1. Chemical Equilibrium & Le ChÂtelier’s Principle

2. Reversible Reactions
Reversible reaction – a reaction in which the conversion of reactants to products and the conversion of products to reactants occurs simultaneously
Most reactions are reversible

3. Reversible Reaction
As the reaction proceeds, you begin with none of the product, SO3
As soon as you create any of the product, the product begins to decompose back to the reactants, SO2 & O2

4. Chemical Equilibrium
Chemical equilibrium – when the rates of the forward and reverse reactions are equal
Occurs in a closed system at a constant temperature
No net change occurs in the actual amounts of the components of the system
Can be established by starting with
reactants only, products only, or
equal amount of products and
reactants

5. Chemical Equilibrium Have both reactions stopped? Dynamic
NO!
Dynamic
Both the forward and reverse reactions continue!!
Forward and reverse reaction rates are equal
Concentrations remain constant
Not necessarily equal

6. Chemical Equilibrium

7. Le ChÂTelier’s Principle
Le Châtelier’s Principle – If a stress is applied to a system at dynamic equilibrium, the system changes in a way that relieves the stress
Applies to chemical reactions at equilibrium
Stresses:
Concentration of reactants & products
Changes in temperature
Changes in pressure

8. Concentration
Changing the concentration of any reactant or product at equilibrium disturbs equilibrium
If you increase the concentration of a reactant, the equilibrium shifts to the products
If you decrease the concentration of a reactant, the equilibrium shifts to the reactants
If you increase the concentration of a product, the equilibrium shifts to the reactants
If you decrease the concentration of a product, the equilibrium shifts to the products

9. Concentration H2CO3 CO2 + H2O At Equilibrium:
Add H2CO3 (reactant), need to make more products
H2CO3
CO2 + H2O
CO2 + H2O
H2CO3
H2CO3
H2CO3
H2CO3
H2CO3
CO2 + H2O
CO2 + H2O

10. Pressure CO(g) + 3H2(g) CH4(g) + H2O(g) Reaction containing all gases
If you increase pressure, the volume available for the reaction decreases
Le Châtelier’s Principle states that the reaction will shift to produce the fewest moles of gas to alleviate the pressure
Pressure – the number of gas particles colliding with the walls of the container

11. Pressure N2(g) + 3H2(g) 2NH3(g) 4 moles gas 2 moles gas
Increase pressure:
Equilibrium shifts to the right
Fewer moles of gas
Relieves pressure

12. Temperature
Every chemical reaction is either exothermic or endothermic
-ΔH Forward reaction: exothermic (heat is a product)
Reverse reaction: endothermic (heat is a reactant)
+ΔH Forward reaction: endothermic (heat is a reactant)
Reverse reaction: exothermic (heat is a product)
Le Châtelier’s Principle states that if heat is added to an equilibrium system, the equilibrium shifts in the direction in which heat is used up
If heat is taken away from a system, equilibrium shifts in the direction in which heat is produced

13. Temperature Exothermic Reaction Equilibrium shifts to the left:
CO(g) + 3H2(g) CH4(g) + H2O(g) + heat
Equilibrium shifts to the right:
Endothermic Reaction
Equilibrium shifts to the right:
heat + N2O4(g) NO2(g)
Equilibrium shifts to the left:
heat
Raise the
temperature
heat
Raise the
temperature
Lower the
temperature
Lower the
temperature

14. Temperature

15. Summary