1. Le Châtelier’s Principle
CHEMICAL EQUILIBRIUM
Le Châtelier’s Principle

2. CHEMICAL EQUILIBRIUM
A state when the rate of the forward reaction is equal to the rate of the reverse reaction.
A + B  C + D
A + B  C + D
A + B ⇄ C + D

3. CHEMICAL EQUILIBRIUM
Dynamic process. Both forward and reverse reactions continue, but there is NO NET CHANGE in the amount of reactants and products.

4. CHEMICAL EQUILIBRIUM M
Time
Products
Reactants
 Equilibrium

5. CHEMICAL EQUILIBRIUM
The equilibrium point is actually a point in a reaction where a specific mixture of reactants and products has obtained a minimum energy state – the most stable state. This is a temperature dependent condition.

6. LE CHATELIER’S PRINCIPLE
When a system at equilibrium experiences a stress, the system responds to relieve that stress by changing the ratio of reactants and products. It shifts right or left!

7. STRESSORS Concentration Changes of R or P Temperature Changes
Pressure/Volume Changes

8. Concentration Changes
Concentration changes only affect reactants or products in the aqueous (aq) or gaseous (g) state – pure solids (s) or liquids (l) don’t have concentrations.
System will consume what is added or replace what is removed.

9. Example 1: Consider the following reaction:
A (g) + B (g) ↔ C (s) + D (g)
Adding more A will cause the reaction to shift to the right, the direction that consumes A.

10. Example 2: Consider the following reaction:
A (g) + B (g) ↔ C (s) + D (g)
Removing some B will cause the reaction to shift to the left, the direction that replaces B.

11. Example 3: Consider the following reaction:
A (g) + B (g) ↔ C (s) + D (g)
Adding or removing C has no effect on the equilibrium position, since it is in the solid state.

12. Example 4: Consider the following reaction:
A (g) + B (g) ↔ C (s) + D (g)
Which direction do you think it will shift if some D is removed?
Yes, that’s right! To the Right!!

13. Temperature Changes
Effect depends on whether the reaction is endothermic or exothermic.
Increasing the temperature is like adding energy.
Decreasing the temperature is like removing energy.

14. Temperature Changes
In endothermic reactions, energy is written on the reactant side:
Energy + A + B  C + D
In exothermic reactions, energy is written on the product side:
A + B  C + D + Energy

15. Example 5:
Consider the following reaction: A + B ↔ C + D + Energy Is this endothermic or exothermic? Which way will it shift if the temperature is increased?
Yes, that’s right! To the Left!!

16. Example 6:
Consider the following reaction: 25 kJ + A + B ↔ C + D Is this endothermic or exothermic? Which way will it shift if the temperature is decreased?
Yes, that’s right! To the Left!!

17. Pressure/Volume Changes
Pressure/Volume changes only affect reactants or products in the gaseous (g) state.
Systems that differ in the number of moles of gaseous reactants and products respond by shifting to a more or less compact state.

18. Pressure/Volume Changes
Consider the following reaction:
A (g) + B (g) ↔ C (s) + D (g)
How many moles of gaseous reactants/products are represented on each side of the arrow?
2 moles on left; 1 mol on right

19. Example 7: A (g) + B (g) ↔ C (s) + D (g)
Which way will it shift if the volume is decreased? (Pressure increased)
Yes, that’s right! To the Right!!

20. Factors That DO NOT Affect Equilibrium Positions
Adding a Catalyst
Adding an Inert substance, like a noble gas.

21. Affect of Catalyst
Speeds up the rate of establishing equilibrium, but will NOT affect the equilibrium position.

22. Affect of Inert Substance
Does not change any reactant or product concentration, does not change temperature, does NOT affect any individual gas pressure – thus, NO EFFECT on equilibrium position.

23. EQUILIBRIUM EXPRESSIONS
An equilibrium expression computes an equilibrium constant, Keq. This constant is the ratio of products to reactants raised to their stoichiometric coefficients once equilibrium has been established. Again, pure solids (s) and liquids (l) are OMITTED from the expression.

24. EQUILIBRIUM EXPRESSIONS
For the following reaction:
aA (g) + bB (g) ⇄ cC (s) + dD (g)
[A]a
[D]d
[B]b
Keq =

25. EQUILIBRIUM EXPRESSIONS
Meaning of K:
If K > 1 then it’s products favored. If K = 1 then it’s neither. If K < 1 then it’s reactants favored.