Le Chatelier’s Principle and Dynamic Equilibrium

1. Le Chatelier’sPrinciple and Dynamic Equilibrium Objective to understand how chemists use stresses to control the amount of product formed

2. Equilibrium

3. Chemical Equilibrium • chemical equilibrium a dynamic state where the concentrations of all reactants and products remain constant.

4. Dynamic • Marked by continuous activity or change.

5. What is Dynamic equilibrium? • When the rate of formation of the reactants = rate of formation of the products.

6. Dynamic Equilibrium • When reactions occur at such rates that the composition of the mixture does not change with time. Reactions do in fact occur, sometimes vigorously, but to such an extent that changes in composition cannot be observed.

7. Jar with lid • There are more molecules evaporating than condensing in the open jar. It is not at dynamic equilibrium.

8. Instant Question #1 • In which jar do the liquid molecules stop turning into gas molecules? • Both • just (a) • Just (b) • Neither

9. Instant Question #2 • In which jar do the are the gas molecules turning into liquid molecules? • Both • just (a) • Just (b) • Neither

10. Shifting equilibrium • What would happen to the [H2O(g)] (in the covered jar) if the temperature were increased? You would get more H2O(g)

11. Shifting equilibrium • What would happen to the amount of H2O((in the covered jar) if the pressure were increased by making the volume smaller? You would get more H2O(L)

12. Adding More H2O(g) • Adding more H2O(g) would (at first) make more gas molecules but they would soon turn into liquid molecules

13. Question #3 • What would happen to the pressure if the temperature were increased? • A. It would go up • B. It would go down • C. It would stay the same

14. Stress • What changes can cause the equilibrium to shift? • (1) Changing the temperature. • (2) Changing the concentration. • (3)Changing the pressure (volume) •  Collectively, what are these factors referred to as Stresses.

15. Le Chatelier’s Principle • When a stress is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that stress.

16. Question #4 • Which would cause more “C” to start forming according to the equation below? • A + B  C + D + heat • (a) adding more D • (b) removing A or B • (c) removing D • (d) adding heat

17. Using the Collision Theory, explain why adding MnO2 to H2O2 makes the rate of the reaction increase. • H2O2 molecules must to collide with each other with enough force to break bonds. • In the presence of MnO2 the molecules absorb onto the catalyst’s surface making the H2O2 bonds weaker

19. Questions #5 and 6 • 5. True or False: A catalyst speeds up a chemical reaction by increasing the concentration of the reactants. • 6. Determine which is the catalyst • 1st A + B  AB, then AB + C  AC + B • Which is the catalyst “A” “B” or “C”

20. The Water Game • Make a graph like so …. 100 80 60 40 20 0 V O L 0 1 2 3 4 5 6 7 8 9 10 transfers

21. The Water Game • Get two 100 ml graduated cylinders. Put 100 ml of water into one of them. Label it “A” and the other (empty one) label it “B” • Also get two empty 250 ml beakers. Label one “A” and the other “B” • A Micro pipette and a paper towel.

22. The Water Game • Get three colored pencils or crayons (one red and one blue and one green) from the supply. • Using Red to represent the volume in graduated cylinder “A”, put a point on your graph. • Do the same using blue to represent the volume in “B”

23. The Water Game • After zero transfers, your graph should look like this. 100 80 60 40 20 0 0 1 2 3 4 5 6 7 8 9 10

24. The Water Game • Now take ½ of the water in grad “A” and put it into beaker “A” • Then take ¼ of the water in grad “B” and put it into beaker “B”. Yes, ¼ of 0.0 = 0 • Make sure you have the volumes correct. • Now put the water in beaker “A” in Grad “B” and the water in beaker “B” in grad “A”

25. The Water Game • Record the volume in the grads. • After adding the water to the grads, using red to represent the volume in graduated cylinder “A”, put a point on your graph. • Do the same using blue to represent the volume in “B” • Repeat six more times after each transfer, make a mark on your graph showing the volumes of “A” and “B”

26. The Water game • If you have not yet figured it out, this represents a reversible reaction. • Define a reversible reaction where reactant “A” turns into reactant “B” • Write an equation representing this reaction • Identify the forward and reverse reactions.

27. The Water Game • Calculate the volume of “B”/”A” at each point along your graph. • Identify with a vertical green line, the point on your graph where the forward and reverse reaction are equal to each other. • What is “B”/”A” at this point? This ratio is called the equilibrium constant. • For chemical reactions it stays constant unless the temperature is changed.

28. The Water Game • Now add 30.0 ml of water to Grad “A” this represents a “stress”. Calculate the “B”/”A” at this point and graph it onto your graph. • Repeat the water game 3 more times then recalculate the “B”/”A” • Without actually doing the lab, show on your graph what would happen if 30 ml of water was now added to “B”

29. The Graph

30. Final Graph

31. Calculations • Suppose the “B”/”A” = 50, what would the values of “A” and “B” equal? • The equation would be: B/(100 –B) =50

32. Graphs So the size of the equilibrium constant tells us how far the reaction goes to completion

33. The Equilibrium Expression • Write the equilibrium expression for the equations below. • H2 + Cl2 2HCl • H2 + O2 2H2O

34. Calculate the Constant • If A <=> B • [A] at equilibrium = .005 and • [B] at equilibrium = .02 • Calculate the equilibrium constant