1. EQUILIBRIUM

2. What Is Equilibrium?
Consider mixing the reactants nitrogen gas (N2) and hydrogen gas (H2).
When reactants are put together the forward reaction starts.
Since there are no products there is no reverse reaction.

3. What Is Equilibrium?
As the forward reaction proceeds the reactants are used up so the forward reaction slows.
The products build up, and the reverse reaction speeds up. 3

4. Equilibrium
Eventually you reach a point where the reverse reaction is going as fast as the forward reaction.
This is dynamic equilibrium.

5. Equilibrium
The double arrow implies the process is dynamic. 5

6. Equilibrium
The rate of the forward reaction is equal to the rate of the reverse reaction.
The concentration of products and reactants stays the same, but the reaction is still running. 6

7. Equilibrium
Catalysts speed up both the forward and reverse reactions.

8. Equilibrium
where K is the equilibrium constant 8

9. Equilibrium
Note that the equilibrium constant expression has products over reactants.
Coefficients become exponents.
Square brackets [ ] mean concentration/molarity (moles/liter).

10. Equilibrium
Pure liquids and pure solids are omitted from the equation!
Pure liquids and solids have a fixed concentration which will always be constant, so there is no need to include that in the equilibrium constant expressions. 10

11. 2C (aq) + D (aq) ↔ 3A (aq) + 2B (s)
Equilibrium
For example
the equilibrium constant expression is
2C (aq) + D (aq) ↔ 3A (aq) + 2B (s) 11

12. Writing Equilibrium Expressions
Write the equilibrium expression for the following reaction.
a) 3H2(g) + N2(g) 2NH3(g)
[NH3]2
K =
[H2]3 [N2]

13. Writing Equilibrium Expressions
Write the equilibrium expression for the following reaction.
b) NiO (s) + CO (g) ↔ Ni (s) + CO2 (g)
[CO2]
K =
[CO] 13

14. Writing Equilibrium Expressions
Write the equilibrium expression for:
c) 2NO (g) ↔ N2 (g) + O2 (g)
K =
[N2] [O2]
[NO]2

15. Writing Equilibrium Expressions
Write the equilibrium expression for:
d) 2O3 (g) ↔ 3O2 (g)
K =
[ O ]3
[ O ]2 3 2

16. Writing Equilibrium Expressions
Write the equilibrium expression for:
e) NH3 (aq) + H2O (l) ↔
NH4+ (aq) + OH- (aq)
K =
[NH4+] [OH-]
[NH3]

17. Writing Equilibrium Expressions
Write the equilibrium expression for:
f) 2H2 (g) + O2 (g) ↔ 2H2O (l)
K = 1
[H2]2 [O2]

18. Le Châtelier’s Principle
Regaining Equilibrium

19. Le Châtelier’s Principle
If something is changed in a system at equilibrium, the system will respond to relieve the stress until a new equilibrium is established.
Three types of stress can be applied.

20. Changing Concentration
Adding a reactant or product shifts the equilibrium away from the increase.
Removing a reactant or product shifts the equilibrium towards the decrease.

21. Changing Concentration
1) Consider the reaction
X + Y ↔ A + B
If the concentration of X is increased, the reaction will shift to the _____.
right

22. Changing Concentration
2) Consider the reaction
X + Y ↔ A + B
If the concentration of Y is decreased, the reaction will shift to the _____.
left 22

23. Changing Concentration
3) Consider the reaction
X + Y ↔ A + B
If the concentration of A is increased, the reaction will shift to the _____.
left 23

24. Changing Concentration
4) Consider the reaction
X + Y ↔ A + B
If the concentration of B is decreased, the reaction will shift to the _____.
right 24

25. Changing Temperature Reactions either require or release heat.
For an endothermic reaction, H > 0 and heat can be considered as a reactant.
For an exothermic reaction, H < 0 and heat can be considered as a product. 25

26. Changing Temperature
Adding heat (i.e. increasing the temperature) favors away from the increase.
Removing heat (i.e. decreasing the temperature) favors toward the increase.

27. Changing Temperature 5) Consider the reaction
2 SO2 + O2 ↔ 2 SO3 + heat
If the temperature is decreased, the reaction will shift to the _____.
right 27

28. Changing Temperature 6) Consider the reaction N2 + O2 + heat ↔ 2 NO
If the temperature is decreased, the reaction will shift to the _____.
left 28

29. Changing Temperature 7) Consider the reaction 2 SO2 + O2 ↔ 2 SO3
ΔH is negative
If the temperature is increased, the reaction will shift to the _____.
left 29

30. Changing Temperature 8) Consider the reaction N2 + O2 ↔ 2 NO
ΔH is positive
If the temperature is increased, the reaction will shift to the _____.
right 30

31. Changes in Pressure
An increase in pressure favors the direction that has fewer moles of gas.
A decrease in pressure favors the direction that has the greatest moles of gas.
In a reaction with the same number of product and reactant moles of gas, pressure has no effect. 31

32. Changing Pressure 9) Consider the reaction
If the pressure is increased, the reaction will shift to the _____.
left 32

33. Changing Pressure 10) Consider the reaction 2H2(g) + O2(g) ↔ 2 H2O(g)
If the pressure is decreased, the reaction will shift to the _____.
left 33

34. Changing Pressure 11) Consider the reaction
CO2 (g) + H2 (g) + heat ↔ CO (g) + H2O (g)
If the pressure is decreased, the reaction will shift to the _____.
No shift 34

35. Adding a Catalyst If a catalyst is added, the reaction will NOT shift.
Equilibrium will only be reached sooner. 35

36. Altering the Yield
The yield of a product will be increased if the reaction shifts to the product side (the forward reaction). 36

37. Altering the Yield 12) N2 (g) + 3H2 (g) ↔ 2NH3 (g) ΔH = – 92.4 kJ
How can the yield of NH3 be increased by manipulating the following?
a) (add or remove) more N2
b) (increase or decrease) the pressure
c) (increase or decrease) the temperature 37

38. Calculating Equilibrium
If K > 1, products are favored. In other words, forward reaction is favored.
If K < 1, reactants are favored. The reverse reaction is favored.

39. Calculating Equilibrium
K, the equilibrium constant, it is only affected by temperature.
K increases when a temperature change favors the products (the forward reaction).
K decreases when a temperature change favors the reactants (the reverse reaction). 39

40. Calculating Equilibrium
13) H2(g) + I2(g) ↔2 HI ΔH = −10.4 kJ
If temperature is increased, the reaction will shift _____.
left
Shifting this direction will cause the value of K to ______.
decrease 40

41. Le Chatelier and Entropy
Entropy (S) is a measure of the disorder or randomness of the particles that make up a system.
A transformation (shift) will not proceed in any direction that reduces the amount of total entropy. 41

42. Entropy Entropy of a solid Entropy of a liquid Entropy of a gas
A solid has an orderly arrangement.
A liquid has the molecules next to each other.
A gas has molecules moving all over the place. 42

43. Problem
14a) Which side of the reaction has greater entropy?
(left)

44. Problem
b) Which side of the reaction has greater entropy?
(right)

45. Problem
16a) Predict the change in entropy (increase or decrease).
(increase)

46. Problem N2 (g) + 3 H2 (g) ↔ 2NH3 (g) (decrease)
16b) Predict the change in entropy (increase or decrease).
N2 (g) + 3 H2 (g) ↔ 2NH3 (g)
(decrease)