Le Chateliers's Principle

Le Chatelier's Principle states that when a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that change.

Le Chatelier's Principle: Changes in Concentration
As4O6(s) + 6C(s) = As4(g) + 6CO(g)
If we added carbon monoxide, the equilibrium will shift away from the CO because its concentration is increased, so it will shift to the left to relieve the added stress.

If we added or removed carbon or arsenic, the equilibrium will not be affected because a pure solid does not have a concentration, so the equilibrium will not have to shift due to an increase or decrease in concentration of the solids.

If Arsenic in the gas state was removed, the equilibrium position will shift to the right, forming more products to make up for the removal of gaseous arsenic.
Le Chatelier's Principle: Changes in Volume
The principle states that the equilibrium will shift in the direction that gives the smaller number of gas molecules.

N2(g)+3H2(g) = 2NH3(g)
1 N2 and 3 H2 make 4 molecules total for the left side. There are only 2 molecules of gas to the right side.
The reaction will shift to the right to reduce the total number of gaseous molecules present.

PCl3(g) + 3NH3(g) = P(NH2)3(g) + 3HCl(g)
Since both sides of the equation have 4 molecules, the equilibrium will not shift in either direction.
Le Chatelier's Principle: The Effect of a Change in Temperature
The value of K changes along with the temperature-the principle allows us to predict the direction in the change of K.
In an exothermic reaction, heat is a product. In an endothermic reaction, the reaction absorbs heat.

Exothermic Reaction:
N2(g) + 3H2(g) = 2NH3(g) + 9

When we add energy to the system at equilibrium by eating it, the shift will be in the direction that consumes the energy, so it will shift to the left.

Endothermic Reaction:
CaCO3(s) + 556kJ = CaO(s) + CO2(g)
This reaction requires energy to make the reaction, so the equilibrium will shift to the right.

Overview of Temperature- treat the energy as a reactant or product and predict the direction of shift in the same way you would if an actual reactant or product were being added or removed.

Review: Try these.

N2O4(g) + Energy = 2NO2(g)

What would happen if you...
1. Added N2O4(g)
2. Added NO2(g)
3. Removed N2O4(g)
4. Removed NO2(g)
5. Decreased the container's volume
6. Increased the container's volume
7. Increased the Temperature
8. Decreased the Temperature

Answer Key:

The equilibrium would shift to the...
1. Right
2. Left
3. Left
4. Right
5. Left
6. Right
7. Right
8. Left