1. 14.9 Le Châtelier’s Principle: How a System at Equilibrium Responds to Disturbance
Check out this graph of the concentration versus time for the reaction N2 (g) + 3 H2 (g) <=> 2 NH3 (g)

2. Le Châtelier’s Principle
Le Châtelier’s Principle: When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance.
In what ways do you feel that equilibrium can be disturbed?
Change in concentration of a reactant or product.
If increased, shifts equilibrium away from the concentration
increase.
Changing the volume or pressure
If volume is decreased or pressure is increased (remember these two variables have an inverse relationship), the equilibrium shifts in the direction that has less moles of gas.
If an inert gas is added at a fixed volume, this has no effect on the equilibrium.
Changing the temperature
If temperature is increased, the equilibrium shifts away from the temperature increase. (Here, the equilibrium constant changes!)
A few side notes
1.) The exact opposite applies when one of the variables above is decreased
2.) If for example there are multiple reactants, and the concentration of one reactant
is increased, the other reactant(s) are decreased to produce more product.

3. Let’s Try a Few Practice Problems!!!
Consider the following reaction in chemical equilibrium: 2BrNO(g) 2NO(g) + Br2(g) What is the effect of adding additional Br2 to the reaction mixture? The concentration of Br increases and the equilibrium shifts left, producing more reactant (BrNO). The concentration of nitrogen monoxide decreases. What is the effect of adding additional BrNO? The concentration of BrNO (nitrosyl bromide) increases and the equilibrium shifts right to produce more product (nitrogen monoxide and bromine gas).

4. Let’s Try Another!!!
Consider the following reaction at chemical equilibrium: 2KClO3(s) 2KCl(s) + 3O2(g) What is the effect of decreasing the volume of the reaction mixture? Since there are more moles of gas on the product side, the reaction will shift left producing more KClO3. What if the volume was increased? The reaction would shift right. What if an inert gas was added? No effect!

5. Let’s Try One More!!!
The following reaction is exothermic: 2SO2(g) + O2(g) 2SO3(g) What is the effect of increasing the temperature of the reaction mixture? The reaction shifts left, producing more reactants. The equilibrium constant gets smaller. Decreasing the temperature? The reaction shifts right, producing more product. The value of the equilibrium constant gets larger.

6. 14.9 pg. 692 #’s 64, 66, 70 and 72 Study for Quiz Chapter 14