1. Tutorial 3-4: Le Chatelier’s Principle
UNIT II
Tutorial 3-4: Le Chatelier’s Principle

2. OBJECTIVES 1. What LeChatelier's Principle is.
2. How LeChatelier's Principle can be used to explain or predict shifts in equilibrium caused by changes in:
 temperature
pressure or volume
concentration or partial pressure of a reactant

3. LE CHATELIER’S PRINCIPLE
LeChatelier's Principle: If a closed system at equilibrium is subjected to a change, processes will occur that tend to counteract that change.

4. LE CHATELIER’S PRINCIPLE
"Processes" usually mean that the equilibrium will shift to the left or right.
 "Counteract" means that if you do something to a system at equilibrium, the system will shift in such a way as to try to "undo" what you did.
 Equilibrium only occurs in a closed system.

5. EFFECT OF TEMPERATURE CHANGES
NOTE: When using LeChatelier's Principle, it is easier to have all reactions with “heat” in “Thermochemical Form.”
Example:
The reaction: A + B C ∆H = -56 kJ is exothermic (∆H is negative), so heat is given off or written on the right.
Thermochemical form:
A + B C kJ

6. EFFECT OF TEMPERATURE CHANGES
The reaction: D + E F + G ∆H = 43 kJ
is endothermic (∆ H is positive), so heat is absorbed, or written on the left.
 Thermochemical form:
D E kJ F G
Recall: When a chemical system is at equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.

7. EFFECT OF TEMPERATURE CHANGES
Example:
A B heat C + D
In an endothermic reaction, the forward reaction needs heat, so its rate will be increased more by an increase in temperature.
The products are formed faster than they are used up, so their concentration increases.
The reactants are used up faster than they are formed, so their concentration decreases.

8. EFFECT OF TEMPERATURE CHANGES
Eventually, the rate of the reverse reaction will catch up and again equal the rate of the forward reaction.
When this happens, we have equilibrium (a new equilibrium!) in which the concentration of the products are higher than they were before and the concentration of the reactants are lower than they were before.
The equilibrium has shifted to the right.

9. EFFECT OF TEMPERATURE CHANGES
Or, to explain it another way...
If we increase the temperature of this system, we are adding heat. In order to counteract our change, the equilibrium will move in such a way as to use up heat.  
Since heat is on the left, the forward reaction uses up heat, so it will predominate and the equilibrium will shift toward the right.

10. EFFECT OF TEMPERATURE CHANGES
Which means a new equilibrium will be established in which there is more C and D and less A and B than in the original equilibrium.
To summarize: When the temperature is increased, the equilibrium will shift away from the side with the heat term.

11. EFFECT OF TEMPERATURE CHANGES
Using the same example:
A B heat C + D
If the temperature is decreased, the rate of the reverse reaction will be faster than the rate of the forward reaction for a while.
The reactants are formed faster than they are used up, so their concentration increases.
The products are used up faster than they are formed, so their concentration decreases.

12. EFFECT OF TEMPERATURE CHANGES
Eventually, the rate of the forward reaction will catch up and again equal the rate of the reverse reaction.
When this happens, we have equilibrium (a new equilibrium!) in which the concentration of the reactants are higher than they were before and the concentration of the products are lower than they were before.
The equilibrium has shifted to the left.

13. EFFECT OF TEMPERATURE CHANGES
Or, to explain it another way...
If the temperature is decreased, the equilibrium will shift in such a way that produces heat (to counteract the change).
To do this, it will shift toward the side with the heat term (in other words, produce heat).
To summarize: When the temperature is decreased, the equilibrium will shift toward the side with the heat term.

14. CHECK YOUR UNDERSTANDING
Example 1. Given the reaction at equilibrium: A(g) + B(g) kJ C(g) a) If the temperature was increased, which way would this equilibrium shift: _________ b) If the temperature was decreased, which way would this equilibrium shift: _________

15. CHECK YOUR UNDERSTANDING
2. Given the reaction: X(g) + Y(g) W(g) + Z(g) ∆H = -75 kJ a) Rewrite this as a thermochemical reaction _____________________________________ b) If the temperature was increased, which way would this equilibrium shift: _________ c) If the temperature was decreased, which way would this equilibrium shift: _________

16. EFFECT OF CHANGES IN CONCENTRATION OR PARTIAL PRESSURE
Consider the equilibrium equation:
H2(g) I2(g) HI(g)
 If we add some H2 to a flask containing this mixture at equilibrium, [H2] will  immediately increase.
In order to counteract this change, the equilibrium will shift to the right in order to "use up" some of the extra H2. (In other words to decrease the [H2]).

