1. Reaction Equilibrium
Unit 17
Daily 2

2. Key Question:
What is Le Chatelier’s Principle?

3. Today’s Goals: Explore factors affecting equilibrium
Predict direction of equilibrium shift
Predict resulting changes in concentration
Make equilibrium serve your own evil purposes

4. Factors Affecting Equilibrium
Equilibrium is a RATE balance.
Anything that can affect the rate of the forward or reverse reaction can throw equilibrium off.
What are the factors that affect reaction rate?
Concentration
Pressure or Volume (gases)
Temperature
Surface Area
Catalysts/Inhibitors

5. Le Chatelier’s Principle
“If a system at equilibrium is subjected to an external stress, the equilibrium will shift to minimize the effects of that stress”
The Steps of Le Chatelier’s Principle:
Stress: Adding or removing a reactant, adding or removing a product, changing the pressure, volume, or temperature.
Shift: The equilibrium will shift away from what is added and towards what is removed.
Change in Concentration: The concentration of what is being shifted towards increases, the concentration of what is being shifted away from decreases.

6. Changes in Concentration
Fe3+(aq) + SCN-(aq)  FeSCN2+(aq)
(colorless) (red)
If more Fe3+ is added to the system, what will happen?
equilibrium shifts to the right
equilibrium shifts to the product side
the forward reaction is favored (forward shift)
How are concentrations affected?
[Fe3+] increases, [SCN-] decreases, [FeSCN2+] increases
How is Keq affected?
Keq does not change when changes in concentration cause a shift in equilibrium

7. Changes in Concentration
Fe3+(aq) + SCN-(aq)  FeSCN2+(aq)
(colorless) (red)
If more FeSCN2+ is added to the system, what will happen?
equilibrium shifts to the left
equilibrium shifts to the reactant side
the reverse reaction is favored (reverse shift)
How are concentrations affected?
[Fe3+] increases, [SCN-] increases, [FeSCN2+] increases

8. Changes in Concentration
Fe3+(aq) + SCN-(aq)  FeSCN2+(aq)
(colorless) (red)
If SCN- is removed from the system (by adding AgNO3 so that AgSCN(s) precipitate forms), what will happen?
equilibrium shifts to the left
equilibrium shifts to the reactant side
the reverse reaction is favored (reverse shift)
How are concentrations affected?
[Fe3+] increases, [SCN-] increases (but also decreases since it was removed), [FeSCN2+] decreases

9. Changes in Volume & Pressure
Only affect an equilibrium system if gases are present
What is the relationship between moles of gas and volume/pressure?
P = (nRT)/(V)
If pressure is increased (or volume decreased):
equilibrium will shift towards the side with FEWER moles of gas to bring the pressure back down
If pressure is decreased (or volume increased):
equilibrium will shift towards the side with MORE moles of gas to bring the pressure back up

10. Changes in Volume & Pressure
N2(g) + 3H2(g)  2NH3(g)
(4 moles) (2 moles)
If pressure is increased (volume decreased) on the system, what will happen?
equilibrium shifts to the right
equilibrium shifts to the product side
the forward reaction is favored (forward shift)
How are concentrations affected?
[N2] decreases, [H2] decreases, [NH3] increases

11. Changes in Volume & Pressure
Notes:
If the number of moles of gas are equal on both sides, then the equilibrium can’t shift
H2(g) + I2(g)  2HI(g)
Make sure to only count the moles of GAS when considering pressure effects on equilibrium

12. Changes in Temperature
If temperature is increased in the system, what will happen?
equilibrium will shift in the endothermic direction
the endothermic direction is away from the side with heat
If temperature is decreased in the system, what will happen?
equilibrium will shift in the exothermic direction
the exothermic direction is towards the side with heat

13. For Those Who Prefer Visuals…

14. See Saw
H2O H OH

15. Add More Hydrogen H2O OH H H
Which way does the see saw have to shift to reach equilibrium again?

16. Add More Hydrogen H2O OH H H
Which way does the see saw have to shift to reach equilibrium again?

17. Add More Hydrogen H2O OH SHIFT LEFT H H
More H2O is produced due to the shift in equilibrium

18. Synthesis of Ammonia H2 H2
NH3
NH3 N2 H2
N2 (g) + 3H2 (g)  2NH3 (g)

19. SHIFT will occur to side with the least number of MOLES
Increase Pressure
SHIFT RIGHT H2
NH3
NH3 H2 H2 N2
SHIFT will occur to side with the least number of MOLES
N2 (g) + 3H2 (g)  2NH3 (g)

20. Decreasing Volume (Think about how P and V relate with Boyle’s Law)
NH3
NH3 N2 H2
N2 (g) + 3H2 (g)  2NH3 (g)

21. Decrease Volume = Increase Pressure
SHIFT RIGHT H2
NH3
NH3 H2 H2 N2
SHIFT will occur to side with the least number of MOLES
N2 (g) + 3H2 (g)  2NH3 (g)

22. Change in Temperature
Depends if reaction is Endothermic or Exothermic.
Endothermic: Heat on LEFT
Exothermic: Heat on RIGHT

23. Exothermic Increase Temperature… which way will the see saw tip? D A C
HEAT D A C B

24. Which way will the reaction shift?
Exothermic A
HEAT B C D
Which way will the reaction shift?

25. Reaction will SHIFT LEFT
Exothermic A
HEAT B C D
Reaction will SHIFT LEFT

26. Exothermic DecreaseTemperature… which way will the see saw tip? D A C
HEAT D A C B
DecreaseTemperature… which way will the see saw tip?

27. Which way will the reaction shift?
Exothermic
HEAT D C B A
Which way will the reaction shift?

28. Reaction will SHIFT Right
Exothermic
HEAT D C B A
Reaction will SHIFT Right

29. Endothermic Increase Temperature… which way will the see saw tip? D A
HEAT D A C B

30. Which way will the reaction SHIFT?
Endothermic
Which way will the reaction SHIFT?
HEAT D C B A

31. Reaction will SHIFT Right
Endothermic
Reaction will SHIFT Right
HEAT D C B A

32. Endothermic Decrease Temperature… which way will the see saw tip? D A
HEAT D A C B

33. Which way will the reaction SHIFT?
Endothermic
HEAT
Which way will the reaction SHIFT? A B C D

34. Reaction will SHIFT Left
Endothermic
HEAT
Reaction will SHIFT Left A B C D

35. Demo: Changes in Temperature
2NO2(g)  N2O4(g) + energy (Dark Brown) (Clear)