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Le Chatelier's Principle
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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.
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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.
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Types of Stress
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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. [Add] – system shifts to use it up. [Remove] – system shifts to make more.
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More C means increased rate of reverse reaction. K c = [C] [A][B] CBA+ K c = 1.35 We say “shifts left” We mean: Excess C used up until ratio of product to reactant concentrations is equal to K c once again. Increase [C]:
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K c = [C] [A][B] BC A+ K c = 1.35 Forward reaction is favoured We say “shifts right” We mean: New concentrations re-establish K c. Increase [B]:
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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, K c the same. Decrease [A]:
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2 NO 2 (g) N 2 O 4 (g) car exhaust smog Huge spike indicates that [ ] was changed by adding more particles.
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2 NO 2 (g) N 2 O 4 (g) car exhaust smog A huge spike indicates that [ ] was changed by removing particles.
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Temperature
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Temperature stress addressed the SAME way as concentration by changing collision rates. **Re-establishes new eqlbm (with new [ ]s) at new temperature – SO…changes the K c. Exothermic: A B (- ∆H ) Endothermic: A B (+ ∆H) HEAT + + HEAT 2. Temperature stress
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Temperature increase / add heat Reaction shifts left. Endothermic collisions (reverse) favored. Temperature decrease / removing heat Reaction shifts right. Exothermic collisions (forward) favored. +heat AB + A B = [B] [A] = [B] [A] KcKc KcKc
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∆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.
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∆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.
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Volume/Pressure
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Changing the pressure of a system only affects those equilibria with gaseous reactants and/or products. 3. Volume stress Rates of collisions change with pressure and effect all concentrations – BUT, K c will re-establish***. A + 2 B C
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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 (pressure increases with more particles) B B A
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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 (pressure reduced with fewer particles)
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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)
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Equilibrium position unchanged. H 2(g) + I 2(g) 2 HI (g) Which way with the system shift IF the pressure is decreased? 1 + 1 : 2 Pressure changes have NO effect on this eqlbm – Same # of particles, same collision effects.
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Factors (stresses) that do not affect Equilibrium Systems
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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.
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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.
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Le Chatelier's AND life
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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 + + 2 SO 4 2- 2 PbSO 4 + 2 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.
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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
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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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