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Le Chatelier's Principle. Additional KEY Terms Use Le Chatelier’s principle to predict and explain shifts in equilibrium. Include: temperature, pressure/volume,

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Presentation on theme: "Le Chatelier's Principle. Additional KEY Terms Use Le Chatelier’s principle to predict and explain shifts in equilibrium. Include: temperature, pressure/volume,"— Presentation transcript:

1 Le Chatelier's Principle

2 Additional KEY Terms Use Le Chatelier’s principle to predict and explain shifts in equilibrium. Include: temperature, pressure/volume, reactant/product concentration, catalyst, inert gas Interpret concentration versus time graphs. Include: temp, concentration, catalyst changes. Describe practical applications of Le Chatelier’s principle.

3 Le Chatelier's Principle (1884) When a system at equilibrium is subjected to a stress, the system will adjust to relieve the stress and return to equilibrium. Remember: K c value is constant. BEFORE the stress, and AFTER the reaction adjusts.

4 Types of Stress

5 1. Concentration Stress Stress: a change in concentration of products or reactants by adding or removing. Adjustment: change in collision rate and redistribution of particles. K c value is re-established after concentrations are adjusted

6 More C means increased rate of reverse collisions K c = [C] [A][B] CBA+ K c = 1.35 We say “shifts left” We mean: Excess [C] used, [A] and [B] increase Re-establish K c Increase [C]:

7 K c = [C] [A][B] BC A+ K c = 1.35 Forward reaction is favoured We say “shifts right” We mean: Redistribute excess particles Re-establish K c Increase [B]:

8 K c = [C] [A][B] BC A+ K c = 1.35 Removing a particle is like decreasing [ ]. Decreased rate of forward reaction collisions We say “shifts left” We mean: Reverse is favoured, ↑ [reactants] Re-establish K c Decrease [A]:

9 2 NO 2 (g) N 2 O 4 (g) car exhaust smog Huge spike indicates that [ ] was changed by adding more particles.

10 2 NO 2 (g) N 2 O 4 (g) car exhaust smog A huge spike indicates that [ ] was changed by removing particles.

11 Temperature

12 2. Temperature stress Stress: a change in temperature by adding or removing heat. Adjustment: change in collision rate and redistribution of particles. *Different eqlbm at new temperature – SO…changes the K c * Exothermic: A  B (- ∆H ) Endothermic: A  B (+ ∆H) HEAT + + HEAT

13 Temperature increase / add heat Endothermic collisions (reverse) favored shifts left Temperature decrease / removing heat Exothermic collisions (forward) favored shifts right +heat AB + A B = [B] [A] = [B] [A] KcKc KcKc

14 ∆H = -58 kJ 2 NO 2 (g) N 2 O 4 (g) car exhaust smog Initial drop in ALL rates can only occur through temperature decrease.

15 ∆H = -58 kJ 2 NO 2 (g) N 2 O 4 (g) car exhaust smog Initial spike in ALL rates can only occur through temperature decrease.

16 Volume/Pressure

17 3. Volume stress Stress: a change in pressure that only affects those systems with gaseous reactants and/or products. Adjustment: change in collision rate and redistribution of particles. K c value is re-established after concentrations are adjusted

18 A + 2 B C Volume increase – (↓P ): A B B C Decreased rate of forward reaction. (fewer collisions, in larger space) Reverse rate favoured – shifts left B B A

19 A + 2 B C A B B C C Volume decrease– (↑P ): Increased rate of forward reaction. (MORE collisions, in smaller space) Forward rate favoured – shifts right

20 Which way with the system shift IF the size of the container is cut in half? Reverse reaction favoured increased likelihood of collisions in a smaller space Shifts left 2 NH 3(g) N 2(g) + 3 H 2(g)

21 No shift H 2(g) + I 2(g) 2 HI (g) Which way with the system shift IF the pressure is decreased? : 2 Pressure has NO effect on this eqlbm Same # of particles, same collision effects

22 Factors that do not affect Equilibrium Systems

23 Catalysts Lowers activation energy for both forward and reverse reaction equally. Equilibrium established more quickly, but position and ratios of concentrations will remain the same. K value remains the same.

24 Inert Gases (noble gases) Unreactive – are not part of a reaction, therefore can not affect equilibrium of a concentration-based equation. Catalysts, inert gases, pure solids or pure liquids do NOT appear in the Equilibrium Law - so they have no effect if altered.

25 Le Chatelier's AND life

26 Appliance - NO energy - forward reaction favored Energy released to run appliance. Outlet (recharge) – HIGH energy - reverse favored Reformes reactants, storing energy for use. Rechargeable Batteries Lead-acid PbO 2 + Pb + 4 H SO 4 2-  2 PbSO H 2 O + energy Nickel-cadmium Cd + 2 NiO(OH) + 2 H 2 O  2 Ni(OH) + Cd(OH) 2 + energy Electrical energy (like heat) is written in the reaction.

27 Haemoglobin protein used to transport O 2 from lungs to body tissue. Lungs - [O 2 ] is high - forward reaction favored Haemoglobin binds O 2 Tissue - [CO 2 ] is high and [O 2 ] is low - reverse reaction favored. Hb releases O 2 Hb (aq) + O 2 (g)  HbO 2 (aq) Haemoglobin AND Oxygen

28 CAN YOU / HAVE YOU? Use Le Chatelier’s principle to predict and explain shifts in equilibrium. Include: temperature, pressure/volume, reactant/product concentration, catalyst, inert gas Interpret concentration versus time graphs. Include: temp, concentration, catalyst changes. Describe practical applications of Le Chatelier’s principle.


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