Applications of KApplications of K

Chapter 15 part Chapter 15 part IIIIII

Knowing the K of a Knowing the K of a Reaction allows one to Reaction allows one to

predict:predict:

1.1. The tendency for a reaction to occur. The tendency for a reaction to occur. (but not the kinetics) (but not the kinetics)

2.2. If the given set of concentrations are If the given set of concentrations are at equilibrium.at equilibrium.

3.3. The equilibrium position that will be The equilibrium position that will be achieved from a given set of achieved from a given set of concentrations.concentrations.

Consider the reaction:Consider the reaction:

►Assume this reaction has a K=16.Assume this reaction has a K=16.►K= K= (N )(N ) (N )(N ) = 16= 16► (N )(N )(N )(N )► N represents the number of molecules of each type.N represents the number of molecules of each type.

►Assume:Assume:► Initial conditions:Initial conditions: New Conditions:New Conditions:►99 -x-x 9-5=49-5=4►1212 -x-x 12-5=712-5=7►00 +x+x 0+5=50+5=5►00 +x+x 0+5 = 50+5 = 5 What is Q?What is Q?

+ +

The new value for Q is:The new value for Q is:►Q= (5)(5) = 0.9 K=16► (4)(7)

►We have moved closer to equilibrium than► the initial value of zero, but we are not

there►with only a change of x=5.►We know that from the initial

concentrations► the change x must be some number

greater ► than 5 such that:►K=16= (x)(x)► (9-x)(12-x)► We know the value of x must be between 5 &9

By trial and error we find the value to By trial and error we find the value to be 8be 8

► If x =8, then 16= If x =8, then 16= (8)(8)(8)(8)► (9-8)(12-8)(9-8)(12-8)►Remember:Remember:► The size of K and the time required to reach The size of K and the time required to reach

equilibrium are not related.equilibrium are not related.►Q, the reaction quotient, is obtained by the Q, the reaction quotient, is obtained by the

initial concentrations.initial concentrations.►Q=K system at equilibrium, no shift.Q=K system at equilibrium, no shift.►Q>K, system shifts towards reactants (left) Q>K, system shifts towards reactants (left) ►Q<K, system shifts towards products (right)Q<K, system shifts towards products (right)

Consider the Haber reaction in Equilibrium

►Or NOr N22 + 3H + 3H22 2 NH 2 NH33

First consider the reaction where +

K = (N )2

(N )(N )3

K = (NH3 )2

(N2)(H2)3

Examples, Find the Q of each situation and Examples, Find the Q of each situation and the direction of the shift. Kc= 6.0 x 10 the direction of the shift. Kc= 6.0 x 10 -2-2

► AA BBCC

► [NH[NH33]]ii=1.0 x 10=1.0 x 10-3-3MM 2.00 x 102.00 x 10-4-4MM 1.0 x 101.0 x 10--

44MM► [N[N22]]ii= 1.0 x 10= 1.0 x 10-5-5MM 1.50 x 101.50 x 10-5-5MM 5.0 M5.0 M

► [H[H22]]ii= 2.0 x 10= 2.0 x 10-3-3MM 3.54 x 103.54 x 10-1-1 MM 1.0 x 1.0 x 1010-2-2 M M

Change in concentration is Change in concentration is dependant on stoichiometrydependant on stoichiometry

1.1. Balanced equation HBalanced equation H22(g)+F(g)+F22(g) 2HF(g)(g) 2HF(g)

2.2. Write the equilibrium expression K=Write the equilibrium expression K=(HF)(HF)22

(H(H22)(F)(F22))

3.3. List initial concentrationsList initial concentrations

4.4. List change.List change.

5.5. Define equilibrium as a function of changeDefine equilibrium as a function of change

6.6. Substitute equilibrium into the equilibrium Substitute equilibrium into the equilibrium expression.expression.

7.7. Check!!Check!!

ICE tablesICE tables

► For this: For this: HH22(g)+F(g)+F22(g) 2HF(g)(g) 2HF(g)

► K=1.15 x 10K=1.15 x 1022= = [HF][HF]22

► [H[H22][F][F22]]

► In this experiment 3.00 moles of In this experiment 3.00 moles of each component was added to a each component was added to a 1.500 L flask. Calculate the 1.500 L flask. Calculate the equilibrium of each species.equilibrium of each species.

Make an ICE tableMake an ICE table

► II

►CC

►EE

►K expression is=K expression is=

Follow the steps.Follow the steps.

1.1. Balanced equationBalanced equation

2.2. Write the equilibrium expressionWrite the equilibrium expression

3.3. Find QFind Q

4.4. ICE tableICE table

5.5. SolveSolve

6.6. Check.Check.

Answer:Answer:

►X=1.528X=1.528►Find equilibrium []’sFind equilibrium []’s►Check to see if K=KCheck to see if K=K

This problem was a perfect This problem was a perfect squaresquare

►That math was easy. What happens if That math was easy. What happens if it is not a perfect square?it is not a perfect square?

►Quadratic equation!Quadratic equation!►What is That??What is That??

►X = X = -b-b±√b2-4ac±√b2-4ac► 2a2a

ReviewReview

La Châtelier’s La Châtelier’s PrinciplePrinciple

► YOUR SUBTOPIC GOES HEREYOUR SUBTOPIC GOES HERE

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