Chem. 1B – 10/6 Lecture
Announcements I
• Exam 1 – Results– Average was 69%– Distribution (see Table)– Fewer A range students
than I would like, but otherwise good
– Two questions not written ideally
– Questions with lowest % correct:
Score Range
Number of Students
90-99 7
80s 29
70s 37
60s 38
50s 15
30-49 17
A ver. # 15 1 23 7 19 21
B ver. # 16 3 21 5 17 22
% correct 27% 27% 28% 41% 44% 46%
Announcements II
• Exam 1 – cont.– I was planning on going over questions students
did worst at, but I need to first finish titrations for this week’s lab
• Today’s Lecture– Titrations
• finish strong acid – strong base• weak acid – strong base• weak base – strong acid• polyprotic acids and indicators
– If time, I will go over a few exam questions (after weak base – strong acid titrations)
Chem 1B – Aqueous Chemistry
Titrations (Chapter 16)• Strong Acid – Strong
Base Titration– How does pH change as
NaOH is added?– 3 regions to titrations
(different calculations in each region):
• before equivalence point
• at equivalence point• after equivalence point
– Show pH at 5 mL, 12.5 mL, and 15 mL
0.100 M NaOH
0.050 M HCl, 25 mL
Did last time
Chem 1B – Aqueous Chemistry
Titrations (Chapter 16)
0.100 M NaOH
0.050 M HCl, 25 mL
Titration Plot
Extremely sharp titration (large slope at equivalence point)
Weak Acid – Strong Base Titration
• Weak Acid – Strong Base Titration– How does pH Change as
NaOH is added?– Reaction:HA + OH- ↔ A- + H2O K = 1/Kb
Example – acetic acid:Ka = 1.76 x 10-5
– 4 regions to titrations (different calculations in each region):
• initial pH (skipping calculation)
• before equivalence point• at equivalence point• after equivalence point
– Go to Board to show pH at 5 mL, 12.5 mL, and 15 mL
0.100 M NaOH
0.050 M HA, 25 mL
Chapter 10 – Acid Base Titrations
Weak Acid – Strong Base Titrations• What affects shape of
curve?– pKa values (low pKa or
stronger weak acid gives sharper titration)
– pKa affects position of curve before and at equivalence point
– Note: at 6.25 mL (half of equivalence point), pH = pKa
50 mM example
Chem 1B – Aqueous ChemistryTitrations (Chapter 16)
• Weak Base – Strong Acid Titration– How does pH Change as HCl
is added?– Reaction:B + H+ ↔ BH+ K = 1/Ka
Example – ammonia:Ka (BH +) = 5.69 x 10-10
– 4 regions to titrations (different calculations in each region):
• initial pH• before equivalence point• at equivalence point• after equivalence point
0.100 M HCl
0.050 M B, 25 mL
Chem 1B – Aqueous ChemistryTitrations (Chapter 16)
• Weak Base – Strong Acid– Has similar 4
regions as weak acid – strong base
• initial (weak base)• before equiv. point
(buffer problem)• equiv. point (weak
acid)• after equiv. point
(excess strong acid)
pH
V(acid)
7
Exam Questions
• A version problems 1, 7, 15, 19, 21, and 23
Chem 1B – Aqueous ChemistryTitrations (Chapter 16)
• Qualitative Understanding Question: Based on the shape of this titration curve the flask/buret contains?– Weak acid/strong base– Strong base/strong acid– Strong acid/strong base– Weak base/strong acid
pH
V(acid)
7Equiv. ptpH
Chem 1B – Aqueous ChemistryTitrations (Chapter 16)
• More complex titrations– polyprotic acid by a strong base (e.g. H2SO3 + OH-)
– This example has pKa1 = 1.81 and pKa1 = 6.97
– Titration involves 2 reactions:1)H2A + OH- ↔ HA- + H2O
2)HA- + OH- ↔ A2- + H2O
Veq1
Veq2
Veq2 = 2Veq1
V(NaOH)
Also has 2 buffer regions: 1) H2A + HA- present, 2) HA- + A2- present
buffer region 1)
buffer region 2)
Chem 1B – Aqueous ChemistryTitrations (Chapter 16)
• Indicators– One of the reasons to bother to learn the
shape of the titration curves is to be able to select an indicator
– Indicators are colored compounds that exist in acidic and basic forms
– Example: methyl orangeAcid form HInBase form In-
Called Methyl Orange, because at pH = pKa(HIn), equal amounts of each form
Chem 1B – Aqueous ChemistryTitrations (Chapter 16)
• Indicators – cont.– Indicators change color over a narrow pH range
(visible over 1 to 2 pH units)
– Methyl Orange pKa = 3.5
– At pH < 2.5 main species = HIn (pink) vs. at pH > 4.5– What type of titrations is it useful for?
pH
V(acid)
7
Yellow (pH > 4.5)
Pink (pH < 2.5)
indicator is only useful where it changes color
Chem 1B – Aqueous ChemistryTitrations (Chapter 16)
• Titration Errors– The observed equivalence point (where the
indicator changes color) is called the end point– Titration errors occur when the end point
volume is before or after the equivalence point– Example: Use of bromothymol blue indicator
(pKa = 6.7) for a weak acid – strong base titration
pH
7 indicator range
end point equivalence point
equivalence point
In this example, end point comes early