1. Chemical Equilibrium

2. Reversible Reactions
Most chemical reactions are able to proceed in both directions. They are reversible.
Example:
Fe3O4 (s) H2 (g) ↔ 3 Fe(s) + 4 H2O(g)
dKsYY

3. Reversible Reactions cont.
As products are produced they will react in the reverse reaction until the rates of the forward and reverse reactions are equal.
Ratefwd = Raterev
This is called chemical equilibrium.

4. Chemical Equilibrium Rateforward reaction = Ratereverse reaction
Concentration of reactants and products are constant NOT necessarily equal.
[reactants] and [products] is constant.

5. The Concept of Equilibrium
As a system approaches equilibrium, both the forward and reverse reactions are occurring.
At equilibrium, the forward and reverse reactions are proceeding at the same rate.

6. Le Chatelier’s Principle
Whenever stress is applied to a reaction at equilibrium, the reaction will shift its point of equilibrium to offset the stress.
A “shift” means that the forward or reverse direction will occur more.
Stresses include:
Temperature, pressure, changes in reactant or product concentrations

7. Example: The Haber Process
N2 (g) + 3 H2 (g)  2 NH3 (g) + heat
 [N2]
 [H2]
 [NH3]
 [NH3]
 pressure
 pressure
 temperature
 temperature
shift towards products (right)
shift towards reactants (left)
shift towards reactants (left)
shift towards products (right)
shift towards products (right)
shift towards reactants (left)
shift towards reactants (left)
shift towards products (right)

8. Equilibrium shifts due to stresses:
Concentration increase: shift away from increase
Concentration decrease: shift toward decrease
 pressure: shifts in direction of fewer gas molecules.
 pressure: shifts in direction of more gas molecules
 temperature favors endothermic reaction
Shift away from heat
 temperature favors exothermic reaction
Shift towards heat

9. The Haber Process Application of LeChatelier’s Principle
N2 (g) + 3 H2 (g)  2 NH3 (g) + 92 kJ
increase pressure
Shift 
decrease Temp
remove NH3 add N2 and H2
****Maximum yields of NH3 occurs under high pressures, low temperatures and by constantly removing NH3 and adding N2 & H2