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# 2012 topic 7.2

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### 2012 topic 7.2

1. 1. IB Chemistry Power Points Topic 7 Equilibriumwww.pedagogics.ca Topic 7.2
2. 2. The Position of EquilibriumConsider A +A B B+D C A reacts with B in a sealed flask. What happens to [A], [B], [C], and [D]?
3. 3. The Position of Equilibrium A +A B B+D CAs 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.
5. 5. Data for these (and other) scenarios might look like [NO2]2 KC = = 4.63 x 10-3 [N2O4]
6. 6. 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
7. 7. The Equilibrium Constantthe 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. 8. Homogenous equilibrium applies to reactions inwhich 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. 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. 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. 11. Practice:The equilibrium concentrations for the reaction betweencarbon 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. 12. Heterogenous equilibrium applies to reactions in whichreactants 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. 13. CaCO3 (s) CaO (s) + CO2 (g) [CO2] = Kc[CO2] does not depend on the amount of CaCO3 or CaO
14. 14. Practice – heterogeneous equilibrium• Write the equilibrium expression for the following reaction: PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
15. 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. 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. 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. 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. 19. Changes in ConcentrationN2 (g) + 3 H2 (g) 2 NH3 (g)Equilibrium Addshifts left to NH3offset stress
20. 20. Le Châtelier’s Principle• Changes in Concentration continued Add aA + bB cC + dD Change Shifts the EquilibriumIncrease concentration of product(s) left
21. 21. Le Châtelier’s Principle• Changes in Concentration continued Remove aA + bB cC + dD Change Shifts the EquilibriumIncrease concentration of product(s) leftDecrease concentration of product(s) right
22. 22. Le Châtelier’s Principle• Changes in Concentration continued Add aA + bB cC + dD Change Shifts the EquilibriumIncrease concentration of product(s) leftDecrease concentration of product(s) rightIncrease concentration of reactant(s) right
23. 23. Le Châtelier’s Principle• Changes in Concentration continued Remove aA + bB cC + dD Change Shifts the EquilibriumIncrease concentration of product(s) leftDecrease concentration of product(s) rightIncrease concentration of reactant(s) rightDecrease concentration of reactant(s) left
24. 24. Le Châtelier’s PrincipleChanges 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. 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 heatRoom temperature. NO2 is brown colder hotter
26. 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 heatRoom temperature. NO2 is brown colder hotter
27. 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 decreasesRoom temperature. NO2 is brown colder hotter
28. 28. Le Châtelier’s PrincipleCatalysts • 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. 29. Example
30. 30. Examplea) shifts left to favor productsb) water vaporizes – shift right to favor reactantsc) shifts right to favor reactantsd) no effect (solids not part of equilibrium expression)
31. 31. Chemistry In Action: The Haber Process – see fact sheet and video N2 (g) + 3H2 (g) 2NH3 (g) ΔH0 = -92.6 kJ/mol
32. 32. Le Châtelier’s Principle (summary) Change Equilibrium Change Shift Equilibrium ConstantConcentration yes no Pressure (g) yes no Volume (g) yes noTemperature yes yes Catalyst no no