Le Châtelier’s Principle

Le Châtelier’s Principle1. System starts at equilibrium.2. A change/stress is then made to system at

equilibrium.• Change in concentration• Change in volume• Change in pressure• Change in Temperature• Add Catalyst

3. System responds by shifting to reactant or product side to restore equilibrium.

If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.

Le Châtelier’s Principle

• Changes in Concentration

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

AddNH3

Equilibrium shifts left to offset stress

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Le Châtelier’s Principle

• Changes in Concentration continued

Change Shifts the EquilibriumIncrease concentration of product(s) leftDecrease concentration of product(s) right

Decrease concentration of reactant(s)Increase concentration of reactant(s) right

left14.5

aA + bB cC + dD

AddAddRemove Remove

Changes in pressure

• Pressure is caused by gas molecules hitting the sides of their container.

• The more molecules you have in the container, the higher the pressure will be.

• Pressure can increase or decrease by changing the volume of the container

• Decreasing the volume of the container _______ the pressure; increasing the volume _____ the pressure

• The system can reduce the pressure by reacting in such a way as to produce fewer molecules.

2NO2(g) ↔ N2O4(g)

Brown colourless

Chemistry; The Science in Context;by Thomas R. Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 752

Le Châtelier’s Principle

• Changes in Volume and Pressure

A (g) + B (g) C (g)

Change Shifts the EquilibriumIncrease pressure Side with fewest moles of gasDecrease pressure Side with most moles of gas

Decrease volumeIncrease volume Side with most moles of gas

Side with fewest moles of gas

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Changes in temperatureIf we consider heat as a product in the following reaction:

A + B C + heatand increasing the temperature of a system as increasing “concentration” of that product, then increasing the temperature will shift the equilibrium to the ________ and decreasing the temperature will shift the equilibrium to the _______.What happens if the heat term is on the reactant side?

Le Châtelier’s Principle; Temperature Effect

Chemistry; The Science in Context;by Thomas R. Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 764

Keq is temperature dependent• What effect does an increase in temperature have on

Keq? • It depends on which side the heat term is written.• Consider A + B C + heatIncreasing temperature causes [C] to ____ and [A] and [B] to _____ What effect does this have on Keq?What effect does a decrease in temperature have on Keq?How do these changes in temperature effect Keq for the reaction: A + B + heat C?

uncatalyzed catalyzed

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Catalyst lowers Ea for both forward and reverse reactions.

Catalyst does not change equilibrium constant or shift equilibrium.

• Adding a Catalyst• does not change K• does not shift the position of an equilibrium system• system will reach equilibrium sooner

Le Châtelier’s Principle

Le Châtelier’s Principle

Change Shift EquilibriumChange Equilibrium

Constant

Concentration yes no

Pressure yes no

Volume yes no

Temperature yes yes

Catalyst no no

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