1. Chem. 31 – 4/22 Lecture

2. Announcements Lab Reports –Soda Ash report due 4/27 –Will be posting information about Formal Report soon Today’s Lecture –Chapter 8: Acid Base Chemistry Buffer Solutions –Chapter 9: Polyprotic Acids Types of acids/functional groups Dissolution in water

3. Acid – Base Equilibria Buffer Solutions: –A buffer solution is designed so that a small addition of acid or base will only slightly change the pH –Most buffer solutions have a weak acid and its conjugate base both present –Example: Determine pH of a mix of 0.010 M HCO 2 H and 0.025 M Na + HCO 2 - solution (ignoring activity) –Go to board to show if ICE approach is needed

4. Buffer Solutions: –Question: Was the ICE Problem set up needed? –Answer: No. The assumption of x << [HA], [A - ] is valid for all “traditional” buffers –Traditional Buffer Weak acid (3 < pK a < 11) Ratio of weak acid to conjugate base in range 0.1 to 10 mM+ concentration range Acid – Base Equilibria

5. Buffer Solutions: –Since ICE not needed, can just use K a equation –K a = [H + ][A - ]/[HA] = [H + ][A - ] o /[HA] o (always valid) (valid for traditional buffer) –But log version more common –pH = pK a + log([A - ]/[HA]) –Also known as Henderson-Hasselbalch Equation

6. Acid – Base Equilibria Buffer Solutions: –Ways to make buffer solution: Mix weak acid and conjugate base (done in making reference solution for soda ash lab) Add strong base to weak acid (weak acid must be in excess) – this converts some of the weak acid to its conjugate base Add strong acid to weak base (weak base must be in excess) – this converts some of weak base to its conjugate acid

7. Buffer Solutions: –Why are they needed?/useful? –The main reason is to keep the pH constant so that the ratio of species of acids and bases is constants –Some examples: in water hardness titration, we want [Y 4- ]/[Y] total constant so sample pH won’t affect results spectroscopy: Beer’s law only applies to single species (e.g. separate laws for HIn and In - ) chromatography: at pH = 2, benzoic acid is a molecule and retained (reversed-phase HPLC) Acid – Base Equilibria

8. Buffer Solutions: –Ways to make buffer solution: Mix weak acid and conjugate base (done in making reference solution for soda ash lab) Add strong base to weak acid (weak acid must be in excess) – this converts some of the weak acid to its conjugate base Add strong acid to weak base (weak base must be in excess) – this converts some of weak base to its conjugate acid

9. Acid – Base Equilibria Example Problems: –How many moles of hydroxyl ammonium chloride (HONH 3 + Cl - ) needs to be added to 500 mL of 0.020 M HONH 2 to obtain a buffer solution with a pH of 6.20? The pK a for HONH 3 + is 5.96. –What is the pH of a solution made from mixing 400 mL of 0.018 M CH 3 CO 2 H (pK a = 4.75) with 100 mL of 0.024 M NaOH? (assume additive volumes)

10. Acid – Base Equilibria Example Qualitative Question: –Which of the following mixtures will result in a traditional buffer: 0.010 M CH 3 CO 2 H+ 0.014 M KOH 0.014 M NH 4 Cl + 0.010 M KOH 0.020 M NH 2 CH 2 CH 2 OH + 0.010 M NaOH 0.010 M Na 2 HPO 4 + 1.0 x 10 -4 M Na 3 PO 4 NH 2 CH 2 CH 2 OH is a weak base

11. Chapter 9 – Polyprotic Acids Polyprotic Acid – An acid that is capable of releasing 2 or more moles of H + per molecule. Diprotic Acid Types: –Standard Acid: H 2 A examples - H 2 CO 3, H 2 SO 3 –Both Acid and Base Constituents: (H 2 L + = acid form) Examples: NH 2 CH 2 CO 2 H, NH 2 C 6 H 4 OH One example is a zwitter ion (exist with both positive and negative charges: NH 3 + CH 2 CO 2 - ), but the other exists in an uncharged form (e.g. NH 2 C 6 H 6 OH) Zwitter ion will occur when pK a (NH 3 + R group) > pK a (RCO 2 H group) –Difunctional Bases: (H 2 L 2+ = acid form) Example: NH 2 CH 2 CH 2 NH 2

12. Chapter 9 – Polyprotic Acids Diprotic Acid Forms: –Acidic Form: H 2 A (for standard acid) or H 2 L + or H 2 L 2+ (acid/base and dibasic compounds) –Intermediate form: HA - (for standard acid) –Basic form: A 2- (for standard acid)

13. Chapter 9 – 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: H 6 Y 2+, H 5 Y +, …, HY 3-, Y 4- Most acidic form most basic form

14. Chapter 9 – Polyprotic Acids Example Questions: 1.Indicate what the “intermediate” form of NH 2 CH 2 CH 2 NH 2 (a compound with two base groups) looks like 2.Is H 3 NC 6 H 4 OH + (shown in the acidic form) a zwitter ion? The pK a for the NH 3 group is 4.8 and the pK a for the OH group is 9.7.

15. Chapter 9 – Polyprotic Acids Dissolution in Water –Acid Forms (e.g. 0.010 M HO 2 CCH 2 CO 2 H – malonic acid) –Intermediate Forms (0.010 M HO 2 CCH 2 CO 2 - ) –Basic Forms (0.010 M PO 4 3- ) –Go Over Examples on Board