Transcript
Topic 7.2
IB Chemistry Power Points
Topic 7
Equilibriumwww.pedagogics.ca
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]?
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
N2O4 (g) 2NO2 (g)
Start with NO2 Start with N2O4 Start with NO2 & N2O4
equilibrium
equilibrium
equilibrium
3 scenarios for the reactionsee simulation
= 4.63 x 10-3KC = [NO2]2
[N2O4]
Data for these (and other) scenarios might look like
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
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
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
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)
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
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
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.
[CO2] = Kc
CaCO3 (s) CaO (s) + CO2 (g)
[CO2]does not depend on the amount of CaCO3 or CaO
Practice – heterogeneous equilibrium• Write the equilibrium expression for the
following reaction:
PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
Practice – heterogeneous equilibrium• Write the equilibrium expression for the
following reaction:
PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
2 2[ ][ ]eqk Pb Cl
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?
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.
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
Changes in Concentration
N2 (g) + 3 H2 (g) 2 NH3 (g)
AddNH3
Equilibrium shifts left to offset stress
Le Châtelier’s Principle
• Changes in Concentration continued
Change Shifts the Equilibrium
Increase concentration of product(s) left
aA + bB cC + dD
Add
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
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
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
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
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
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
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
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
Example
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)
Chemistry In Action: The Haber Process – see fact sheet and video
N2 (g) + 3H2 (g) 2NH3 (g) ΔH0 = -92.6 kJ/mol
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
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