1. Equilibrium

2. Reaction Graphs Initially … A B or Reactants  Products Then A  B Ao
‘Equilibrium is established’
Then A  B Ao Bo
Concentrations
Time vs
Finally … A  B Rates equal
Note: the process is dynamic

3. Physical vs. Chemical Physical - between states of matter
H2O(l) H2O(g)
Chemical - When the rates of forward and reverse reactions are equal.
N2O4(g)  2NO2 (g)

4. Equilibrium Constant (Kc)
N2O4(g)  2NO2 (g)
Equilibrium concentrations (in M/L)
[NO2]
[N2O4]
Exp #
[NO2]2
[N2O4]
[NO2]
[N2O4]
0.0547
0.0457
0.0475
0.0523
0.0204
0.643
0.448
0.491
0.594
0.0898 1 2 3 4 5
0.0851
0.102
0.0967
0.0880
0.227
Average = 4.63 x 10-3

5. Equilibrium Constant (Kc)
aA + bB  cC + dD
Kc =
Products
reactants =
[C]c[D]d
[A]a[B]b
This works for partial pressures, too.

6. The meaning of K What does it mean if K is greater than 1?
(Hint: How is K derived?)
What does it mean if K is less than 1? Ao Bo
Concentrations
Time

7. KP
bB  cC Kc =
[C]c
[B]b P KP =

8. There is a relationship between pressure and concentration.
PV=nRT
Rearranges to … P= n V ( ) RT
Note: n/V = concentration

9. Relationship between Kc & KP
KP = Kc(RT)Δn
Δn from the coefficients of the balanced equation
T(temperature) in kelvin
What’s “R”?

10. Reaction quotient (Qc)
The quantity obtained by substituting the initial concentrations into the equilibrium constant expression.
N2 + 3H2  2NH3
Example: At the start of a reaction, there are mol, N , mol H2 and mol NH3 in a 3.50L reaction vessel at 200ºC. If the equilibrium constant for the reaction is .065, decide whether the system is at equilibrium.

11. Reaction quotient (Qc)
Qc > Kc proceeds from right to left
Qc = Kc at equilibrium
Qc < Kc proceeds from left to right
Products
Reactants

12. Problems
Page #10, 13, 19, 20a, 21 – 24, 27, 29, 31, 32, 35, 37, 39

13. Part II
ICE & Le Chat

14. Solving Equilibrium Problems
1. Balance the equation.
2. Write the equilibrium expression.
3. List the initial concentrations.
4. Calculate Q and determine the shift to equilibrium.

15. Solving Equilibrium Problems
5. Define equilibrium concentrations.
6. Substitute equilibrium concentrations into equilibrium expression and solve.
7. Check calculated concentrations by calculating K.

16. I C E
I = Initial
C = Change
E = Equilibrium

17. I C E
A  B
How do we calculate the concentrations of A and B in equilibrium?
Kc must be given.
Suppose that A is initially mol/L and Kc =24.0
I = Initial
C = Change
E = Equilibrium
-x x
(0.850-x) x
[B]
[A] =
[x]
[0.850-x]
Kc =

18. I C E I = Initial C = Change E = Equilibrium A  B 0.850 0 -x x
-x x
(0.850-x) x =
[x]
[0.850-x]
Kc =
24.0
Now solve

19. Le Châtelier’s Principle
. . . if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.