Summer Research Program for Science Teachers

Jassy Ubhi

Bayard Rustin H.S. for the Humanities, Manhattan

2000

Subject: Chemistry (HS)

Topic:  Chemical Equilibrium

Observing the Effect of a Change in Conditions on a System at Equilibrium by Applying Le Chatelier’s Principle

[9-12 Content Standard B- Chemical reactions]

AIM:

          How can we observe the effect of a change in conditions on a system at equilibrium by applying Le Chatelier’s principle?

 EQUIPMENT:

Safety Goggles                  1 50ml graduated cylinder

5 medium test tubes         1 100ml beaker

1 test tube rack                1 250ml beaker

1 dropper pipet                1 white card 3in x 5in.  

MATERIALS:

          Saturated Potassium nitrate solution KNO₃

            0.1M Iron (111) chloride, FeCl₃

            0.1M Potassium thiocyanate KSCN

            Potassium chloride crystals, KCl

            Potassium nitrate crystals KNO₃

            Ice

            Distilled water

  [Teaching Standard D- Make accessible science tools]

PROCEDURE:

1.      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.

2.      Cool the test tube in a 250-ml beaker of ice water for about 10 minutes. Then record the results. [9-12 Content Standard B- Properties of matter]

3.      Remove the test tube from the ice water and place in the test tube rack. Record what happens as the solution warms to room temperature.

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]

5.      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]

DATA RECORD

ANALYSIS and CONCLUSIONS:

[Teaching Standard B- Orchestrate scientific discourse]

1.      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.

  [Content Standard Unifying Concepts- Change, constancy, and measurement]

3.      Did increasing the temperature (step3) disturb the equilibrium? What evidence do you have for your answer?

  [Content Standard Unifying Concepts- Evidence and equilibrium]

4.      Explain what happened in the potassium nitrate system in terms of Le Chatelier’s principle?

5.      Write a balanced equation for the equilibrium that existed after the ferric and thiocyanate ions were combined in the beaker.

6.      What evidence was there that the equilibrium shifted when iron (III) chloride was added?  In which direction did it shift?

7.      What evidence was there that the equilibrium shifted when potassium thiocyanate was added?  In which direction did it shift?

8.      Explain the effect of adding potassium chloride to the system.