1. Le Chatelier’s Principle

2. Reversible Reactions
Thus far, we have considered only one-way reactions:
A + B → C + D
Some reactions are reversible: a chemical reaction in which the products can react to re-from the reactants
They go forward (“to the right”) : A + B → C + D
and backwards (“to the left”) : A + B ← C + D
Written with a two-way arrow:
A + B ↔ C + D

3. Examples of reversible reactions:
Melting & Freezing
Recharging a rechargeable battery
Examples of irreversible reactions:
Burning paper
Dropping an egg

4. Chemical Equilibrium A + B ↔ C + D + + + + + +
For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established.
A + B ↔ C + D A B + A B + A B + C D + C D + C D +

5. Le Chatelier’s Principle
When a stress is applied to a system (i.e. reactants and products) at equilibrium, the system responds to relieve the stress.
The system shifts in the direction of the reaction that is favored by the stress.
A stress is a change in:
Concentration
Temperature (ENERGY)
Volume
Pressure

6. Shifts
If you INCREASE volume, concentration or temperature from the REACTANT side, there will be a shift RIGHT towards the products.
If you DECREASE volume, concentration or temperature from the REACTANT side, there will be a shift LEFT towards the reactants.
*** Opposite is true for compounds on the products side***
If you INCREASE pressure, it shifts to the side with the least amount of moles.
If you DECREASE pressure, it shifts to the side with the most amount of moles.
If a catalyst is present NO shift happens

7. Example CO (g) + 2 H2 (g)  CH3OH (g) + energy
Methanol, CH3OH, can be manufactured using the following equilibrium reaction:
CO (g) + 2 H2 (g)  CH3OH (g) + energy
Predict the direction that the reaction will shift for each of the following stresses.
Decrease in temperature
Increase in pressure
Add more concentration of H2
Add a catalyst
Decrease volume CO