1. Anthony Gates AP Chemistry Dubuque Senior High School
Chemical Equilibrium
Anthony Gates
AP Chemistry
Dubuque Senior High School

2. Bellwork… be prepared to share your responses
Discuss with those around you, your responses to the following:
In the last unit, why was it so important to focus on the initial rate of reaction?
Do all reactions go until completion? Why?
Are all reactions final (they can’t be undone)? Why?
What would happen if molecules were people too?
Answer:

3. Chemical Reactions are Reversible
Many observable processes are reversible.
Example: include the evaporation and then condensation of water.
Chemical reactions can behave similarly.
Example: Transfer of electrons in redox reactions, and transfer of protons in acid-base reactions.
Nature prefers a sense of equilibrium… and so all reversible reactions work to reach a chemical equilibrium.
Chemical Equilibrium: the state where the concentrations of all reactants and products remain constant with time.

4. Chemical Equilibrium
When equilibrium is reached, no observable changes occur in the system.
Reactant and product molecules are present.
Concentration of all species remain constant.
The rate of the forward reaction is equal to the rate of the reverse reaction.

5. Law of Mass Action For the reaction: 𝑗𝐴+𝑘𝐵 ↔𝑙𝐶+𝑚𝐷
Where K is the Equilibrium Constant, and is unitless.

6. Equilibrium Plays Favorites
K > 1 … Equilibrium favors the product
0  K < 1 … Equilibrium favors the reactant

7. Writing an Equilibrium Expression
Write an equilibrium expression for the following reaction:
2NO2(g)  2NO(g) + O2(g)
K=???

8. Reverse Reaction… Inverse Constant
2NO2(g)  2NO(g) + O2(g)
2NO(g) + O2(g)  2NO2(g)

9. Change the Stoich, Change the Constant
2NO2(g)  2NO(g) + O2(g)
NO2(g)  NO(g) + ½O2(g)

10. Concentration
Concentration is a commentary on both the amount of something per volume of space it fits in.
Moles per liter perhaps…. Molarity (M)
PV=nRT
P=nRT/V
P=(n/V)RT
P=(C)RT

11. Equilibrium Pressure… like no way, Oh yeah bro, WAY
We can express the equilibrium constant in terms of pressure.
For the gas phase reaction:
3H2(g) + N2(g)  2NH3(g)

12. How do we relate the two???
Kp = K(RT)-2

13. Heterogeneous Equilibria
The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present.
Write the equilibrium expression for the reaction:
PCl5(s)  PCl3(l) + Cl2(g)
Pure
solid
Pure
liquid

14. Homework Pg
# 18, 25-30, 32

15. Just a reminder… You have a quiz today.
Feel free to look over your notes during announcements.
For all problems assume the reaction is at 298K, when at equilibrium.

16. jA + kB  lC + mD Reaction Quotient
Reaction Quotient is the application of the law of mass action to initial concentrations.
jA + kB  lC + mD

17. Significance of the Reaction Quotient
jA + kB  lC + mD
If Q = K, the system is at equilibrium.
If Q > K, the system shifts to the left, consuming products and forming reactants until equilibrium is achieved.
If Q < K, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved.

18. Solving for Equilibrium Concentration
Consider this reaction at some temperature:
H2O(g) + CO(g)  H2(g) + CO2(g) K = 2.0
Assume you start with 8 molecules of H2O and 6 molecules of CO. How many molecules of H2O, CO, H2, and CO2 are present at equilibrium?

19. Solving cont. H2O(g) + CO(g)  H2(g) + CO2(g) K = 2.0
Step #1: We write the law of mass action for the reaction:

20. Solving cont. K = 2.0 H2O(g) + CO(g)  H2(g) + CO2(g) Initial: 8 6
Step #2: We “ICE” the problem, beginning with the Initial concentrations
K = H2O(g) + CO(g)  H2(g) + CO2(g)
Initial: 8 6
Change: -x +x
Equilibrium:
8-x
6-x x

21. x = 4 Solving cont. K = 2.0 H2O(g) + CO(g)  H2(g) + CO2(g)
Step #3: We plug equilibrium concentrations into our equilibrium expression, and solve for x
K = H2O(g) + CO(g)  H2(g) + CO2(g)
x = 4
Equilibrium:
8-x
6-x x

22. Solving cont. K = 2.0 H2O(g) + CO(g)  H2(g) + CO2(g) Equilibrium: 8-x
Step #4: Substitute x into our equilibrium concentrations to find the actual concentrations
K = H2O(g) + CO(g)  H2(g) + CO2(g)
X = 4
Equilibrium:
8-x
6-x x
Equilibrium:
8-4=4
6-4=2 4

23. Homework
Pg 615
# 33-46

24. Le Chatelier’s Principle
If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.
These changes result in a change in the equilibrium position, but not the equilibrium constant

25. Le Chatelier’s Principle: Concentration
Increasing the concentration of a reactant causes a shift to the right.
Increasing the concentration of the product causes a shift to the left.

26. Le Chatelier’s Principle: Pressure
Change in pressure can be caused by:
Add or remove a gaseous reactant or product
Add an inert gas (one not involved in the reaction)
No change in equilibrium position
Change the volume of the system
Increase volume, shift towards side with greater number of moles
Decrease volume, shift towards side with least number of moles

27. Le Chatelier’s Principle: Pressure cont.
Change the volume of the system
Increase volume, shift towards side with greater number of moles
Decrease volume, shift towards side with least number of moles
2NO2(g)  2NO(g) + O2(g)

28. Le Chatelier’s Principle: Temperature
With a change in Temperature, the Equilibrium constant will change, but we can still predict the direction of the change.
An increase in temperature will shift towards the side that consumes energy.
A decrease in temperature will shift towards the side that produces energy.

29. Le Chatelier’s Principle: Temperature cont.
Ex. Exothermic production of ammonia from hydrogen and nitrogen gas.
𝑁 2 𝑔 +3 𝐻 2 𝑔 ↔2 𝑁𝐻 3 𝑔 +92 𝑘𝐽
Increase temp. shifts left
Decrease temp shifts right

30. Le Chatelier’s Principle in the Lab
The principle can give insight into observations in the laboratory setting.
𝑁 2 𝑔 +3 𝐻 2 𝑔 ↔2 𝑁𝐻 3 𝑔 +92 𝑘𝐽
Predict the shift in the system that might cause the following observations:
The laboratory equipment becomes cold to the touch.
A strong odor fills the room.

31. Homework
Pg. 617
#57-64

32. Announcements…
The lab will be collected at the end of the period, in case after our class discussion there are any changes you wish to make any changes.
Authorities found a man climbing in the Himalaya's who was cooled to absolute zero. He is 0k now.

33. Equilibrium Review
Equilibrium Expression:

34. Review cont. 𝑁 2 𝑔 +3 𝐻 2 𝑔 ↔2 𝑁𝐻 3 𝑔 +92 𝑘𝐽 Equilibrium Position:
𝑁 2 𝑔 +3 𝐻 2 𝑔 ↔2 𝑁𝐻 3 𝑔 +92 𝑘𝐽
Equilibrium Position:
Shifting to the right means that more products is being produced than reactants.
Shifting to the left means that more reactants is being created than products.

35. Review cont. 𝑁 2 𝑔 +3 𝐻 2 𝑔 ↔2 𝑁𝐻 3 𝑔 +92 𝑘𝐽 Le Chatelier’s Principle
𝑁 2 𝑔 +3 𝐻 2 𝑔 ↔2 𝑁𝐻 3 𝑔 +92 𝑘𝐽
Concentration
Pressure
Inert gas
Volume
Temperature