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