intro to equilibrium abbrev alg
TRANSCRIPT
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