LeChatelier's Principle by fjwuxn


									                                          LeChatelier's Principle
Chemical reactions can be described as reversible or nonreversible. A reversible reaction is one in which the products
can react to give the reactants, just as the reactants can react to give the products. A common example of a
reversible reaction is the reaction of iron(III) ions and thiocyanate ions to give ferrothiocyanate ions.
                         Fe3+ +SCN-  FeSCN2+
The double arrows are one way of showing that the reaction can proceed in both directions.

Consider the reversible reaction above. As you have learned, the rate of the forward reaction depends on the
temperature and the concentration of the reactants, as well as, the nature of the reactants. Similarly, the rate of the
reverse reaction depends on the factor of the products. What will happen if the rate of the forward reaction equals
the rate of the reverse reaction?. Both reactions will continue to occur, but the net effect will be that there is no
change in the concentrations of the reactants or products. For every pair of Fe:+3 and SCN- ions that combine to
form an FeSCN+2 ion, one FeSCN2+ ion will break apart to give Fe3 and SCN-. Consequently, although changes

are occurring constantly within the system, no net change will be observed. This condition is called dynamic

LeChatelier's principle applies to systems in equilibrium. It states that if a stress is placed on a system that is at
equilibrium, the system will shift to relieve the stress. For example, if additional Fe 3+ ions are added to an
equilibrium mixture of the three ions, the reaction will shift in the direction that will reduce that additional amount
of Fe+3 . In other words, the rate of the forward reaction will increase, due to the increase in the concentration of
iron. Since the forward reaction is now faster than the reverse reaction, we say that the reaction "shifts toward the
products." This will have the effect of using up some of the iron (III) ions that were added, thereby reducing the
stress that was applied to the system. Eventually a new equilibrium is established.

The Cobalt Chloride Equilibrium equation:
[Co(H2O)6]2+ + 4 Cl-  [CoCl4]2- + 6 H2O
To observe and explain LeChatelier's principle.


safety goggles                                 dropper pipet                           ice
6 medium test tubes                            0.1M potassium thiocyanate,             spatula
test tube rack                                 potassium nitrate                       50-mL graduated cylinder
100-mL beaker                                  distilled water                         plastic wash bottle
250-mLbeaker                                   potassium chloride crystals             white card, 3" x 5"
saturated potassium nitrate solution,          0.1M iron(Ill) chloride,


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 act as a seed crystal. (This crystal should not dissolve. If it does, inform the teacher
        that the solution is not saturated.)
2. Cool the test tube in an ice bath for 10 minutes. Record
3. Remove the tube from thc ice bath and place it in the test tube rack. Record what happens as the solution
        warms to room temperature.
Part B.
4. Using a graduated cylinder, measure out 50 mL of distilled water and pour into a 100-mL beaker. Add 1 mL of
   0.1M iron(III) chloride and 1 ml of 0.1M potassium thiocyanate. Stir the mixture. The reaction observed is
   described above. The color that appears is due to the presence of ferrothiocyanate ions, FeSCN + . Record your
5. Pour 5 mL of the mixture from Step 4 into each of four identical clean dry test tubes labeled with the numbers 1-
6. Hold the tubes over a white background and look down through them. The color of the solution in each tube
        should appear equally dark.
7. Tube 1 is the control in this experiment. It will not be changed. To Tube 2, add 20 drops of 0.1M iron(III)
chloride. To Tube 3, add 20 drops of 0.1M potassium thiocyanate. Flick each tube to mix the solutions. To Tube 4,
add 1 g of solid potassium chloride and flick the tube to dissolve the crystals. Compare the colors of the solutions in
Tubes 2, 3, and 4 with the color of the solution in the tube 1. Record your observations,
8. Discard the solutions into the sink.
Part C.
9. Take one sealed beral pipette of CoCl2. Note color. Place in hot water bath for 2 minutes, note color of solution.
10. Remove and place in ice bath for two minutes. Note color
Reaction                             Observations
KNO3 cooled
KNO3 warmed
Fe3+ + SCN-
Fe3+ +SCN-     plus iron(III)
Fe3+ +SCN-     plus SCN-
Fe3+ +SCN- + KCl
CoCl2 soln  room temp
CoCl2 soln warmed
CoCl2 soln cooled

1. Write a balanced equation For the equilibrium that existed before the saturated potassium nitrate was cooled.

2. What evidence was there. that the equilibrium shifted when the temperature of the potassium nitrate was
lowered? In which direction did it shift?

3. Was the system at equilibrium just before it was removed from the ice water bath? What evidence do you have for
your answer'?
4. In which direction did the equilibrium shift when you again increased the temperature?

5. What was the stress applied to the potassium nitrate system?

6. Explain what happened in tile potassium nitrate system in terms of LeChatelier's principle.

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

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

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

10. What evidence provided in the procedures show that all 3 ions were present in step 4?

11. Explain the effect that the addition of KCl has on the system.

12. Explain the changes observed in this system based on LeChatelier's principle

13. What stress was applied in Part C. Rewrite the equation [Co(H2O)6]2+ + 4 Cl-  [CoCl4]2- + 6 H2O using heat as a
term(either reactant or product) (no numbers just the word) Is this rxn endo or exo thermic?

14. Explain the results of Part C in terms of LeChatelier's principle

To top