1. Chemical Equilibrium Chapter 13
Equilibrium Expression and Constant

2. 13.2 The Equilibrium Constant
Law of Mass Action – a general description of equilibrium
Equilibrium position – a set of equilibrium concentrations
There is only one equilibrium constant for a system at a certain temperature.
There are infinite number of equilibrium positions.
Equilibrium positions depend on initial concentrations
Equilibrium constant does not
aA + bB  cC + dD cC + dD  aA + bB n[aA + bB  cC + dD] K=
K’=
K’’=
[C]x [D]y
[A]n [B]m
K = =
products
reactants
K’ = 1/K
K’’ = Kn

3. Equilibrium Positions
Table Results of Three Experiments for the Reaction N2(g) + 3H2(g) -- 2NH3(g)

4. 13.3 Equilibrium Expressions Involving Pressures
K or Kc = Equilibrium constant in terms of concentration
Kp = equilibrium constant in terms of partial pressures
P is substituted for [concentration]

5. Problem #26 N2 (g) + 3H2 (g)  2NH3 (g)
The following equilibrium pressures were observed at a certain temperature for the reaction below. PNH3 = 3.1 x atm, PN2= 8.5 x 10-1 atm, PH2 = 3.1 x atm. Calculate the value for the equilibrium constant Kp at this temperature
N2 (g) + 3H2 (g)  2NH3 (g)
Answer

6. Problem #55
At 25C, Kp = 2.9 x 10-3 for the reaction below. In an experiment carried out at 25C, a certain amount of NH4OCOHN2 is placed in an evacuated rigid container and allowed to come to equilibrium. Calculate the total pressure in the container at equilibrium.
NH4OCOHN2 (s)  2NH3 (g) + CO2 (g)
0.27 atm

7. 13.4 Heterogeneous Equilibria
Heterogeneous Equilibrium: more than one phase
The position of heterogeneous equilibrium doesn’t depend on the amounts of pure solids or liquids.
Solids and liquids are not included in the equilibrium expression
Why?
You cannot compress solid and liquid.
If you change the volume, you must also change the moles so concentration is constant.
Not included in equilibrium expression

8. Equilibrium Positions

9. 13.5 Application of Equilibrium Constant
Review
K>1 then products are favored – the equilibrium lies to the right
K<1 then reactants are favored – the equilibrium lies to the left
Equilibrium constant tells
the tendency of a reaction to occur
whether a set of concentrations represent equilibrium condition
the equilibrium position achieved from a set of initial concentrations
Will the system shift toward the right or left?
The size of K is not directly related to the time to reach equilibrium.

10. Reaction Quotient, Q Q is like K but using initial concentrations
If Q = K, the system is at equilibrium – no shift will occur.
If Q > K, the system shifts to the left – too much product initially
If Q < K, the system shifts to the right – too much reactant initially
Sample 13.7 page 593
For the synthesis of ammonia at 500C, the equilibrium constant is 6.0 x Predict the direction in which the system will shift to reach equilibrium in each of the following cases.
a. [NH3]0 = 1.0 x 10-3 M; [N2]0 = 1.0 x 10-5 M; [H2]0 = 2.0 x 10-3 M

11. Sample Page 598 K & Q
Assuming that the reaction for the formation of gaseous hydrogen fluoride from hydrogen and fluorine has an equilibrium constant of 1.15 x 102 at a certain temperature. In a particular experiment, mol of each component was added to a L flask. Calculate the equilibrium concentration of all species.
[H2] = [F2] = 0.472M ; [HF] = 5.056M

12. 13.6 Using the Quadratic Equation
K = 115
Find Q. Which way does it go?
Fill in ICE with x.
Set up equilibrium expression.
You must use the quadratic equation to solve for x.
Get two x’s x=2.14M and x=0.968M
Which is correct?
H F2  2 HF
1.000
2.000

13. Problem #47
At 2200C, Kp = for the reaction below. What is the partial pressures of NO in equilibrium with N2 and O2 that were placed in a flask at initial pressures of 0.80 and 0.20 atm, respectively?
N  2NO
7.8 x 10-2 atm

14. Systems with Small Equilibrium Constants
K = 1.6 x 10-5
What does the equilibrium expression look like?
Approximating
Since the equilibrium will not proceed far to the right (lots of reactants – fewer products) x will be very small.
If K x 100 < [A]0 (check all reactants) then don’t worry about subtracting x.
Once you find x go back and look at the difference between 0.50 and x. If it is 5% or less you can ignore the -2x in the reactant.
2NOCl  2NO Cl2
0.50

15. Problem #53
At a particular temperature, K = 2.0 x 10-6 for the reaction below. If 2.0 mol CO2 is initially placed into a 5.0L vessel, calculate the equilibrium concentrations of all species.
2CO2 (g)  2 CO (g) O2 (g)
[CO2] = 0.39 M; [CO] = 8.6 x M; [O2] = 4.3 x 10 -3M