1. Chemical Equilibrium
Unit 8

2. Reversible Reactions
Most reactions continue until all the reactants are used up; however some reactions appear to stop after a certain period of time
This is because these reactions can proceed in both directions:
Forward: N2 + 3H2  2NH3
Reverse: N2 + 3H2  2NH3

3. Chemical Equilibrium
When the rate of the forward reaction equals the rate of the reverse reaction, the reaction is said to be in chemical equilibrium
In other words, as soon as some product is formed it reacts to produce more reactant

4. The Concept of Equilibrium
Consider the following reversible reaction:
N2O4(g) NO2(g)
Because the reaction is reversible, it has a rate for both the forward and backward reaction

5. Achieving Equilibrium
At equilibrium, the rate of the forward reaction equals the rate of the backward reaction
Neither reaction stops, they are just in dynamic equilibrium

6. The Equilibrium Constant
The equation below represents an equilibrium expression for the generic reaction:
The equilibrium constant is expressed with no units
aA + bB cC + dD
Example: N2(g) + 3H2(g) NH3(g)

7. Evaluating the Equilibrium Constant, K
Write the equilibrium expression and determine the value of the equilibrium constant for the following reaction. The equilibrium concentrations are listed below:
[SO2] = 0.44 M, [O2] = 0.22 M, [SO3] = 0.11 M
2SO3(g) SO2(g) + O2(g)

8. Equilibrium Constants
Each reaction has a certain equilibrium constant that tells you to what extent the reaction proceeds to produce product
Reactions with K values > 1 favor products
Reactions with K values < 1 favor reactants

9. Le Châtelier's Principle
Le Châtelier's principle essentially states that if a system is at equilibrium and that equilibrium is altered, the system will shift so as to reestablish equilibrium
Most common methods of altering equilibrium:
Adding reactant or product
Temperature changes

10. Addition of Reactant or Product at Equilibrium
N2(g) + 3H2(g) NH3(g)
After addition of H2:

11. Effect of Temperature on Equilibrium
Consider two different types of reactions:
Exothermic & Endothermic
Exothermic: Heat can be considered a product
Endothermic: Heat can be considered a reactant
Endothermic: Increase in T shifts reaction to the right
Exothermic: Increase in T shifts reaction to the left

13. Summary of Shifts in Equilibrium
Some sulfur trioxide is sealed in a container and allowed to equilibrate at a particular temperature. The reaction is endothermic.
SO3(g) SO2(g) + ½ O2(g)
In which direction will the reaction proceed
(a) if more SO3 is added to the system?
(b) if oxygen is removed from the system?
(c) if SO2 is added to the system?
(d) if the temperature is increased?
(e) if the temperature is decreased?