1. Weak Acids/ICE Boxes

2. Evaluation/Assessment:
Objective:
Today I will be able to:
Determine the concentrations of reactants and products intitially and during equilibrium for a reaction by using an ICE table
Calculate the concentrations of weak acids and bases at equilbrium
Evaluation/Assessment:
Informal assessment: monitoring student questions as they complete the practice problems
Formal assessment: analyzing responses to the practice and exit ticket
Common Core Connection
Build strong content knowledge
Reason abstractly and quantitatively
Make sense of problems and persevere in solving them

3. Lesson Sequence Evaluate: Warm – Up Explain: ICE Tables
Elaborate: ICE Table Practice
Evaluate: Exit Ticket

4. Warm - Up
Complete #2 on your Equilibrium Practice (ICE Boxes) worksheet

5. Objective Today I will be able to:
Determine the concentrations of reactants and products intitially and during equilibrium for a reaction by using an ICE table
Calculate the concentrations of weak acids and bases at equilbrium

6. Homework
Finish Practice Problems from Class

7. Agenda Warm – Up Intro to ICE Boxes Notes Ice Box Practice
Weak Acid Notes
Weak Acid Practice Problems
Exit Ticket

8. Take out your homework for last night. Let’s review #3 & #4
ICE Boxes
Take out your homework for last night. Let’s review #3 & #4

9. Weak Acids/Bases and ICE Boxes

10. Remember this next slide? …

11. Strong Acids/Bases vs. Weak Acids/Bases
Ionize almost 100% in water (react to completion)
WEAK:
Do not ionize completely, achieve an equilibrium

12. Q: How can I tell the difference between strong and weak acids?

13. Two different ways: Study the list of strong acids and bases
Look at the arrow in an acid/base balanced equation
If arrow is one direction  it is STRONG
If arrow is two directions   it is WEAK

14. Examples Strong Acid Example Weak Acid Example Strong Base Example:
HNO3(aq) + H2O(l)  H3O+(aq) + NO3-(aq)
Weak Acid Example
CH3COOH(aq) + H2O(l)  H3O+(aq) + CH3COO-(aq)
Strong Base Example:
NaOH(aq) + H2O(l)  Na+(aq) + OH-(aq)
Weak Base Example:
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)

15. Instead of using Keq we use a different constant for weak acids and bases

16. Ionization Constant of a Weak Acid
Ka = acid ionization constant
Ka varies at different temperatures
Generic example of a weak acid reaction:

17. Weak Acid Specific Example
CH3COOH(aq) + H2O(l)  H3O+(aq) + CH3COO-(aq)
Weak acids have a Ka < 1
Leads to small [H3O+]
pH of weak acids is calculated by solving the expression for [H3O+] at equilibrium and then taking the –log[H3O+]

18. Ionization Constant of a Weak Base
Kb = base ionization constant
Weak bases = Kb < 1
Leads to a small [OH-]
To find pOH take -log[OH-]

19. Weak Acid/Base Worked Examples

20. 1) Weak Acid Example HA(aq) + H2O(l)  H3O+(aq) + A-(aq)
Q: Initially, you have 1.00 M HA. Calculate the equilibrium concentrations of HA, H3O+, A-, and calculate the pH. Ka = 1.80 x 10-5
Hint: Use an ICE table!

21. Weak Acid Example Cont. HA
H3O+ A- I
1.00 C -x +x E
1.00 – x

22. Weak Acid Example Cont. Write Ka expression:
Assume x is small because Ka is very small, therefore x can be ignored (5% Rule)
Ka = 1.80 x 10-5 = [H3O+][A-] = x2
[HA] x
Ka = 1.80 x 10-5 = x2
1.00

23. Weak Acid Example Cont. Ka = 1.80 x 10-5 = x2 1.00
x = [H3O+] = [A-] = 4.20 x 10-3 M
pH = - log [H3O+] = 2.37

24. 2) Weak Base Example NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
Initially, you have a concentration of M NH3. Find the concentrations at equilibrium. Calculate the pH. Kb = 1.80 x 10-5
Hint: Use an ICE table!

25. Weak Base Example Cont.
NH3
NH4+
OH- I
0.010 C -x +x E
0.010 – x

26. Weak Base Example Cont. Write Kb expression:
Assume x is small because Kb is very small, therefore x can be ignored
Kb = 1.80 x 10-5 = [NH4][OH-] = x2
[NH3] x
Kb = 1.80 x 10-5 = x2
0.010

27. Weak Base Example Cont. Kb = 1.80 x 10-5 = x2 0.010
x = [NH4+] = [OH-] = 4.20 x 10-4 M
pOH = - log [OH-] = 3.37
pH + pOH = 14
Therefore, = 10.6 = pH

28. Weak Acid/Base Practice
Complete the practice at your desk. Whatever you do not finish will become your homework.

29. Exit Ticket
Which problems on the weak acid/base practice were the most challenging?
One thing I am confused about is ___________________________.