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 s29 70s37 60s38 50s A ver. # B ver. # % correct27% 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 M NaOH M HCl, 25 mL Did last time

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) M NaOH M HCl, 25 mL Titration Plot Extremely sharp titration (large slope at equivalence point)

Weak Acid – Strong Base Titration –How does pH Change as NaOH is added? –Reaction: HA + OH - ↔ A - + H 2 O K = 1/K b Example – acetic acid: K a = 1.76 x –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 M NaOH M HA, 25 mL

Chapter 10 – Acid Base Titrations Weak Acid – Strong Base Titrations What affects shape of curve? –pK a values (low pK a or stronger weak acid gives sharper titration) –pK a affects position of curve before and at equivalence point –Note: at 6.25 mL (half of equivalence point), pH = pK a 50 mM example

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) Weak Base – Strong Acid Titration –How does pH Change as HCl is added? –Reaction: B + H + ↔ BH + K = 1/K a Example – ammonia: K a (BH + ) = 5.69 x –4 regions to titrations (different calculations in each region): initial pH before equivalence point at equivalence point after equivalence point M HCl M B, 25 mL

Chem 1B – Aqueous Chemistry Titrations (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 Chemistry Titrations (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) 7 Equiv. pt pH

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) More complex titrations –polyprotic acid by a strong base (e.g. H 2 SO 3 + OH - ) –This example has pK a1 = 1.81 and pK a1 = 6.97 –Titration involves 2 reactions: 1)H 2 A + OH - ↔ HA - + H 2 O 2)HA - + OH - ↔ A 2- + H 2 O V eq1 V eq2 V eq2 = 2V eq1 V(NaOH) Also has 2 buffer regions: 1) H 2 A + HA - present, 2) HA - + A 2- present buffer region 1) buffer region 2)

Chem 1B – Aqueous Chemistry Titrations (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 orange Acid form HInBase form In - Called Methyl Orange, because at pH = pK a (HIn), equal amounts of each form

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) Indicators – cont. –Indicators change color over a narrow pH range (visible over 1 to 2 pH units) –Methyl Orange pK a = 3.5 –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 Chemistry Titrations (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 (pK a = 6.7) for a weak acid – strong base titration pH 7indicator range end pointequivalence point In this example, end point comes early