1. Le Chatelier's
Principle

2. What if the conditions of the equilibrium changed?
Le Chatelier’s principle states that if a closed system at equilibrium is subjected to a change(stress), things will happen to counteract that change (regain back balance).

3. The effect of Temperature changes
Temperature affects heat terms in a reaction.
If you increase temperature, you are increasing the heat term(energy). Therefore, the reaction will shift to use up this “extra” heat/energy and go back to original state.

4. The Effect of Temperature Changes
A B ⇌ C + Energy
Change: increase Temp
reaction- shifts left

5. The Effect of Temperature Changes
A B Energy ⇌ C
Change: increase Temp
reaction- shifts right

6. The Effect of Temperature Changes
A B + Energy ⇌ C
Change: decrease Temp
reaction- shifts left

7. The Effect of Concentration Changes

8. Adding a reactant or product shifts the reaction in the direction that removes them/uses them up.
A B ⇌ C + Energy
change- increase [A]
reaction- shifts right

9. Adding a reactant or product shifts in the direction that removes them.
A B ⇌ C + Energy
stress- increase [C]
reaction- shifts left

10. Removing a reactant or product shifts reaction to make MORE.
A B ⇌ C + Energy
change- decrease [A]
reaction- shifts left

11. Removing a reactant or product shifts reaction to make more.
A B ⇌ C + Energy
change- decrease [C]
reaction- shifts right

12. Effect of pressure changes

13. A(g) + B(g) ⇄ AB(g)
2 mols mol
more pressure less pressure
Usually we refer only to gases(g) when we talk about pressure changes. Those pressure changes we control by changing the volume.

14. As you know… If you decrease the volume, you increase the pressure.
If you increase the volume, you decrease the pressure.
Also, increasing pressure increases the concentrations of ALL gaseous species found.

15. Where to start?
First identify where there is more pressure or where there is less pressure by counting up the molecules of gas you have on each side.

16. 4HCl(g) + 2O2(g) ⇄ 2H2O(l) + 2Cl2(g)
6 mols
2 mols
more pressure
less pressure
If we decreased the volume, pressure will ________ and so the reaction will go to low pressure side.

17. increasing pressure again and goes to more pressure side
2SO3(g) ⇄ SO2(g) O2(g)
2 gas
3 gas
more pressure
less pressure
If we increased the volume, pressure will _________, so the reaction responds by…
increasing pressure again and goes to more pressure side

18. 4HCl (g) + 2O2 (g) ⇄ 2H2O (l) + 2Cl2 (g) + 98kJ
Increasing the pressure by decreasing the volume shifts to the eqlm fewest gas molecules. Only (g) count for gas particles.
4HCl (g) O2 (g) ⇄ H2O (l) Cl2 (g) kJ
 We decrease the volume, we increase pressure!
The reaction responds by shifting to right to decrease the pressure!
All concentrations go up!

19. 7 9 5A(g) + 2B(g) ⇌ 4C(g) + 5D(g) We ⇓ V We ⇑ P
7 9
5A(g) B(g) ⇌ 4C(g) + 5D(g)
We ⇓ V
We ⇑ P
The Reaction opposes and goes to low pressure side⇓ P
Shifts left

20. 6 5 4A(g) + 2B(g) ⇌ 3C(g) + 2D(g) What volume change shifts right?
6 5
4A(g) B(g) ⇌ 3C(g) + 2D(g)
What volume change shifts right?
We ⇑ P
We ⇓ V

21. 4 7 2A(g) + 2B(g) ⇌ 3C(g) + 4D(g) What volume change shifts right?
4 7
2A(g) B(g) ⇌ 3C(g) + 4D(g)
What volume change shifts right?
We ⇓ P
We ⇑ V

22. 4HCl (g) + 2O2 (g) ⇄ 2H2O (l) + 2Cl2 (g) + 98kJ
Describe the shift in the above equilibrium system for each stress indicated.
Stress Shift [HCl] [O2] [Cl2]
↑ [HCl] → ↑ ↓ ↑
↓ [O2] ← ↑ ↓ ↓
Add water ― ― ― ―
↑ [Cl2] ← ↑ ↑ ↑ ↓ ↓ ↓
↑ V ←
When the volume increases, pressure decreases and all concentration go down

23. Adding a catalyst does not shift the equilibrium
Forward and reverse rates are increased by the same amount
Equilibrium is attained faster
reactants
products

24. Adding an inert (non-reactive) gas does not shift the equilibrium.
2NH3(g) ⇌ N2(g) H2(g)
Adding Ne(g)
No shift!

25. Homework
Exercises on page 54. Exercises 24-26, do NOT graph, only state what each change/stress would do to the equilibrium (ie; where it shifts it)