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Chemical EquilibriumChapter 13

- Equilibrium Expression and Constant

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

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

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)

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

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

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

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

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

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

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