1. Section 8.4—Le Chatelier’s Principle
How can we push a reaction to make more products?

2. Le Chatelier’s Principle
Le Chatelier’s Principle – when a stress is applied to a system in equilibrium – it will shift to relieve the stress.

3. What kinds of stresses can be put on a system in equilibrium?
Adding or removing a reactant or product (i.e. changing the concentration)
Increasing or decreasing the pressure
Adding or removing heat

4. Increasing or Decreasing Concentrations
Fe+3 + SCN-1 ↔ FeSCN+2
Colorless Red-Brown
This is a reversible reaction that has colorless reactants and a red-brown product. You can tell the direction of shift from the color change you witness!
If we add or remove the reactants and products what should happen?
Add some Fe The reaction should shift to use up this added Fe+3 . It will shift to the RIGHT!
Add some FeSCN The reaction should shift to use up this added FeSCN It will shift to the LEFT
Remove FeSCN The reaction should shift to create more FeSCN It will shift to the RIGHT
Remove SCN-1 - The reaction will want to make more so it will shift to the LEFT.
What color changes should happen as you add or remove these?
Notice this: When we add – it runs away and when we remove it goes back towards that substance!
Q becomes too large
There’s too many products

5. Changes in Pressure – This only matters if you have GASES in the Reaction!
Pressure increases – In the reaction the pressure comes from the gases!
Reaction shifts to the side with least moles of gas to decrease pressure
Decrease volume
What would happen if we decreased the volume for this system?
2 SO2 (g) + O2 (g)  2 SO3 (g)
It will shift to the side that decreases the pressure – The left has 3 moles of gas the right has 2 moles of gas so the shift will go Right!
Reactions shifts to the side with the most moles of gas to increase pressure
Pressure decreases
Increase volume
What would happen if we increased the volume for this system?
2 SO2 (g) + O2 (g)  2 SO3 (g)
It will shift to the side that increases the pressure – Left

7. Endo & Exothermic
Endothermic Reaction – The reaction takes in energy…it absorbs energy and the products now have more energy than the reactants
Energy is a reactant in the reaction
Exothermic Reaction – The reaction gives off energy…it releases energy and the products have less energy than the reactants
Energy is a product in the reaction

8. Changing temperature—Endothermic
Reaction shifts to right
(get rid of extra reactants and make more products)
Increase temperature of endothermic reaction
Increasing a reactant
181kJ + 2HgO ↔ 2 Hg + O2
If we increase the temperature on this system it is as if we are adding a reactant so it will shift away from the heat and GO RIGHT!
Decrease temperature of endothermic reaction
Reaction shifts to left
(make more reactants)
Remove a reactant
181kJ + 2HgO ↔ 2 Hg + O2
If we decrease the temperature on this system it is as if we are removing a reactant so it will shift towards the heat and GO LEFT!

9. Changing Temperature—Exothermic
Reaction shifts to left
(get rid of extra products and make more reactants)
Increase temperature of exothermic reaction
Increasing a product
Reaction shifts to right
(make more products)
Decrease temperature of exothermic reaction
Remove a product

10. Some changes have no effect!
Adding a pure solid or liquid reactant or product
They’re not in the equilibrium constant expression
Increasing pressure by adding an inert gas
Changing the volume of a reaction with an equal number of moles of gas on each side of the reaction
The system won’t gain anything by shifting since both sides will cause the same pressure
Adding a catalyst
A catalyst will speed up how fast equilibrium is established—but not the number of reactants and products once it’s at equilibrium

11. Which way will the reaction shift for each of the following changes:
Examples
Example:
Which way will the reaction shift for each of the following changes:
NH4Cl (s)  NH3 (g) + HCl (g)
Removing some NH4Cl
Adding HCl
Adding Ne (g)
Decreasing volume

12. Which way will the reaction shift for each of the following changes:
Examples
Example:
Which way will the reaction shift for each of the following changes:
NH4Cl (s)  NH3 (g) + HCl (g)
Removing some NH4Cl
Adding HCl
Adding Ne (g)
Decreasing volume
No change (it’s a solid)
(Adding a product)
No change (it’s an inert gas)
(Goes to side with least gas moles)

13. Which way will the reaction shift for each of the following changes:
Let’s Practice
Which way will the reaction shift for each of the following changes:
2 SO2 (g) + O2 (g)  2 SO3 (g) an exothermic reaction
Increasing volume
Raising temperature
Adding O2
Removing SO2

14. Which way will the reaction shift for each of the following changes:
Let’s Practice
Which way will the reaction shift for each of the following changes:
2 SO2 (g) + O2 (g)  2 SO3 (g) an exothermic reaction
Increasing volume
Raising temperature
Adding O2
Removing SO2
(Goes to side with most gas moles)
(Energy is a product)
(Adding a reactant)
(Removing a reactant)