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Chemical Equilibrium Chapter 13


Chemical Equilibrium Chapter 13 Equilibrium Expression and Constant – PowerPoint PPT presentation

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Title: Chemical Equilibrium Chapter 13

Chemical EquilibriumChapter 13
  • Equilibrium Expression and Constant

13.2 The Equilibrium Constant
  • Law of Mass Action a general description of
  • Equilibrium position a set of equilibrium
  • There is only one equilibrium constant for a
    system at a certain temperature.
  • There are infinite number of equilibrium
  • Equilibrium positions depend on initial
  • Equilibrium constant does not
  • aA bB ? cC dD cC dD ? aA bB
    naA bB ? cC dD

K 1/K
K Kn
Equilibrium Positions
Table 13.1 Results of Three Experiments for the
Reaction N2(g) 3H2(g) -- 2NH3(g)
13.3 Equilibrium Expressions Involving Pressures
  • K or Kc Equilibrium constant in terms of
  • Kp equilibrium constant in terms of partial
  • 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)

13.3 Equilibrium Expressions Involving Pressures
  • K or Kc Equilibrium constant in terms of
  • Kp equilibrium constant in terms of partial
  • P is substituted for concentration

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)

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
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

Equilibrium Positions
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.

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

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
13.6 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
  • 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

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,
  • N2 02 ? 2NO

7.8 x 10-2 atm
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

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

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
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