Le Chatelier's Principle

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Le Chatelier's Principle

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. Additional KEY Terms

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: Kc value is constant. BEFORE the stress, and AFTER the reaction adjusts.

Types of Stress

Kc value is re-established after concentrations are adjusted
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. Kc value is re-established after concentrations are adjusted

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

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

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

Huge spike indicates that [ ] was changed by adding more particles.
2 NO2 (g) N2O4 (g) car exhaust smog Huge spike indicates that [ ] was changed by adding more particles.

A huge spike indicates that [ ] was changed by removing particles.
2 NO2 (g) N2O4 (g) car exhaust smog A huge spike indicates that [ ] was changed by removing particles.

Temperature

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

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

Initial drop in ALL rates can only occur through temperature decrease.
2 NO2 (g) N2O4 (g) car exhaust smog ∆H = -58 kJ Initial drop in ALL rates can only occur through temperature decrease.

2 NO2 (g) N2O4 (g) car exhaust smog ∆H = -58 kJ Initial spike in ALL rates can only occur through temperature decrease.

Volume/Pressure

Kc value is re-established after concentrations are adjusted
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. Kc value is re-established after concentrations are adjusted

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

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

2 NH3(g) N2(g) + 3 H2(g) 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

H2(g) + I2(g) 2 HI(g) : Which way with the system shift IF the pressure is decreased? Pressure has NO effect on this eqlbm Same # of particles, same collision effects No shift

Factors that do not affect Equilibrium Systems

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.

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.

Le Chatelier's AND life

Rechargeable Batteries
Electrical energy (like heat) is written in the reaction. Lead-acid PbO2 + Pb H SO42-  2 PbSO H2O + energy Nickel-cadmium Cd NiO(OH) H2O  2 Ni(OH) + Cd(OH)2 + energy Appliance - NO energy - forward reaction favored Energy released to run appliance. Outlet (recharge) – HIGH energy - reverse favored Reformes reactants, storing energy for use.

Haemoglobin AND Oxygen
Hb (aq) + O2 (g)  HbO2 (aq) Haemoglobin protein used to transport O2 from lungs to body tissue. Lungs - [O2] is high - forward reaction favored Haemoglobin binds O2 Tissue - [CO2] is high and [O2] is low - reverse reaction favored. Hb releases O2

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.