2012 topic 7.2
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Shifts equilibrium to counteract stress and restore original Kc value. Does not change position of equilibrium or value of Kc. Lowers activation energy of both forward and reverse reactions, allowing equilibrium to be established more quickly.
![The Position of Equilibrium
Consider
A +A
B B+D
C
A reacts with B in a sealed flask.
What happens to [A], [B], [C], and [D]?](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-2-320.jpg)
![The Position of Equilibrium
A +A
B B+D
C
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.](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-3-320.jpg)
![Data for these (and other) scenarios might look like
[NO2]2
KC = = 4.63 x 10-3
[N2O4]](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-5-320.jpg)
![The Equilibrium Constant
the equilibrium constant expression is
c d
[C ] [D]
KC a b
[ A] [B]
where Kc is the equilibrium constant
and is dependent on temperature.](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-7-320.jpg)
![Homogenous equilibrium applies to reactions in
which all reacting species are in the same phase.
N2O4 (g) 2NO2 (g)
[NO2]2
Kc =
[N2O4]
CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)
[CH3COO-][H3O+]
Kc‘ = [H2O] = does not change
[CH3COOH][H2O] significantly so is
omitted
[CH3COO-][H3O+]
Kc =
[CH3COOH]](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-8-320.jpg)
![Practice – homogeneous equilibrium
• Write the equilibrium expressions for the
following reactions:
H2 (g) + I2 (g) Ý 2 HI (g)
2
[ HI ]
keq
[ H 2 ][ I 2 ]
HF (aq) Ý H+ (aq) + F- (aq)
[ H ][ F ]
keq
[ HF ]](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-10-320.jpg)
![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](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-11-320.jpg)
![Heterogenous equilibrium applies to reactions in which
reactants and products are in different phases.
CaCO3 (s) CaO (s) + CO2 (g)
[CaO][CO2] [CaCO3] = constant
Kc‘ = [CaO] = constant
[CaCO3]
Kc = [CO2]
The concentration of solids and pure liquids do not
change hence they are not included in the expression
for the equilibrium constant.](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-12-320.jpg)
![CaCO3 (s) CaO (s) + CO2 (g)
[CO2] = Kc
[CO2] does not depend on the amount of CaCO3 or CaO](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-13-320.jpg)
![Practice – heterogeneous equilibrium
• Write the equilibrium expression for the
following reaction:
PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
keq [Pb2 ][Cl ]2](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-15-320.jpg)
![What does the Kc value mean?
Recall: for the reaction
aA + bB cC + dD
[C]c[D]d
K= a b
[A] [B]
Equilibrium Will
K >> 1 Lie to the right Favor products
K << 1 Lie to the left Favor reactants](https://image.slidesharecdn.com/2012topic7-2-120426063428-phpapp01/85/2012-topic-7-2-16-320.jpg)
2012 topic 7.2
- 1. IB Chemistry Power Points Topic 7 Equilibrium www.pedagogics.ca Topic 7.2
- 2. The Position of Equilibrium Consider A +A B B+D C A reacts with B in a sealed flask. What happens to [A], [B], [C], and [D]?
- 3. The Position of Equilibrium A +A B B+D C 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.
- 4. see simulation 3 scenarios for the reaction N2O4 (g) 2NO2 (g) equilibrium equilibrium equilibrium Start with NO2 Start with N2O4 Start with NO2 & N2O4
- 5. Data for these (and other) scenarios might look like [NO2]2 KC = = 4.63 x 10-3 [N2O4]
- 6. The Equilibrium Constant BIG 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
- 7. The Equilibrium Constant the equilibrium constant expression is c d [C ] [D] KC a b [ A] [B] where Kc is the equilibrium constant and is dependent on temperature.
