1. Unit 2.6: Equilibrium

2. The Concept of Equilibrium
Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.

3. The Concept of Equilibrium
As a system approaches equilibrium, both the forward and reverse reactions are occurring.
At equilibrium, the forward and reverse reactions are proceeding at the same rate.

4. A System at Equilibrium
Once equilibrium is achieved, the amount of each reactant and product remains constant

5. Reactions are Reversible
A + B  C + D ( forward)
C + D  A + B (reverse)
Initially there is only A and B so only the forward reaction is possible
As C and D build up, the reverse reaction speeds up while the forward reaction slows down.
Eventually the rates are equal

6. What is at Equilibrium? Rates are equal
Concentrations are not.
Rates are determined by concentrations and activation energy.
The concentrations do not change at equilibrium.
or if the reaction is verrrry slooooow.

7. Depicting Equilibrium
Since, in a system at equilibrium, both the forward and reverse reactions are being carried out, we write its equation with a double arrow.
N2O4 (g)
2 NO2 (g)

8. Equilibrium Constant Forward reaction: Rate Law: N2O4 (g)  2 NO2 (g)
Rate = kf [N2O4]

9. Equilibrium Constant Reverse reaction: Rate Law: 2 NO2 (g)  N2O4 (g)
Rate = kr [NO2]2

10. Equilibrium Constant kf kr [NO2]2 [N2O4] = Therefore, at equilibrium
Ratef = Rater
kf [N2O4] = kr [NO2]2
Rewriting this, it becomes kf kr
[NO2]2
[N2O4] =

11. Equilibrium Constant Keq = kf kr [NO2]2 [N2O4] =
The ratio of the rate constants is a constant at that temperature, and the expression becomes
Keq = kf kr
[NO2]2
[N2O4] =

12. aA + bB cC + dD The equilibrium expression becomes:
Equilibrium Constant
aA + bB cC + dD The equilibrium expression becomes:
Kc =
[C]c[D]d
[A]a[B]b

13. Equilibrium Constant

14. Equilibrium Constant

15. Equilibrium Constant Expression
In an equilibrium constant expression
All concentrations are equilibrium values
Product concentrations appear in the numerator, and reactant concentrations appear in the denominator
Each concentration is raised to the power of its stoichiometric coefficient in the balanced chemical equation
The value of the constant K depends on the particular reaction and on the temperature
Units are never given with K

16. Writing Equilibrium Constant Expressions
Reactions involving solids
S(s) + O2(g) SO2(g)
Concentration of a solid is determined by its density… and density is fixed
Meaning the concentration of a solid is essentially a constant and not included in the expression
The concentrations of any solid reactants and products are not included in the equilibrium constant expression

17. Writing Equilibrium Constant Expressions

18. Writing Equilibrium Constant Expressions
Reactions in Aqueous solutions
The real question here isn’t about aqueous… it’s about pure liquids
Both the concentrations of liquids and solids can be obtained by multiplying the density of the substance by its molar mass — and both of these are constants at constant temperature.

19. Writing Equilibrium Constant Expressions

20. Writing Equilibrium Constant Expressions

21. Writing Equilibrium Constant Expressions
Reactions involving Gases: Kc and Kp
Since pressure is proportional to concentration for gases in a closed system, the equilibrium expression can also be written
Kp =
(PCc) (PDd)
(PAa) (PBb)

22. Writing Equilibrium Constant Expressions
Relationship between Kc and Kp
From the Ideal Gas Law we know that
PV = nRT
Rearranging it, we get
P = RT n V

23. Writing Equilibrium Constant Expressions
Plugging this into the expression for Kp for each substance, the relationship between Kc and Kp becomes
where
Kp = Kc (RT)n
n = (moles of gaseous product) - (moles of gaseous reactant)

24. Practice

25. Practice
Write the equilibrium constant expressions for the following reactions:
N2(g) + 3H2(g) NH3(g)
H2CO3(g) + H2O(l) HCO3-(aq) + H3O+(aq)

26. Unit 2.6: Equilibrium Constants

27. Equilibrium Can Be Reached from Either Direction
As you can see, the ratio of [NO2]2 to [N2O4] remains constant at this temperature no matter what the initial concentrations of NO2 and N2O4 are.

28. Equilibrium Can Be Reached from Either Direction
This is the data from the last two trials from the table on the previous slide.

29. Equilibrium Can Be Reached from Either Direction
It doesn’t matter whether we start with N2 and H2 or whether we start with NH3: we will have the same proportions of all three substances at equilibrium.

30. The Meaning of the Equilibrium Constant, K
If K>>1
the reaction is product-favored; product predominates at equilibrium; system contains almost all product
“reaction has gone to completion”

31. The Meaning of the Equilibrium Constant, K
If K<<1
the reaction is reactant-favored; reactant predominates at equilibrium; system contains almost all reactant

32. Practice
Are the following reactions product- or reactant-favored at equilibrium?
[Cu(NH3)4]+2  Cu+2(aq) + 4NH3(aq)
K = 1.5 x 10-13
[Cd(NH3)4]+2  Cd+2(aq) + 4NH3(aq)
K = 1.0 x 10-7
If each reaction has a reactant concentration of
0.10M, in which solution is the NH3
concentration greater?

36. Equilibrium Constants
The equilibrium constant denotes the progress of the reaction
If the equilibrium constant is greater than 1,the rxn favors the formation of the products. More products are formed than reactants remain.
If the equilibrium constant is less than 1, the rxn favors the formation of the reactants. Most of the original reactant remains as reactant, little product is formed.
The equilibrium constant is temperature dependent, only
The value of the equilibrium constant is not affected by changes in the pressure or concentration of any of the reactants or products.