General Chemistry IIGeneral Chemistry IICHEM 152 Unit 2CHEM 152 Unit 2

Week 5

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Week 5 Reading Assignment

Chapter 14 – Sections 14.2 (dynamic equilibrium), 14.3 (equilibrium constant), 14.5 (heterogeneous equilibria)

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CHEMICAL CHEMICAL EQUILIBRIUMEQUILIBRIUM

How far does the reaction go?

What is the final concentration of reactants and products?

We have seen that when the rate forward equals the rate backwards in a chemical reaction, the system reaches a

state of equilibrium. But,

These are the types of questions we now want to

answer.

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Chemical EquilibriumChemical Equilibrium

If NO2 is reddish brown and N2O4 is colorless:What is happening here?

What properties are changing?What is happening over time? After a long time?

Consider this reaction:

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The final state depends on:

1)The chemical nature of the reactants and products

2)The conditions of the system (temperature,

pressure, volume).

Get time progressionCheck silberberg

N2O4 2NO2

Low T High T

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Analyzing Reaction Progress

Product-or Reactant Favored Process?

If we consider the two possible chemical processes in this system:

A B k1=exp(-Ea1/RT) Rateforward=k1[A]

B A k2=exp(-Ea2/RT) Ratereverse=k2[B]

Reaction progress

A

B

Ea1 Ea2

.

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Analyzing Reaction Progress

What happens with Rateforward and Ratereverse as a function of time?

When does the reaction “stop”?

A B k1=exp(-Ea1/RT)

Rateforward=k1[A]

B A k2=exp(-Ea2/RT)

Ratereverse=k2[B]

TWO 1st order reactions in OPPOSITE directions with DIFFERENT size k’s

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Analyzing Reaction Progress

A B k1=exp(-Ea1/RT) Rateforward=k1[A]

B A k2=exp(-Ea2/RT) Ratereverse=k2[B]

The system reaches a state of chemical equilibrium when:

Rateforward=Ratereverse or k1[A]=k2[B]

k1/k2=[B]/[A] = Keq (Equilibrium Constant)

What does it mean?

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Week 5 Reading Assignment

Chapter 14 – Sections 14.2 (dynamic equilibrium), 14.3 (equilibrium constant), 14.5 (heterogeneous equilibria)

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Announcements

•Exams are graded•will be returned in lab this week

•Will post powerpoints this afternoon•Quiz on Friday•Mechanisms and equilibrium

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Chemical EquilibriumLet’s try the following system:

A100

BProbability

A B 40 %

B A 10 %

Analyze the time evolution of this

system

How many particles of each

species do we have at chemical

equilibrium?

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Chemical EquilibriumA

100

B

A B 40 %B A 10 %

Time A B

0 100 0

AB 40 BA 0

1 60 40

AB 24 BA 4

2 40 60

AB 16 BA 6

3 30 70

AB 12 BA 7

4 25 75

Etc.

Equilibrium?

A*0.4 = B*0.1 A + B = 100 A = 20 B = 80 Keq = 80/20 = 4 = 0.4/0.1

The reaction keeps going but [A] and [B] remain

constant.

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Over time the concentrations reach levels where they do NOT change

This is the equilibrium condition

N2O4 2NO2

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Let’s Simulate: A B

What is Keq= [B]/[A]

at equilibrium? Does the Keq depend

on the initial

values of [A] and

[B]?

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Conclusion?

No matter what the starting concentrations at equilibrium.

The ratio [B]eq/[A]eq is a constant

Keq

EQUILIBRIUM CONSTANT

A B

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Another ReactionN2O4 2NO2

What is the value of

[NO2]eq /

[N2O4]eq?

Does this value remain constant as

we change the initial

concentrations?

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Another ReactionN2O4 2NO2

Let’s try again with

[NO2]2eq /

[N2O4]eq?

Does this value remain

constant as we change the

initial concentrations

?

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The Equilibrium Constant

The Equilibrium Constant

For a reaction of the typeFor a reaction of the typea A + b B a A + b B c C + d D c C + d D

the following is a CONSTANT (at a given T)the following is a CONSTANT (at a given T)

K =[C]c [D]d

[A]a [B]b

conc. of products

conc. of reactantsequilibrium constant

If KIf Keqeq is known, then we can PREDICT is known, then we can PREDICT concentrations of products or concentrations of products or

reactantsreactants

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Your Turn

.

Equilibrium mixWhat is the value of

Keq?

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Keq ~ 92/(1 x 1) = 81

Your Turn

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Your Turn

. .

Equilibrium mix

What is the value of Keq?

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Your Turn

.

Equilibrium mix

Keq ~ 22/(6 x 4) = 0.17

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Chemical Equilibrium Chemical Equilibrium

Some things to keep in mind about Some things to keep in mind about chemical equilibrium:chemical equilibrium:• DYNAMIC DYNAMIC (in constant motion);(in constant motion);• REVERSIBLE; REVERSIBLE; • Can be approached from either Can be approached from either direction;direction;• After a period of time, the After a period of time, the concentrationsconcentrations of reactants and products are constant; of reactants and products are constant; • The forward and reverse reactions The forward and reverse reactions continue continue after equilibrium is attained; after equilibrium is attained;• Catalysts do not affect equilibrium Catalysts do not affect equilibrium concentrations (Why?) concentrations (Why?)

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Let us consider the conversion of Let us consider the conversion of cis (A) to trans (B) butene: cis (A) to trans (B) butene:

Effect of Catalysts

Reaction progress

E

N

E

R

G

Y

A

B

Ea1 Ea

2

.

