)
2
(aq) until a change is noted.
Record your observations and save the test tube for later tests.
3.
Based on your work in steps 1 and 2, decide which is more soluble, barium chromate or barium dichromate.
4.
Add drops of HCl with shaking to the tube from step 1 until a change is observed. Record your observations.
5.
Add drops of NaOH with shaking to the tube from step 2 until a change is observed. Record your observations.
6.
Suggest a way to reverse the changes observed in steps 4 and 5. Attempt to do this with the tubes from those steps.
7.
Place 10 drops each of K
2
CrO
4
(aq) and K
2
Cr
2
O
7
(aq) in separate test tubes.
Add 5 drops of Ba(NO
3
)
2
to each tube. Record your observations.

Analysis
1.
Write the reversible equation that shows solid BaCrO
4
reversibly dissolving into its ions. Using that equation and the results from Part I,, explain your observations in steps 4-6 in Part II.
2.
From your observations in step 7, how does the equilibrium [CrO
4
2-
] compare in potassium chromate and potassium dichromate before the Ba(NO
3
)
2
is added?
Part III: Adding other substances to the chromate/dichromate equilibrium
For each of the tests listed below, place 10 drops of K
2
CrO
4
(aq) and 10 drops of
K
2
Cr
2
O
7
(aq) into different test tubes, then add 5 drops of the 1 M solution listed below and record your observations.
1.
acetic acid (CH
3
COOH)
2.
nitric acid (HNO
3
)
3.
potassium hydroxide (KOH)
4.
ethanol (C
2
H
5
OH)
5.
ammonia (NH
3
)
Analysis
1.
Which substances caused the color to change from chromate to dichromate?
What do those substances have in common when you compare their chemical formulas?
2.
Which substances caused the color to change from dichromate to chromate?
What do those substances have in common when you compare their chemical formulas?
3.
Explain your observations of changes (or no changes) when ethanol and ammonia was added.
4.
Predict some substances that would have the same effect on the equilibrium as the substances in question #1.