Learning objectives

Week 9
Chemical Equilibria
Elley Rudebeck
[email protected]
Menti: 2275 037
Learning objectives
9.1 explain that many chemical reactions do not lead to complete conversion of reactants
to products but to equilibrium mixtures containing both.
9.2 write expressions for Kc, Kp and Qc.
9.3 use the relationship between K and ΔrG ο to calculate one quantity given the other.
9.4 predict how an equilibrium mixture will change in response to the addition or removal
of a product or reactant or a change in the pressure or temperature.
9.5 calculate equilibrium constants and equilibrium concentrations.
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9.1 Chemical equilibrium
• Many chemical reactions result in the virtually complete conversion of reactants into
products.
• When the reactants and products are of approximately equal stability, the reaction is
reversible.
• A reversible reaction is one that can go in either direction, from products to reactants
or reactants to products.
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9.1 Chemical equilibrium
4
When the number of people going up
is the same as the number going
down, the number of people on each
floor is constant.
9.1 Chemical equilibrium
• Rates of the forward and reverse reactions are equal.
• There is no net change in the overall composition of the reaction mixture.
• Dynamic equilibrium.
 ‘Reactants’ substances on the left.
 ‘Products’ substances on the right.
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9.1 Chemical equilibrium
 The reaction read from left to right is referred to as the forward reaction.
 The reaction from right to left is referred to as the reverse reaction.
• After some time, the concentrations of reactants and products undergo no further
change although both reactions continue.
• At this point, the reaction vessel contains a mixture of all reactants and products, and
the reaction is said to be in a state of chemical equilibrium.
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9.1 Chemical equilibrium
7
• The forward reaction takes place rapidly at the beginning of the reaction then
slows down as reactant concentrations decrease.
• The reverse reaction takes place slowly at the beginning but then speeds up
as product concentrations increase.
• Ultimately, the forward and reverse rates become equal.
9.1 Chemical equilibrium
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9.1 Chemical equilibrium
9
• Chemical equilibrium is an active, dynamic condition.
• All substances present are appearing and disappearing at the same rate, so
their concentrations are constant at equilibrium.
 The reaction is continuing in both directions.
• It is not necessary for the concentrations of reactants and products at
equilibrium to be equal.
9.1 Chemical equilibrium
10
• The extent to which the forward or reverse reaction is favoured is a
characteristic property of a reaction under given conditions.
• It is possible to predict what the equilibrium conditions will be for any given
reaction.
 The concentrations of reactants and products are fixed at equilibrium.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
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• For a given overall system composition equilibrium concentrations are
independent of direction of approach.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
12
Reactions at equilibrium:
• The following holds when equilibrium is established:
• Equilibrium constant expression.
 K
c – equilibrium constant.
 K
c dependent on temperature, always specify temperature when Kc
reported.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
13
• For a given overall system composition equilibrium concentrations are
independent of direction of approach.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
14
The magnitude of the equilibrium constant:
• Product concentrations are in the numerator of K
c
• The size of K
c gives a measure of how far the reaction proceeds towards
completion when equilibrium is reached.
• Applies to reactions involving only gasses or species in solution.
Worked example 9.1
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• Write the expression for Kc for the following reaction:
• Does this reaction favour reactants or products at equilibrium?
9.2 The equilibrium constant, K, and the reaction
quotient, Q
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Reactions at equilibrium:
• The value of the equilibrium constant indicates the position of a reaction at
equilibrium.
 K
c >> 1 indicates the equilibrium reaction mixture contains more product
than reactant (reaction is product favoured).
 K
c than product (reaction is reactant favoured).
 K
c ≈ 1 indicates equilibrium reaction mixture contains approximately equal
amounts of product and reactant.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
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Quick check
18
Practice exercise 9.1:
Write the equilibrium constant expression Kc for the following reaction:
At 25 °C, the Kc value for this reaction is 9.1 x 1080. Does this reaction favour
products or reactants?
Menti: 2275 037
Worked example 9.1
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• Find the value of K
c for the following reaction at 472 °C:
• The concentrations at equilibrium were measured to be:
• 3.1 x 10‐2 mol L‐1 for N
2
• 8.5 x 10‐1 mol L‐1 for H
2
• 3.1 x 10‐3 mol L‐1 for NH
3
Quick check
20
Calculating equilibrium constant K:
• Concentrations measured at equilibrium:
 H
2 = 1.2 mol L‐1
 I
2 = 1.2 mol L‐1
 HI = 0.35 mol L‐1
• Find the value of K.
Menti: 2275 037
Quick check
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Calculating equilibrium constant K:
• Does this system favour products or reactants at equilibrium?
Worked example
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Calculating equilibrium concentration:
• Given the following data:
 [PCl3] = [Cl2] = 0.10 M
 K
c = 4.2 x 10‐2
• Find [PCl5] at equilibrium.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
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Equilibria involving gases:
• Example: N2O4(g) 2NO2(g)
• We can also use partial pressure to specify the quantity of the two gases.
• Therefore, we can also have:
K
c
=
NO
2
   2
N
  2O4 
⇋
KP

P
NO
2
  P 2
PN2
O4
  P
9.2 The equilibrium constant, K, and the reaction
quotient, Q
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Equilibria involving gases:
• In general, for a reaction:
aA(g) + bB(g) ⇋ cC(g) + dD(g)
c d
C D
p a b
A B
p p
p p
K
p p
p p

