1. Writing and Solving Equilibrium Constant Expressions

2. Objectives:
Today I will be able to:
Write and solve equilibrium constant expressions for reactions
Formal assessment – analyzing student responses to the practice problems and closure
Informal assessment – monitoring student questions and interactions as they complete the practice problems

3. Lesson Sequence Evaluate: Warm Up Explain: Equilibrium Notes Review
Elaborate: Practice Problems
Evaluate: Closure

4. Warm Up

5. Objectives Today I will be able to:
Write and solve equilibrium constant expressions for reactions

6. Homework
Study for Ch. 5/ 19 Exam Tomorrow

7. Agenda Warm Up Equilibrium Review Notes Equilibrium practice problems
Closure

8. Equilibrium Notes
Chapter 15

9. What is equilibrium?

10. Equilibrium is … The forward and reverse reaction rates are equal
Referred to as dynamic
Equilibrium DOES NOT mean the concentrations of reactants and products are equal

12. Writing Equilibrium Constant Expressions
Based on the Law of Mass Action
a A + b B  c C + d D
Law of Mass Action – there is a relationship between concentrations of reactants and products at equilibrium

13. What is the difference between heterogeneous equilibrium and homogeneous equilibrium?

14. Heterogeneous vs. Homogeneous
Homogeneous – all substances involved in equilibrium system are in the same phase
N2(g) + 3H2(g)  2NH3(g)
Heterogeneous – some substances in the equilibrium system are in a different phase
PbCl2(s)  Pb2+(aq) + 2Cl1- (aq)

15. What does this mean for writing equilibrium expressions?

16. In heterogeneous equilibrium…
Liquids and solids are omitted from the equilibrium constant expression
Why can they be omitted?
Think about the molarity relationship
Double mass, double the volume so the ratio stays the same

17. Two types of equilibrium constant expressions
Concentrations
Symbol = Kc
Use [ ] to represent molarities in expression
Partial Pressures
Symbol = Kp
Use ( ) to represent partial pressures in expression
Note: Kp ≠ Kc

18. What do K values mean? K >1 K = 1 K < 1
Reaction is product favored at equilibrium
K = 1
Reaction has equal concentrations of reactants and products at equilibrium
K < 1
Reaction is reactant favored at equilibrium
Think about the magnitude of the value in the reaction

19. Equilibrium Problems
Write the equilibrium expression for the following reaction
2 SO2(g) + O2(g)  2 SO3(g)
[SO3]2
[SO2]2 [O2]
_______________

20. Keq = .18 (reactant favored)
Equilibrium Problems
If M of O2, M of SO2 and M of SO3 at equilibrium, calculate Keq
[SO3]2
[SO2]2 [O2]
[.00140]2
[.0250]2 [.0172]
_______________
_______________
Keq = .18 (reactant favored)

21. Equilibrium Problems
The equilibrium constant can also be used to calculate equilibrium concentrations from initial concentrations
Use an ICE table!
To start with, we will use a generic equation
A(g) + B(g)  2 C(g)
[A] = .100 M and [B] = .100 M

22. A(g) + B(g)  2 C(g) A B 2 C Initial .100 M Change -x +2x Equilibrium
Change -x
+2x
Equilibrium
.100 – x 2x
A and B are assigned –x values because they are being reacted, while C gets a +x because products are being formed. The 2x comes from the coefficient in front of the C

23. Sometimes we are given concentrations in a reaction and need to determine if the system is at equilibrium

24. Use the Reaction Quotient (Q)
Number obtained by substituting reactant and product concentrations or partial pressures at any point during a reaction into an equilibrium constant expression
Qp or Qc

25. Comparing K and Q

26. Closure – Practice Problems
Using the 10th edition of the textbook
Equilibrium; the Equilibrium Constant 14, 15, 19, 21, 22, 24
Calculating Equilibrium Constants , 31-33
Applications of Equilibrium Constants , 41-44, 49

27. Closure – Practice Problems
Using the 15th edition of the textbook
Equilibrium; the Equilibrium Constant
15,17,21,23,25
Calculating Equilibrium Constants , 35-37
Applications of Equilibrium Constants , 41-44, 47, 49, 51