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)

Answer
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
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 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)

answer
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
  dont worry about subtracting x.
• 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