1. Qualitative Changes in Equilibrium Systems
Chapter 7.4

2. Disturbing Equilibrium
A chemical equilibrium can be disturbed by changes in:
Concentration
Temperature
Pressure/Volume

3. Restoring Balance: Le Châtelier’s Principle
An equilibrium shift is a change in concentrations of reactants and products in order to restore an equilibrium state
If you somehow change a system at equilibrium, it will shift left or right to restore equilibrium
Henry Louis Le Chatelier ( )
French chemist

4. Le Chatelier’s Principle: Seesaw Analogy
Recall that at equilibrium a system is in balance;
Forward rate = reverse rate
[reactants] and [products] are constant
A chemical reaction at equilibrium is like a seesaw;
If we added more reactants, the reaction will become unbalanced;
To restore equilibrium, the reaction will shift right to make more products and bring the seesaw back to balance!

5. N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol
Le Chatelier’s Principle: Changes in Concentration
For example, the Haber process:
N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol
What happens when concentration changes are made to equilibrium?
Add [N2]
Decrease [H2]
Add [NH3]
System will want to produce more NH3;
Equilibrium shifts right!
System will want to produce more H2;
Equilibrium shifts left!
System will want to produce more reactants;
Equilibrium shifts left!

6. Le Chatelier’s Principle: Changes in Temperature
The Haber process again:
N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol
What happens when temperature changes are made to equilibrium?
Increase temperature
Decrease temperature
Rewrite to include heat in reaction. Since it is exothermic:
N2(g) + 3H2(g) ↔ 2NH3(g) kJ/mol
System wants to absorb extra heat;
Since reverse reaction absorbs heat (endothermic)
Equilibrium shifts left!
System wants to release more heat;
Since forward reaction releases heat (exothermic)
Equilibrium shifts right!

7. Le Chatelier’s Principle: Changes in Pressure
Changes in pressure only apply to reactions involving gases
The Haber process again:
N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol
What happens when pressure changes are made to equilibrium?
Increase pressure
Decrease pressure
What if there are equal number of moles on left and right?
Look at number of moles on each side of reaction.
1N2(g) + 3H2(g) ↔ 2NH3(g)
∴ 4 molecules ↔ 2 molecules
System wants to lower the pressure;
Since fewer molecules reduces pressure
Equilibrium shifts right!
System wants to increase the pressure;
Since more molecules increases pressure
Equilibrium shifts left!
The system cannot move in any way to reduce the pressure again! Therefore, there is no shift!

8. Increasing the temperature Decreasing the pressure Adding more O2
Practice: Predict the effect of the following changes on the following reaction
Increasing the temperature
Decreasing the pressure
Adding more O2
Removing SO2
Adding SO3
Decreasing the container volume

9. Analyzing Concentration vs Time Graphs
At equilibrium, more CO2 added.
System responds by shifting right; thus the concentration of products increases
At equilibrium, CO2 removed.
System responds by shifting left; thus the concentration of products decreases

10. Analyzing Concentration vs Time Graphs
At equilibrium, temperature is decreased.
For this exothermic reaction, system responds by shifting right to restore the energy; thus the concentration of products increases
At equilibrium, temperature is decreased.
For this endothermic reaction, system responds by shifting left to restore the energy; thus the concentration of reactants increases

11. Factors that Do Not Affect Equilibrium Position
Catalysts
In a reversible reaction, catalysts increase forward and reverse rates by following the lower energy path
However, it does not change ∆H so the overall energy change is not affected
As a result, the position of equilibrium is not affected

12. Factors that Do Not Affect Equilibrium Position
2. Inert Gas
Inert gases are not reactive and will not enter a chemical reaction
Adding an inert gas will result in more collisions but since those collisions do not result in reactions, the actual number and frequency of collisions between reactants and products stays the same
Therefore, adding an inert gas does not affect equilibrium position

13. Factors that Do Not Affect Equilibrium Position
3. State of Reactants
Equilibrium is only affected by changes in concentration of entities that are in the same state of matter
For example: H2(g) + I2(s) ↔ 2HI(g)
Adding more solid iodine DOES NOT change the concentration, therefore it has NO EFFECT on equilibrium!

14. Homework Watch the following video: Questions p. 446 #1-4
Le Chatelier's Principle Explained
Questions p. 446 #1-4
Assignment (Thurs and Friday)