2012 topic 7.2
TRANSCRIPT
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Topic 7.2
IB Chemistry Power Points
Topic 7
Equilibriumwww.pedagogics.ca
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The Position of EquilibriumConsider
A BA + B C + D
A reacts with B in a sealed flask.
What happens to [A], [B], [C], and [D]?
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The Position of Equilibrium
As the reaction progresses–[A] & [B] decrease to constant values,
–[C] & [D] increase from zero to constant values
–When [A], [B], [C] and [D] are all constant values, equilibrium is achieved.
A BA + B C + D
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N2O4 (g) 2NO2 (g)
Start with NO2 Start with N2O4 Start with NO2 & N2O4
equilibrium
equilibrium
equilibrium
3 scenarios for the reactionsee simulation
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= 4.63 x 10-3KC = [NO2]2
[N2O4]
Data for these (and other) scenarios might look like
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The Equilibrium ConstantBIG IDEA : At a given temperature, no matter
the starting composition of reactants and products, the same ratio of concentrations is achieved at equilibrium.
• For a general reaction
aA bB cC dD
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The Equilibrium Constant
the equilibrium constant expression is
where Kc is the equilibrium constant
and is dependent on temperature.
[ ] [ ][ ] [ ]
c d
C a b
C DK
A B
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Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.N2O4 (g) 2NO2 (g)
Kc = [NO2]2
[N2O4]
CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)
Kc =‘[CH3COO-][H3O+][CH3COOH][H2O]
Kc = [CH3COO-][H3O+]
[CH3COOH]
[H2O] = does not change significantly so
is omitted
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Practice – homogeneous equilibrium• Write the equilibrium expressions for
the following reactions:
H2 (g) + I2 (g) Ý 2 HI (g)
HF (aq) Ý H+ (aq) + F- (aq)
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Practice – homogeneous equilibrium• Write the equilibrium expressions for
the following reactions:
H2 (g) + I2 (g) Ý 2 HI (g)
HF (aq) Ý H+ (aq) + F- (aq)
2
2 2
[ ]
[ ][ ]eq
HIk
H I
[ ][ ]
[ ]eq
H Fk
HF
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Practice:The equilibrium concentrations for the reaction between carbon monoxide and molecular chlorine to form COCl2 (g) at 740C are [CO] = 0.012 M, [Cl2] = 0.054 M, and [COCl2] = 0.14 M. Calculate Kc
CO (g) + Cl2 (g) COCl2 (g)
220 M-1
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Heterogenous equilibrium applies to reactions in which reactants and products are in different phases.
CaCO3 (s) CaO (s) + CO2 (g)
Kc =‘[CaO][CO2]
[CaCO3][CaCO3] = constant[CaO] = constant
Kc = [CO2]
The concentration of solids and pure liquids do not change hence they are not included in the expression for the equilibrium constant.
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[CO2] = Kc
CaCO3 (s) CaO (s) + CO2 (g)
[CO2]does not depend on the amount of CaCO3 or CaO
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Practice – heterogeneous equilibrium• Write the equilibrium expression for the
following reaction:
PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
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Practice – heterogeneous equilibrium• Write the equilibrium expression for the
following reaction:
PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
2 2[ ][ ]eqk Pb Cl
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aA + bB cC + dD
K = [C]c[D]d
[A]a[B]b
K >> 1K << 1
Lie to the right Favor productsLie to the
leftFavor reactants
Equilibrium Will
Recall: for the reaction
What does the Kc value mean?
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Summary Writing Equilibrium Expressions
• The concentrations of the reacting species in the condensed phase are expressed in M (mol dm3) In the gaseous phase, the concentrations can be expressed in M or in atm.
• The concentrations of pure solids, pure liquids and other solvents do not appear in the equilibrium constant expressions.
• In quoting a value for the equilibrium constant, you must specify the balanced equation and the temperature.
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Le Chatelier’s Principle: if you disturb an equilibrium, it will shift to undo the disturbance.
Remember, in a system at equilibrium, the concentrations will always change to restore the same value for Kc (as long as there is a constant temperature).
7.2 Le Châtelier’s Principle
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Changes in Concentration
N2 (g) + 3 H2 (g) 2 NH3 (g)
AddNH3
Equilibrium shifts left to offset stress
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Le Châtelier’s Principle
• Changes in Concentration continued
Change Shifts the Equilibrium
Increase concentration of product(s) left
aA + bB cC + dD
Add
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Le Châtelier’s Principle
• Changes in Concentration continued
Change Shifts the Equilibrium
Increase concentration of product(s) leftDecrease concentration of product(s) right
aA + bB cC + dD
Remove
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Le Châtelier’s Principle
• Changes in Concentration continued
Change Shifts the Equilibrium
Increase concentration of product(s) leftDecrease concentration of product(s) rightIncrease concentration of reactant(s) right
aA + bB cC + dD
Add
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Le Châtelier’s Principle
• Changes in Concentration continued
Change Shifts the Equilibrium
Increase concentration of product(s) leftDecrease concentration of product(s) right
Decrease concentration of reactant(s)Increase concentration of reactant(s) right
left
aA + bB cC + dD
Remove
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Le Châtelier’s Principle
Changes in Pressure
A (g) + B (g) C (g)
Change Shifts the Equilibrium
Increase pressureDecrease pressure Side with most moles of gas
Side with fewest moles of gas
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Le Châtelier’s Principle Changes in Temperature
Change Response
Increase temperature forward reaction favored - remove heat
Decrease temperaturereverse reaction favored – absorb heat
colder hotter
N2O
4(g) 2 NO
2(g) ΔH is +ive
Room temperature. NO2 is brown
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Le Châtelier’s Principle Changes in Temperature
Change Response
Increase temperature forward reaction favored - remove heat
Decrease temperaturereverse reaction favored – absorb heat
colder hotter
N2O
4(g) + heat 2 NO
2(g)
Room temperature. NO2 is brown
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Le Châtelier’s Principle
ONLY changes in temperature affect Kc
Change Exothermic Rx
Increase temperature K decreasesDecrease temperature K increases
Endothermic Rx
K increasesK decreases
colder hotter
N2O
4(g) 2 NO
2(g) endothermic
Room temperature. NO2 is brown
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uncatalyzed catalyzed
Catalyst lowers Ea for both forward and reverse reactions.
Catalyst does not change equilibrium constant or shift equilibrium.
• does not change the value of Kc
• does not shift the position of an equilibrium system• system will reach equilibrium sooner
Le Châtelier’s Principle Catalysts
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Example
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Example
a) shifts left to favor products
b) water vaporizes – shift right to favor reactants
c) shifts right to favor reactants
d) no effect (solids not part of equilibrium expression)
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Chemistry In Action: The Haber Process – see fact sheet and video
N2 (g) + 3H2 (g) 2NH3 (g) ΔH0 = -92.6 kJ/mol
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Le Châtelier’s Principle (summary)
Change Shift EquilibriumChange Equilibrium
Constant
Concentration yes no
Pressure (g) yes no
Volume (g) yes no
Temperature yes yes
Catalyst no no