1. Le Châtelier’s Principle
System starts at equilibrium.
A change/stress is then made to system at equilibrium.
Change in concentration
Change in volume
Change in pressure
Change in Temperature
Add Catalyst
System responds by shifting to reactant or product side to restore equilibrium.

2. Le Châtelier’s Principle
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.
Changes in Concentration
N2 (g) + 3H2 (g) NH3 (g)
Add
NH3
Equilibrium shifts left to offset stress
14.5

3. Le Châtelier’s Principle
Changes in Concentration continued
Remove
Add
Add
Remove
aA + bB cC + dD
Change
Shifts the Equilibrium
Increase concentration of product(s)
left
Decrease concentration of product(s)
right
Increase concentration of reactant(s)
right
Decrease concentration of reactant(s)
left
14.5

4. 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.

5. 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

6. Le Châtelier’s Principle
Changes in Volume and Pressure
A (g) + B (g) C (g)
Change
Shifts the Equilibrium
Increase pressure
Side with fewest moles of gas
Decrease pressure
Side with most moles of gas
Increase volume
Side with most moles of gas
Decrease volume
Side with fewest moles of gas
14.5

7. Changes in temperature
If we consider heat as a product in the following reaction: A + B  C + heat and 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?

8. 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

9. 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 + heat
Increasing 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?

10. Le Châtelier’s Principle
Adding a Catalyst
does not change K
does not shift the position of an equilibrium system
system will reach equilibrium sooner
uncatalyzed
catalyzed
Catalyst lowers Ea for both forward and reverse reactions.
Catalyst does not change equilibrium constant or shift equilibrium.
14.5

11. Le Châtelier’s Principle
Change Equilibrium
Constant
Change
Shift Equilibrium
Concentration
yes no
Pressure
yes no
Volume
yes no
Temperature
yes
yes
Catalyst no no
14.5