1. Le Châtelier’s Principle

2. His principle states:
Every change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change.
So, what does this really mean?

3. 4 important points to consider
The chemical system MUST be at equilibrium first
Some type of change is involved in a factor that affects equilibrium (we’ll discuss shortly)
The old equilibrium will rearrange itself in response to this change.
Opposite meaning that if something is increased, the change will be for something to decrease. Or the reverse is also true.

4. Modern definition
If a system at equilibrium is subjected to a stress, the equilibrium will shift in attempt to reduce the stress.
The equilibrium position changes but the equilibrium constant does not change UNLESS temperature changes

5. Factors that affect a chem. rxn at equilibrium
Concentration
Temperature
Pressure
Addition of a catalyst

6. Temperature Is the reaction exothermic (-ΔH) or endothermic (+ΔH)?
If exo: increase in temp will make rxn proceed to left (reactants)
If endo: increase in temp will make rxn proceed to right (products)
Increasing temperature INCREASES reaction rate
Changing temp will change Kc

7. Concentration Look at reactants/products
If a [reactant] increases, rxn shifts towards product
If a [product] increases, rxn shifts towards reactants
You can change concentration by
Adding (or removing) a reactant/ product
Adding something that reacts completely with reactant/ product (like something that will cause a precipitate to form)

8. Pressure Check # of moles of GAS Pressure changes: Volume
Decrease in volume increases pressure, cause rxn to proceed towards side with FEWER moles of gas
Decreasing volume increases rxn rate for both forward and reverse rxn
Changing volume does NOT change Kc
Adding inert gas
Changing pressure does NOT change rxn rate
Changing pressure does NOT change Kc

9. Catalyst Speeds up rate of reaction by lowering Ea
NO EFFECT on equilibrium position
NO EFFECT on Kc

10. Examples For the reaction: N2 +3H2 ↔2 NH3
Which way will the equilibrium shift:
If more H2 is added
Some NH3 is removed

11. Examples For the following reaction: 2 SO2 + O2 ↔2 SO3 + heat
Which way will the equilibrium shift:
If the temperature increases?
If heat is removed (temp goes down)?

12. Examples For the following reaction: PCl3 + Cl2↔ PCl5
Which way will the equilibrium shift:
if a catalyst is added?

13. Haber Process: manufacturing ammonia
N2(g) + 3H2(g) <-> 2NH3(g)
Conditions:
High pressure required (200atm)
High pressure = higher density=more collisions
Low temp required (450C)
Exothermic rxn
Low temp = slower rxn rate, takes longer to reach equilibrium
Finely divided pieces of Iron catalyst
Overall yield: 15%

14. Haber Process Ideal situation: High pressure Intermediate temp
Iron catalyst
Return unreacted gases to mix w/ fresh

15. Contact Process: production of sulfuric acid
2SO2(g) + O2(g) <-> 2SO3(g)
Conditions:
High pressure required (2atm)
Low temp required (450C)
Vanadium oxide catalyst used
Overall yield: 99%

16. Haber Process