Le Chatelier's Principle

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Le Chatelier's Principle

Use Le Chatelier’s Principle to explain the effects on the position of a system at equilibrium from:
Changing concentration Changing temperature Changing pressure Adding a catalyst

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

Types of Stress

1. Concentration stress [increase] [decrease]
Any change in concentration of products or reactants from equilibrium values. [increase] System will react by consuming the excess. [decrease] System will react by producing to replace the loss.

Kc = [C] [A][B] A + B C Kc = 1.35 Increase [C] - the reaction shifts left to use it up. More C means increased rate of reverse reaction. Excess C used up until ratio of product to reactant concentrations is equal to Kc once again.

Kc = [C] [A][B] A + B C Kc = 1.35 Increase [B] - the reaction shifts right to use it up. Forward reaction rate increases (favored) and Kc is reestablished. Removing a particle is like decreasing [ ]. Reaction will shift to replace the loss.

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.

2 NO2 (g) N2O4 (g) car exhaust smog

Temperature

2. Temperature stress The system relieves the stress by either replacing lost heat or consuming added heat. Consider heat a component of the system: Exothermic A  B (- ∆H ) Endothermic A  B (+ ∆H) + HEAT HEAT + Reaction reestablishes new eqlbm (with new [ ]s) at new temperature – BUT also changes the Kc.

+ A B heat + A B heat Kc = Kc = [B] [A] [B] [A]
Temperature increase / add heat Reaction shifts left to use up the heat. Endothermic reaction (reverse) favored. + A B heat Kc = [B] [A] Temperature decrease / removing heat Reaction shifts right to produce more heat. Exothermic reaction (forward) favored.

2 NO2 (g) N2O4 (g) car exhaust smog ∆H = -58 kJ

2 NO2 (g) N2O4 (g) car exhaust smog ∆H = -58 kJ

Volume/Pressure

3. Volume stress Changing the pressure of a system only affects those equilibria with gaseous reactants and/or products. Shifts to compensate pressure changes will effect all concentrations – BUT, Kc value will return as equilibrium reestablishes. A B  C

B A C B A + 2 B C C Volume decrease – (↑P )
Reaction shifts to decrease total number of gas particles – reduces pressure.

B A A + 2 B C B B A B C Volume increase– (↓P )
Reaction shifts to increase total number of gas particles – increase pressure.

2 NH3(g) N2(g) + 3 H2(g) 1. IF the size of the container is cut in half? Increases pressure  reduce pressure by reducing the number of molecules in the container. Reverse reaction favoured. The equilibrium shifts left.

2 NO2 (g) N2O4 (g) 2. IF the reaction chamber is increased in volume?
car exhaust smog 2. IF the reaction chamber is increased in volume? Increasing volume, reduces pressure  increase pressure by increasing number of molecules in container. Reverse reaction is favoured. The equilibrium shifts left.

H2(g) + I2(g) 2 HI(g) : 3. Increase container OR increase the pressure? Pressure changes have NO effect on this eqlbm – same # of particles regardless of shift (stoich). Therefore, in response to pressure changes, the equilibrium position remains unchanged.

Factors (stresses) 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 [ ], pressure or volume of a equilibrium system. Catalysts, inert gases, pure solids or pure liquids do NOT appear in the mass action expression - so they cannot have an effect if altered.

Le Chatelier's AND life

Hb (aq) + O2 (g)  HbO2 (aq)
Haemoglobin Production and Altitude 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 to the excess O2. Tissue - [CO2] is high and [O2] is low - reverse reaction favored. Hb releases O2.

Hb (aq) + O2 (g)  HbO2 (aq)
High altitudes - [O2] is very low - reverse reaction favored. Hb release O2, fewer Hb bind oxygen. Result in exaggerated lack of oxygen to the tissues, resulting in headache, nausea and fatigue. Over time, body adjusts by producing more haemoglobin molecules. Increases [Hb] in the blood stream shifts equilibrium right - more O2 bound and transported to the tissue.

Rechargable 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 release to run appliance. Outlet (recharge) - high energy - reverse favored Reforming the reactants, storing the energy for use.

THE HABER PROCESS

N2(g) + 3H2(g) 2NH3(g) ΔH = kJ mol-1 high pressure medium temperature - catayst remove ammonia high reactant concentrations