Summer Research Program for Science Teachers
Bayard Rustin H.S. for the Humanities, Manhattan
Subject: Chemistry (HS)
Topic: Chemical Equilibrium
Observing the Effect of a Change in Conditions on a System at Equilibrium by Applying Le Chateliers Principle
[9-12 Content Standard B- Chemical reactions]
How can we observe the effect of a change in conditions on a
system at equilibrium by applying Le Chateliers principle?
Safety Goggles 1 50ml graduated cylinder
5 medium test tubes 1 100ml beaker
1 test tube rack 1 250ml beaker
1 white card 3in x 5in.
Saturated Potassium nitrate solution KNO3
0.1M Iron (111) chloride, FeCl3
0.1M Potassium thiocyanate KSCN
Potassium chloride crystals, KCl
Potassium nitrate crystals KNO3
Add 2-3 ml of saturated potassium nitrate solution to a clean
test tube. Using a spatula, add one crystal of potassium nitrate
to the solution to act as a seed crystal.
Cool the test tube in a 250-ml beaker of ice water for about 10
minutes. Then record the results.
Remove the test tube from the ice water and place in the test
tube rack. Record what happens as the solution warms to room
4. Use a graduated cylinder to add 50ml of distilled water to a 100ml beaker. Added 1ml of 0.1M iron (III) chloride, FeCl3 and 1ml of 1.0M potassium thiocyanate to the water, and stir it. The color that appeared was due to the presence of ferrothiocyanate ions, FeSCN+. [9-12 Content Standard B- Chemical reactions]
Label four identical test tubes with the numbers 1-4. Pour 5ml of
the mixture from step 4 into these test tubes. Hold the test
tubes against a white background and look to make sure that they
are all equally dark.
6. Remember test tube 1 is the control in this experiment. In tube 2 add 20 drops of 0.1M iron(III) chloride. To tube# 3 add 20 drops of 0.1M potassium thiocyanate. Flick each test tube to mix the solutions. Then to the test tube # 4 add 1g of potassium chloride crystals. Flick the test tube to dissolve the crystal. Finally compare the colors of the solutions in tubes 2,3, and 4 with the color in the solution of test tube 1. [Content Standard Unifying Concepts- Equilibrium]
OBSERVATIONS (color changes)
|KNO3 (sat) warmed||
|Fe / SCN» reaction||
/SCN» mixture + additional Fe
/ SCN» mixture additional SCN»
/ SCN» mixture + KCl(s)
[Teaching Standard B- Orchestrate scientific discourse]
Write a balanced equation for the equilibrium that existed before
the saturated potassium nitrate was cooled.
2. Did lowering the temperature (step2) affect the equilibrium? Explain your answer.
Did increasing the temperature (step3) disturb the equilibrium?
What evidence do you have for your answer?
Explain what happened in the potassium nitrate system in terms of
Le Chateliers principle?
Write a balanced equation for the equilibrium that existed after
the ferric and thiocyanate ions were combined in the beaker.
What evidence was there that the equilibrium shifted when iron
(III) chloride was added? In which direction did it shift?
What evidence was there that the equilibrium shifted when
potassium thiocyanate was added? In which direction did it
Explain the effect of adding potassium chloride to the system.
Explain the changes observed in the ferrothiocyanate ion system in terms of Le Chateliers principle.
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