AssignmentTutorOnline

9.2 The equilibrium constant, K, and the reaction
quotient, Q
25
Reactions not at equilibrium:
• Q
c – reaction quotient.
• Expression for systems not necessarily at equilibrium.
 K
c can have only one positive value at a specific temperature.
 Q
c can have any positive value.
   
   
a b
c d
c
A B
Q  C D
9.2 The equilibrium constant, K, and the reaction
quotient, Q
26
Q
c = Kc at equilibrium
Q
c > Kc system reacts to use up products and generate more reactants
Q
c 9.2 The equilibrium constant, K, and the reaction
quotient, Q
27
Quick check
28
The concentrations of the components of the following reaction mixture
N
2 (g) + 3H2 (g) ⇋ 2NH3 (g)
were measured to be:
[N2] = 4.0 x 10‐2 mol L‐1
[H2] = 8.5 x 10‐1 mol L‐1
[NH3] = 3.1 x 10‐3 mol L‐1
At equilibrium, Kc = 5.1 x 10‐4 for this system
Calculate Q
c for the reaction and predict whether it will proceed towards
reactants or products.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
29
Manipulating equilibrium constant expressions:
• When the direction of an equation is reversed, the new equilibrium
constant is the reciprocal of the original.
 
  3   2
5 C
PCl l
PCl
c 
K

‘ 3 2
5
PCl Cl
 PCl
c
c K
K
‘ 1

9.2 The equilibrium constant, K, and the reaction
quotient, Q
30
Manipulating equilibrium constant expressions:
• When multiplying the stoichiometric coefficients of a reaction, the
equilibrium constant is raised to a power equal to that factor.
 
  3   2
5 C
PCl l
PCl
c 
K

2
” 5
2 2
3 2
PCl
PCl Cl

” 2
c c
K K

9.2 The equilibrium constant, K, and the reaction
quotient, Q
31
Manipulating equilibrium constant expressions:
• When chemical equilibria are added, their equilibrium constants are
multiplied.
 
    2
2
2
2
2
1
N O
N O
c 
K

4
2
2 2 3
2 2
NO
N O O

 
   
 
   
 
   
4
2
2
2
4
2
3
2
2
2
4
2
2
2
2
2
2
N O
NO
N O O
NO
N O
N O
 
 
   
c
K
4
2
3 2 4
2 2
NO
N O

c1 c2 c3
K K K
 
9.2 The equilibrium constant, K, and the reaction
quotient, Q
32
Equilibrium constant expressions for heterogeneous systems:
• Homogeneous reaction, all reactants and products are in the same phase.
• Heterogeneous reaction, more than one phase exists in reaction mixture:
Kc
= [H2O][CO2]
 Do not include the concentrations of pure solids or pure liquids.
9.2 The equilibrium constant, K, and the reaction
quotient, Q
33
Equilibrium constant expressions for heterogeneous systems:
• For any pure liquid or solid at constant temperature, the ratio of amount of
substance to volume of substance is constant.
9.3 Equilibrium and Gibbs energy
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The relationship between ΔrGθ and K:
• If the system is at equilibrium:
 ΔG = 0
 Q = K
rG  rG  RT lnQ
rG  RT ln K
9.4 How systems at equilibrium respond to
change
35
Le Châtelier’s principle:
• If an outside influence upsets an equilibrium, the system undergoes a change
in a direction that counteracts the disturbing influence and, if possible,
returns the system to equilibrium.
• Better to compare equilibrium constant, K, and reaction quotient, Q, when
examining the effect of perturbation to a chemical process at equilibrium.
9.4 How systems at equilibrium respond to
change
36
Le Châtelier’s principle:
• When a stress is applied to a system at equilibrium, the equilibrium shifts to
relieve the stress.
 The word “stress” means anything that disturbs the original equilibrium.
9.4 How systems at equilibrium respond to
change
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Le Châtelier’s principle:
9.4 How systems at equilibrium respond to
change
38
Adding or removing a product or reactant:
• When not a pure solid or liquid, removal or addition of a reactant or product
instantaneously alters the concentration of that species in the reaction
mixture.
• The value of Q changes so that Q ≠ K, the system is no longer at equilibrium.
9.4 How systems at equilibrium respond to
change
39
Adding or removing a product or reactant:

[CuCl4]2‐(aq) + 4H2O(l)
 
Q

 
 
 

   
   
2
4
2 4
2 4
CuCl
Cu OH Cl
9.4 How systems at equilibrium respond to
change
40
Changing the pressure in gaseous reactions:
• Two ways of changing the total pressure:
 changing the volume of the system
 adding an inert gas.
• Consider the equilibrium
N
2 (g) + 3H2 (g) ⇋ 2NH3 (g)
9.4 How systems at equilibrium respond to
change
41
Changing the pressure in gaseous reactions:
• Option 1: Changing the volume of the system:
 
  
Qc
2
3
3
2 2
NH
N H V
n
c

 
 
c
n
Q V
n n
V V


3
2 2
2
NH
2
3
N H
3
 
 

n
 

3
2 2
2
NH 2
3
N H
Qc ∝