Weak Acids/ICE Boxes
 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
Lesson Sequence
•
•
•
•
Evaluate: Warm – Up
Explain: ICE Tables
Elaborate: ICE Table Practice
Evaluate: Exit Ticket
Warm - Up
• Complete #2 on your Equilibrium
Practice (ICE Boxes) worksheet
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
Homework
• Finish Practice Problems from Class
Agenda
•
•
•
•
•
•
Warm – Up
Intro to ICE Boxes Notes
Ice Box Practice
Weak Acid Notes
Weak Acid Practice Problems
Exit Ticket
ICE Boxes
Take out your homework for last
night. Let’s review #3 & #4
Weak Acids/Bases and ICE
Boxes
Remember this next slide? …
Strong Acids/Bases vs.
Weak Acids/Bases
• STRONG:
– Ionize almost 100%
in water (react to
completion)
• WEAK:
– Do not ionize
completely, achieve
an equilibrium
Q: How can I tell the
difference between strong
and weak acids?
Two different ways:
1. Study the list of strong acids and
bases
1. 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
Examples
• Strong Acid 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)
Instead of using Keq we use
a different constant for
weak acids and bases
Ionization Constant of a Weak
Acid
• Ka = acid ionization constant
– Ka varies at different temperatures
• Generic example of a weak acid
reaction:
Weak Acid Specific Example
• CH3COOH(aq) + H2O(l)  H3O+(aq) + CH3COO-(aq)

–
[H3O ][CH3COO ]
Ka 
[CH3COOH]
• 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+]
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-]
Weak Acid/Base Worked
Examples
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!
Weak Acid Example Cont.
HA
H3O+
A-
I
1.00
0
0
C
-x
+x
+x
E
1.00 – x
+x
+x
Weak Acid Example Cont.
• Write Ka expression:
Ka = 1.80 x 10-5 = [H3O+][A-] = x2
[HA]
1.00-x
• Assume x is small because Ka is very
small, therefore x can be ignored (5%
Rule)
Ka = 1.80 x 10-5 = x2
1.00
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
2) Weak Base Example
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
• Initially, you have a concentration of
0.010 M NH3. Find the concentrations
at equilibrium. Calculate the pH. Kb =
1.80 x 10-5
– Hint: Use an ICE table!
Weak Base Example Cont.
NH3
NH4+
OH-
I
0.010
0
0
C
-x
+x
+x
E
0.010 – x
+x
+x
Weak Base Example Cont.
• Write Kb expression:
Kb = 1.80 x 10-5 = [NH4][OH-] = x2
[NH3]
0.010-x
• Assume x is small because Kb is very
small, therefore x can be ignored
Kb = 1.80 x 10-5 = x2
0.010
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, 14 - 3.37 = 10.6 = pH
Weak Acid/Base Practice
Complete the practice at your
desk. Whatever you do not
finish will become your
homework.
Exit Ticket
• Which problems on the weak acid/base
practice were the most challenging?
• One thing I am confused about is
___________________________.