17. EFFECT OF CHANGES IN CONCENTRATION OR PARTIAL PRESSURE
Let's say now that we somehow take away some I2. [I2] will immediately decrease.
 In order to counteract this change, the equilibrium will shift to the left in order to increase [I2] again .
 If we were to add some HI, the [HI] would immediately ____crease.
 In order to counteract this change, the equilibrium would shift to the ____.

18. EFFECT OF CHANGES IN CONCENTRATION OR PARTIAL PRESSURE
If the concentration of a substance in an equilibrium system is increased by us, the equilibrium will shift toward the other side of the equation, in order to counteract the change.
If the concentration of a substance in an equilibrium system is decreased by us, the equilibrium will shift toward the side of the equation with that substance, in order to counteract the change.

19. CHECK YOUR UNDERSTANDING
3. Given the equilibrium equation: PCl5(g) PCl3(g) + Cl2(g) a) If the [PCl5] is increased, the equilibrium will shift to the _________________ b) If the [PCl5] is decreased, the equilibrium will shift to the _________________ c) If the [PCl3] is increased, the equilibrium will shift to the _________________

20. CHECK YOUR UNDERSTANDING
d) If the [PCl3] is decreased, the equilibrium will shift to the _________________
e) If the [Cl2] is increased, the equilibrium will shift to the _________________
f) If the [Cl2] is decreased, the equilibrium will shift to the _________________

21. EFFECT OF CHANGES IN TOTAL PRESSURE OR VOLUME FOR GASEOUS SYSTEMS
The more moles of gas in a certain
volume, the higher the pressure.
If the total pressure of a system at equilibrium is increased, the equilibrium will shift toward the side with less moles of gas (as shown by coefficients) in order to reduce the total pressure.

22. EFFECT OF CHANGES IN TOTAL PRESSURE OR VOLUME FOR GASEOUS SYSTEMS
Given the equilibrium equation:
N2(g) H2(g) NH3(g)
If the total pressure on this system is increased, the equilibrium would shift to the right (the side with fewer moles of gas).
This counteracts the imposed change by reducing the pressure.

23. EFFECT OF CHANGES IN TOTAL PRESSURE OR VOLUME FOR GASEOUS SYSTEMS
To review:
 A shift to the right in this case would mean that once the new equilibrium is established, the [NH3] would be higher than before, the [N2] and the [H2] would be lower than before.
 If the total pressure on this system is decreased, the equilibrium would shift to the left (the side with more moles of gas). This counteracts the imposed change by increasing the pressure.

24. EFFECT OF CHANGES IN TOTAL PRESSURE OR VOLUME FOR GASEOUS SYSTEMS
Increasing the volume of a closed system with gases will decrease the pressure. (The molecules have more room so they exert less pressure on the sides of the container).
So, given changes in volume of the container, just remember that the changes in pressure are just the opposite.

25. EFFECT OF CHANGES IN TOTAL PRESSURE OR VOLUME FOR GASEOUS SYSTEMS

26. EFFECT OF CHANGES IN TOTAL PRESSURE OR VOLUME FOR GASEOUS SYSTEMS
Given the equilibrium equation:
N2(g) H2(g) NH3(g)
If the total volume of the container is increased, this means that the total pressure is decreased. 
The equilibrium will then shift to the side with more moles (to the left in this case), in order to counteract the change and try to increase the pressure again.

27. EFFECT OF CHANGES IN TOTAL PRESSURE OR VOLUME FOR GASEOUS SYSTEMS
If the total volume of the container is decreased, this means that the total pressure is increased.
The equilibrium will then shift to the side with less moles (to the right in this case), in order to counteract the change and try to decrease the pressure again.

