1. Unit 7: Chemical Equilibrium
Le Châtelier’s Principle

2. Le Châtelier’s Principle
Once a reaction has reached equilibrium, if you apply a stress to the system, the system will respond to regain equilibrium
Possible stresses include changes in:
concentration
pressure
temperature

3. Equilibrium and Change in Concentration
Assume constant pressure and temperature
Increasing concentration shifts equilibrium away from the side where a substance is being added
Decreasing concentration shifts equilibrium toward the side where a substance is being removed

4. Equilibrium and Change in Concentration Example
N2 (g) + 3H2 (g) ↔ 2NH3 (g)
If we add H2 which way will equilibrium shift?
To the right – More NH3 will form and N2 will be used up (less N2)

5. Equilibrium and Change in Pressure
Assume constant concentration and temperature
Increasing pressure shifts equilibrium toward the side that has fewer gas molecules
Decreasing pressure shifts equilibrium toward the side that has more gas molecules

6. Equilibrium and Change in Pressure Example
N2 (g) + 3H2 (g) ↔ 2NH3 (g)
If we increase the pressure, which way will equilibrium shift?
To the right – More NH3 will form

7. Equilibrium and Change in Temperature
Assume constant concentration and pressure
Works just like concentration – find the side of the equation that contains the energy:
endothermic = energy on the left (reactant)
exothermic = energy on the right (product)
Increased temperature = increased energy
Decreased temperature = decreased energy

8. Equilibrium and Change in Temperature Example
N2 (g) + 3H2 (g) ↔ 2NH3 (g) + 92kJ (exothermic)
If we heat the container, which way will equilibrium shift?
To the left – LESS NH3 will form

9. Equilibrium Shift Practice
S8 (g) + 12O2 (g)  8 SO3 (g) kcals
Determine if equilibrium will shift to the left or to the right if…
Oxygen gas is added?
Sulfur trioxide is removed?
The reaction vessel is cooled?
The pressure is decreased?
Give students two different colored pieces of paper and have them “vote”

10. Equilibrium Shift Practice
2NaHCO3 (s)  Na2CO3 (s) + H2O (g) + CO2 (g)
Determine if equilibrium will shift to the left or to the right if…
Carbon dioxide was removed?
Pressure is increased?
Na2CO3 was added?
NaHCO3 was removed?
STOPPED HERE and had students do the first four columns of the WS

11. Equilibrium and Change in Temperature
Keq will ONLY change if the temperature changes
Remember: Keq is roughly
A temperature change that shifts equilibrium to the right (products) will increase Keq
A temperature change that shifts equilibrium to the left (reactants) will decrease Keq

13. Equilibrium Visualizations

14. Equilibrium Book Problems
Read Ch. 15 (some parts we won’t cover)
Assigned: 15.2, 15.4, 15.6, 15.9, 15.16, 15.52