A Catalyst lowers BOTH the forward

and reverse reaction activation

energiesproportionally. So, both rates change

but NOT the equilibrium

distributions.

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Adding more C(s) will NOT change the other concentrations

.

Does addingmore solid

carbon changethe equilibrium?

Heterogeneous Equilibrium

WHY?

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What happens if you add sugar to hot water? Will the sugar continue to dissolve indefinitely?

Once the sugar can no longer dissolve will adding more sugar help dissolve additional sugar?

The solid sugar and dissolved sugar are in equilibrium. The

additional sugar has no effect on the equilibrium

Solids and pure liquids do not change their “concentration” at equilibrium and there are

not included in the Keq expression for a “HETEROGENEOUS EQUILIBRIUM”

Heterogeneous Equilibrium

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Heterogeneous Equilibrium

What is the equilibrium constant for

this process?

How would you expect

Keq to change if more H2,

CH4, or C are added?

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Your TurnYour Turn

Write the Equilibrium Write the Equilibrium constant for the process:constant for the process:

S(s) + OS(s) + O22(g) (g) SOSO22(g)(g)

K [SO2 ][O2 ]

K [SO2 ][O2 ]

Solids and liquids Solids and liquids NEVERNEVER appear in equilibrium appear in equilibrium

expressions expressions ((their concentration is their concentration is

constant during the constant during the processprocess).).

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Chemical Equilibrium Chemical Equilibrium

Some things to keep in mind about Some things to keep in mind about chemical equilibrium:chemical equilibrium:• DYNAMIC DYNAMIC (in constant motion);(in constant motion);• REVERSIBLE; REVERSIBLE; • Can be approached from either Can be approached from either direction;direction;• After a period of time, the After a period of time, the concentrationsconcentrations of reactants and products are constant; of reactants and products are constant; • The forward and reverse reactions The forward and reverse reactions continue continue after equilibrium is attained; after equilibrium is attained;• Catalysts do not affect equilibrium Catalysts do not affect equilibrium concentrations (Why?) concentrations (Why?)

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The Equilibrium Constant

The Equilibrium Constant

For a reaction of the typeFor a reaction of the typea A + b B a A + b B c C + d D c C + d D

the following is a CONSTANT (at a given T)the following is a CONSTANT (at a given T)

K =[C]c [D]d

[A]a [B]b

conc. of products

conc. of reactantsequilibrium constant

If KIf Keqeq is known, then we can PREDICT is known, then we can PREDICT concentrations of products or concentrations of products or

reactantsreactants

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Your TurnYour Turn

Write the equilibrium constant for the Write the equilibrium constant for the following process:following process:

NHNH33(aq) + H(aq) + H22O(liq) O(liq) NH NH44++(aq) + OH(aq) + OH--

(aq)(aq)

K [NH4

+][OH- ][NH3 ]

K [NH4

+][OH- ][NH3 ]

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Manipulating KManipulating Keqeq

Consider the equilibrium:

N2(g) + 3H2(g) 2NH3(g) Keq= 3.5 x 108 (25 oC)

What would be the value of the equilibrium constant for these processes?

1/2 N2(g) + 3/2 H2(g) NH3(g)

2NH3(g) N2(g) + 3H2(g)If the chemical equation is multiplied by n, Kc

Kcn

If the chemical equation is reversed, Kc 1/Kc

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Stepwise Equilibrium

(1.) N2(g) + O2(g) 2NO(g)

(2.) 2NO(g) + O2(g) 2NO2(g)

[NO]2

Keq,1 = [N2][O2]

[NO2]2

Keq,2 = [NO]2[O2]

Manipulating KManipulating Keqeq

[NO]2 [NO2]2

Keq = = Keq,1 Keq,2 [N2][O2] [NO]2[O2]

Determine the chemical equation for the overall process

What is the equilibrium constant for the overall process? How does it

depend on the value of Kc1 and Kc2?

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Relevant Relevant Examples of Examples of

Chemical Chemical EquilibriaEquilibria

Formation of Formation of

stalactites and stalagmitesstalactites and stalagmites

CaCOCaCO33(s) + H(s) + H22O(liq) + COO(liq) + CO22(g)(g)

CaCa2+2+(aq) + 2 HCO(aq) + 2 HCO33(aq)(aq)

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Relevant Examples of Relevant Examples of Chemical EquilibriaChemical Equilibria

Phase changes such asPhase changes such as

HH22OO(s) (s) H H22OO(liq)(liq)

In order to reach

equilibrium, the system should be

closed

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Summary Activity

1) Write the equilibrium constant for these processes:

MgO(s) + SO2(g) + ½ O2(g) MgSO4(s)

H2O(l) H2O(g)

CoCl(H2O)5+(aq) + Cl(aq) CoCl2(H2O)2(aq) +

3H2O(l)

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2) If the equilibrium constant for the process

H2(g) + 1/8 S8(s) H2S(g) Kc=7.6 x 105

What is the equilibrium constant associated with

the reaction: 8H2S 8H2(g) + S8(s)?

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3) Given these equilibrium reactions and constants

3/2 O2(g) O3(g) Keq,1=2.5x10-29

2NO(g) + O2(g) 2NO2(g) Keq,2=2.25 x 1012

Calculate the equilibrium constant for the reaction

NO(g) + O3(g) NO2(g) + O2(g)

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4) If Kc=2.5 for the reaction A B, is the system depicted in the figure in equilibrium? If not, how would you represent the equilibrium state?

Apply your Knowledge

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5) If Keq=7 for the reaction A 2B, is the system depicted in the figure in equilibrium? If not, how would you represent the equilibrium state?

Apply your Knowledge