28. CHECK YOUR UNDERSTANDING
4. Given the equilibrium equation: N2O4(g) 2O2(g) + N2(g) a) If the total pressure of this system is increased, the equilibrium will shift ________ b) If the total pressure of this system is decreased, the equilibrium will shift ________ c) If the total volume of this system is increased, the equilibrium will shift ________ d) If the total volume of this system is decreased, the equilibrium will shift ________

29. CHECK YOUR UNDERSTANDING
5. Given the equilibrium equation:
NO(g) CO2(g) CO(g) NO2(g)
a) If the total pressure of this system is increased, the equilibrium will ___________
b) If the total pressure of this system is decreased, the equilibrium will ___________
c) If the total volume of this system is increased, the equilibrium will ____________
d) If the total volume of this system is decreased, the equilibrium will ___________

30. EFFECT OF CATALYSTS Consider the equilibrium system:
N2O4(g) heat NO2(g)
colourless brown
 Adding a catalyst to this system would decrease the activation energy by providing a different route (or mechanism) for the reaction.

31. EFFECT OF CATALYSTS
Uncatalyzed Reaction Catalyzed Reaction NO2 PE (kJ) N2O4

32. EFFECT OF CATALYSTS
Using a catalyst to provide a route will decrease the activation energy for the forward reaction and for the reverse reaction by the same amount. 
This means the forward reaction will speed up, but so will the reverse reaction.
 In fact, the rates of both the forward and reverse reactions will still remain equal to each other (even though they are both faster.)
Therefore, the equilibrium will not shift!

33. EFFECT OF CATALYSTS
Adding a catalyst to a system not at equilibrium will simply speed things up so that equilibrium will be attained faster. It does not alter any of the concentrations etc. at equilibrium!

34. SHIFTING EQUILIBRIUM AND RATE OF REACTION
A "shift to the left “means that once the new equilibrium is reached, there will be more reactants and less products. It does not say anything about the rate of the reaction!!
For example, consider the reaction:
A B C heat
If the temperature of this system was increased, the equilibrium would shift to the left.

35. SHIFTING EQUILIBRIUM AND RATE OF REACTION
This does not mean that the rate will be slower! It simply means that a new equilibrium will be reached which has more A and B and less C.
Increasing the temperature always increases the rate of a reaction (more molecules have the minimum energy necessary for an effective collision).
Increasing the temperature just causes equilibrium to be reached faster.

36. CHECK YOUR UNDERSTANDING
6. Given the equilibrium equation: X + Y + heat Z a) Increasing the temperature will ___________ the rate of reaction. b) Increasing the temperature will cause the equilibrium to shift ___________ c) Decreasing the temperature will ___________ the rate of reaction. d)Decreasing the temperature will cause the equilibrium to shift ___________

37. CHECK YOUR UNDERSTANDING
7. Given the equilibrium equation:
D + E F heat
a) Increasing the temperature will ___________ the rate of reaction.
b) Increasing the temperature will cause the equilibrium to shift ___________
c) Decreasing the temperature will ___________ the rate of reaction.
d) Decreasing the temperature will cause the equilibrium to shift ___________

38. SUMMARY
1. Temperature:
If T is increased, the equilibrium will shift toward the side without the heat term.
If T is decreased the equilibrium will shift toward the side with the heat term.

39. Le Chatelier’s Principle: Graphs Involving Temperature Changes

40. SUMMARY 2. Concentration or Partial Pressure of Gases:
If the [ ] of a species is increased, the equilibrium will shift away from that species.
If the [ ] of a species is decreased, the equilibrium will shift toward the species.

41. Le Chatelier’s Principle: Graphs Involving Concentration Changes

42. SUMMARY 3. Total Volume and Total Pressure:
If pressure is increased (volume decreased), the equilibrium will shift to the side with less moles of gas.
If the pressure is decreased (volume increased), the equilibrium will shift toward the side with more moles of gas.

43. Le Chatelier’s Principle: Graphs Involving Total Volume & Total Pressure Changes

44. SUMMARY
4. Catalysts
Have no effect on equilibrium. They may help a system reach equilibrium faster, that's all!

45. Le Chatelier’s Principle: Graphs Involving The Use of Catalysts

46. ASSIGNMENT