1. AP Chem
Today: We’re starting the big unit of AP Chem…Equilibrium.

2. What happens when a reverse and forward reaction occur simultaneously?
You are going to simulate the forward and reverse reaction of
Simulation #1: Give the 40 paper clips to the reactant side.
Forward reaction: transfer 1/2 of reactant paper clips to product side (round up if it’s a fraction ex. 7.5=8)
Reverse reaction: transfer 1/4 of product paper clips to reactant side (round up if it’s a fraction ex. 7.5=8)

4. Questions
1. Looking at your results, after which transfer does this process (reaction) reach equilibrium?
After 3rd transfer

5. 2. Once the reaction is at equilibrium,
What happens to the concentration (# of clips) of the reactants (A)?
The concentration remains the same
What happens to the concentration (# of clips) of the products (B)?
Are the concentrations of the reactants (A) and the products (B) the same? No
How does the transfer of clips from the reactants to the products side compare to the transfer of clips from the products to the reactants side?
The number of clips transferred to each side is the same

6. 4. How many clips would be transferred on the 100th cycle?
7 paper clips to both sides

7. Dynamic Equilibrium
Equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction in a closed system.
***When equilibrium is reached, the concentration of reactants and products are constant, but they ARE NOT necessarily equal to each other

8. Dynamic Equilibrium
Equilibrium is represented by ↔ arrows instead of a single arrow. This allows us to illustrate the reactions are proceeding in both directions (forward and reverse)
Equilibrium is dynamic which means that it is constantly moving/changing (the reaction doesn’t stop just because it’s at equilibrium)

9. Types of Equilibrium:
Chemical Equilibrium: Ex: Chemical reaction in closed container that has reached equilibrium RATE of Forward Reaction = RATE of Reverse Reaction
Phase Equilibrium: Occurs during a phase change Ex: Closed container of water at 100°C. H2O(l) ↔ H2O (g): RATE of Evaporation = RATE of Condensation
Solution Equilibrium: occurs when a solution is saturated Ex: Closed container of saturated NaCl. NaCl (s) ↔ NaCl (aq): RATE of Dissolving = RATE of Crystallization

10. At Equilibrium What are Equal: Rates of forward & reverse reaction
What are Constant: concentrations of reactants & products

11. Concept Check

12. Deriving the Equilibrium Constant
Consider the following system at equilibrium:
Forward reaction: N2O4 (g)  2 NO2 (g)
Rate Law: Rate = kf [N2O4]
N2O4 (g)
2 NO2 (g)

13. Deriving the Equilibrium Constant
N2O4 (g)
2 NO2 (g)
Reverse reaction: 2 NO2 (g)  N2O4 (g)
Rate Law: Rate = kr [NO2]2

14. Deriving the Equilibrium Constant
At equilibrium,
Ratef = Rater
kf [N2O4] = kr [NO2]2
Rewriting this, it becomes kf kr
[NO2]2
[N2O4] =

15. The Equilibrium Constant
The ratio of the rate constants is a constant at that temperature, and the expression becomes
Keq = kf kr
[NO2]2
[N2O4] =

16. The Equilibrium Constant
Consider the generalized reaction
aA + bB
cC + dD
The equilibrium expression for this reaction would be
Equilibrium constant expression
Keq =
[C]c[D]d
[A]a[B]b
** the equilibrium constant expression depends only on the stoichiometry and temperature, NOT on its mechanism!!

17. Some notes about K… It does not have units
Only gases and aqueous substances are included as part of the equilibrium expression. Solids and liquids are omitted from equilibrium expressions.
There are different types of K constants depending on the reaction

18. Practice:
Write the equilibrium expression for Kc for the following reactions:
2 O3 (g) ⇌ 3 O2(g)
2 NO (g) + Cl2 (g) ⇌ 2 NOCl (g)
Ag+(aq) + 2 NH3 (aq) ⇌ Ag(NH3)2+(aq)
2 Ag (s) + Zn2+ (aq) ⇌ 2 Ag+ (aq) + Zn (s)

19. Le Chatelier’s Principle: Explains how a system at equilibrium will respond to stress
Stress=any change in concentration, temperature, or pressure put upon a system at equilibrium
When a STRESS is added to a system at equilibrium, the system will SHIFT in order to relieve that stress and reach a new equilibrium.
Shift= an increase in the rate of either the forward OR the reverse reaction

20. Consider the following reaction at equilibrium:
(more H2)

21. DECREASE
INCREASE