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## Chemical Equilibrium Chapter 13

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Title: Chemical Equilibrium Chapter 13

1
Chemical EquilibriumChapter 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
naA bB ? cC dD

K
K
K
K 1/K
K Kn
3
Equilibrium Positions
Table 13.1 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
• Rearrange the ideal gas law to solve for n/V
• n/V C molar concentration of the gas
• Substitute P/RT for A
• Do the algebra
• K Kp (RT)?n
• where ?n is the (? products coefficients) (?
reactants coefficients)

5
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

6
Problem 26
• The following equilibrium pressures were observed
at a certain temperature for the reaction below.
PNH3 3.1 x 10 -2 atm, PN2 8.5 x 10-1 atm, PH2
3.1 x 10 -3 atm. Calculate the value for the
equilibrium constant Kp at this temperature
• N2 (g) 3H2 (g) ? 2NH3 (g)

7
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
8
13.4 Heterogeneous Equilibria
• Heterogeneous Equilibrium more than one phase
• The position of heterogeneous equilibrium doesnt
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 molesso concentration is constant.
• Not included in equilibrium expression

9
Equilibrium Positions
10
13.5 Application of Equilibrium Constant
• Review
• Kgt1 then products are favored the equilibrium
lies to the right
• Klt1 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.

11
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 gt K, the system shifts to the left too
much product initially
• If Q lt 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 10-2. Predict the
direction in which the system will shift to reach
equilibrium in each of the following cases.
• a. NH30 1.0 x 10-3 M N20 1.0 x 10-5 M
H20 2.0 x 10-3 M

12
Sample 13.11 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, 3.000 mol of each component was added
to a 1.500-L flask. Calculate the equilibrium
concentration of all species.

H2 F2 0.472M HF 5.056M
13
• 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 xs x2.14M and x0.968M
• Which is correct?

H2 F2 ? 2 HF H2 F2 ? 2 HF H2 F2 ? 2 HF
1.000 2.000 0

14
Problem 47
• At 2200?C, Kp 0.050 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?
• N2 02 ? 2NO

7.8 x 10-2 atm
15
Problem 91
• For the reaction below, K 400. at 35.0?C. If
2.00 mol each of NH3, H2S, and NH4HS are placed
in a 5.00 L vessel, what mass of NH4HS will be
present at equilibrium? What is the pressure of
H2S at equilibrium?
• NH3 (g) H2S (g) ? NH4HS
(g)

16
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 lt A0 (check all reactants) then
• Once you find x go back and look at the
difference between 0.50 and 0.50-2x. If it is 5
or less you can ignore the -2x in the reactant.

2NOCl ? 2NO Cl2 2NOCl ? 2NO Cl2 2NOCl ? 2NO Cl2
0.50 0 0

17
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 10 -3 M O2 4.3
x 10 -3M