- 8. Homogenous equilibrium applies to reactions in which all reacting species are in the same phase. N2O4 (g) 2NO2 (g) [NO2]2 Kc = [N2O4] CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq) [CH3COO-][H3O+] Kc‘ = [H2O] = does not change [CH3COOH][H2O] significantly so is omitted [CH3COO-][H3O+] Kc = [CH3COOH]
- 9. 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)
- 10. Practice – homogeneous equilibrium • Write the equilibrium expressions for the following reactions: H2 (g) + I2 (g) Ý 2 HI (g) 2 [ HI ] keq [ H 2 ][ I 2 ] HF (aq) Ý H+ (aq) + F- (aq) [ H ][ F ] keq [ HF ]
- 11. 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
- 12. Heterogenous equilibrium applies to reactions in which reactants and products are in different phases. CaCO3 (s) CaO (s) + CO2 (g) [CaO][CO2] [CaCO3] = constant Kc‘ = [CaO] = constant [CaCO3] Kc = [CO2] The concentration of solids and pure liquids do not change hence they are not included in the expression for the equilibrium constant.
- 13. CaCO3 (s) CaO (s) + CO2 (g) [CO2] = Kc [CO2] does not depend on the amount of CaCO3 or CaO
- 14. Practice – heterogeneous equilibrium • Write the equilibrium expression for the following reaction: PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
- 15. Practice – heterogeneous equilibrium • Write the equilibrium expression for the following reaction: PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq) keq [Pb2 ][Cl ]2
- 16. What does the Kc value mean? Recall: for the reaction aA + bB cC + dD [C]c[D]d K= a b [A] [B] Equilibrium Will K >> 1 Lie to the right Favor products K << 1 Lie to the left Favor reactants
- 17. 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.
- 18. 7.2 Le Châtelier’s Principle 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).
- 19. Changes in Concentration N2 (g) + 3 H2 (g) 2 NH3 (g) Equilibrium Add shifts left to NH3 offset stress
- 20. Le Châtelier’s Principle • Changes in Concentration continued Add aA + bB cC + dD Change Shifts the Equilibrium Increase concentration of product(s) left
- 21. Le Châtelier’s Principle • Changes in Concentration continued Remove aA + bB cC + dD Change Shifts the Equilibrium Increase concentration of product(s) left Decrease concentration of product(s) right
- 22. Le Châtelier’s Principle • Changes in Concentration continued Add aA + bB cC + dD Change Shifts the Equilibrium Increase concentration of product(s) left Decrease concentration of product(s) right Increase concentration of reactant(s) right
- 23. Le Châtelier’s Principle • Changes in Concentration continued Remove aA + bB cC + dD Change Shifts the Equilibrium Increase concentration of product(s) left Decrease concentration of product(s) right Increase concentration of reactant(s) right Decrease concentration of reactant(s) left
- 24. Le Châtelier’s Principle Changes in Pressure A (g) + B (g) C (g) Change Shifts the Equilibrium Decrease pressure Side with most moles of gas Increase pressure Side with fewest moles of gas
- 25. Le Châtelier’s Principle Changes in Temperature N2O4(g) 2 NO2(g) ΔH is +ive Change Response Increase temperature forward reaction favored - remove heat Decrease temperature reverse reaction favored – absorb heat Room temperature. NO2 is brown colder hotter
- 26. Le Châtelier’s Principle Changes in Temperature N2O4(g) + heat 2 NO2(g) Change Response Increase temperature forward reaction favored - remove heat Decrease temperature reverse reaction favored – absorb heat Room temperature. NO2 is brown colder hotter
- 27. Le Châtelier’s Principle ONLY changes in temperature affect Kc N2O4(g) 2 NO2(g) endothermic Change Exothermic Rx Endothermic Rx Increase temperature K decreases K increases Decrease temperature K increases K decreases Room temperature. NO2 is brown colder hotter
- 28. Le Châtelier’s Principle Catalysts • does not change the value of Kc • does not shift the position of an equilibrium system • system will reach equilibrium sooner uncatalyzed catalyzed Catalyst lowers Ea for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium
- 29. Example
- 30. 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)
- 31. Chemistry In Action: The Haber Process – see fact sheet and video N2 (g) + 3H2 (g) 2NH3 (g) ΔH0 = -92.6 kJ/mol
- 32. Le Châtelier’s Principle (summary) Change Equilibrium Change Shift Equilibrium Constant Concentration yes no Pressure (g) yes no Volume (g) yes no Temperature yes yes Catalyst no no