Chem. 31 – 12/4 Lecture

Announcements I Remaining Lab Due Dates Final Exam Formal Lab Report – part B – due 12/6 Soda Ash – due 12/8 (need to get in soon for resubmissions) IC #2 – due 12/8 All Resubmissions – due 12/8 Final Exam Dec. 11, 10:15-12:15 Will allow you to bring in 1 sheet of notes (8.5”x11” or smaller) with notes 175 points; about half new material

Announcements II Final Exam – cont. Today’s Lecture New material Starting with relative retention and band broadening in Ch. 23 Ending with Chapter 10 (ran out of time to get to titrations) Today’s Lecture Complete buffer calculations Chapter 10 Polyprotic acid basics/definitions Not covering determining pH when a form of polyprotic acid is dissolved in water

Announcements III Today’s Lecture – cont. Wednesday (last lecture) Chapter 10 – cont. Polyprotic acids used to make buffers Monoprotic and Polyprotic acids in buffer solutions (if time) Wednesday (last lecture) Complete Chapter 10 material Review for Final Exam Teaching Evaluations

Acid – Base Equilibria Example Problems: What is the pH of a solution made from mixing 400 mL of 0.018 M CH3CO2H (pKa = 4.75) with 100 mL of 0.024 M NaOH? (assume additive volumes) 5

Acid – Base Equilibria Example Qualitative Question: Which of the following mixtures will result in a traditional buffer: 0.010 M CH3CO2H+ 0.014 M KOH 0.014 M NH4Cl + 0.010 M KOH 0.020 M NH2CH2CH2OH + 0.010 M NaOH 0.010 M Na2HPO4 + 1.0 x 10-4 M Na3PO4 NH2CH2CH2OH is a weak base 6

Chapter 10 – Polyprotic Acids Polyprotic Acid – An acid that is capable of releasing 2 or more moles of H+ per molecule. Diprotic Acid Types: Standard Acid: H2A examples - H2CO3, H2SO3 Difunctional Bases: (H2L2+ = acid form) Example: NH2CH2CH2NH2 Both Acid and Base Constituents: (H2L+ = acid form) Examples: NH2CH2CO2H, NH2C6H4OH One example is a zwitter ion (exist with both positive and negative charges: NH3+CH2CO2-), but the other exists in an uncharged form (e.g. NH2C6H6OH) Zwitter ion will occur when pKa (NH3+R group) > pKa (RCO2H group)

Chapter 10 – Polyprotic Acids Diprotic Acid Forms: Acidic Form: H2A (for standard acid) or H2L+ or H2L2+ (acid/base and dibasic compounds) Intermediate form: HA- (for standard acid) Basic form: A2- (for standard acid)

Chapter 10 – Polyprotic Acids Polyprotic acids can be even more complex Example EDTA – has 4 acid functional groups and 2 base functional groups So 7 possible forms: H6Y2+, H5Y+, …, HY3-, Y4- Most acidic form most basic form

Chapter 10 – Polyprotic Acids Example Questions: Indicate what the “intermediate” form of NH2CH2CH2NH2 (a compound with two base groups) looks like Can H3NC6H4OH+ (shown in the acidic form) exist as a zwitter ion? The pKa for the NH3 group is 4.8 and the pKa for the OH group is 9.7.

Chapter 10 – Polyprotic Acids Use as Buffers Similar to buffers covered in Chapter 9 With most polyprotic acids where Ka1 >> Ka2 >> Ka3, the Henderson-Hasselbalch equation can be used with a single pKa selected) The only difficulty can be in deciding which species exist and how to get to those species Example 1: A mixture of 0.010 mol NaH2PO4 and 0.045 mol Na2HPO4 in 1.00 L solution. Determine the pH Example 2: How many mL of 1.00 M HCl needs to be added to 500 mL of 0.0080 M Na3PO4 to get a pH = 6.5 buffer?

Chapter 10 – Polyprotic Acids Distribution of forms at fixed pH (e.g. when in a buffer) Let’s start with monoprotic acid HA HA H+ + A- Fraction present as HA = a(HA) = [HA]/[HA]total = [HA]/([HA] + [A-]) Can calculate ratio ([A-]/[HA]) using Henderson-Hasselbalch equation, but other method calculating a values is more general Go over method on board a(A-) = Ka/([H+] + Ka) (can do derivation yourself)

Chapter 10 – Polyprotic Acids On to a diprotic acid H2A From H2A form From HA- form From A2- form Notice that denominator is the same Can you guess a(A2-)?

Chapter 10 – Polyprotic Acids Alpha Plot pKa2 pKa1