Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2

1. Chemistry 223 Chapter 21: Buffers and the Titration of Acids and Bases PART 2

2. Generally, “x is small” will work when both of these are true: • initial [ ]’s of acid & salt are • Ka is _______________ • Most problems: initial acid & salt [ ]’s should be _____ to ______ > value of Ka

3. How Much Does the pH of a Buffer Change When an Acid or Base Is Added? • Calculating new pH after adding acid or base requires doing 2 calculations:

4. stoichiometry calculation for added chemical with HA or A- of buffer to reduce initial [ ] and increase [ ] of the other • added acid reacts with • added base reacts with

5. 2. an equilibrium calculation (I.C.E.) of [H3O+] using

6. What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + OH− C2H3O2 + H2O

7. HC2H3O2 + OH− C2H3O2 + H2O

8. HC2H3O2 + OH− C2H3O2 + H2O

9. 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L with 0.010 mol NaOH added. HC2H3O2 + OH− C2H3O2 + H2O

10. 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L with 0.010 mol NaOH added. HC2H3O2 + OH− C2H3O2 + H2O −0.010 +0.010 −0.010 0.110 0 0.090 0.090 0.110 0

11. HC2H3O2 + H2O  C2H3O2 + H3O+qq

12. HC2H3O2 + H2O  C2H3O2 + H3O+ x +x +x x 0.090 x 0.110 + x

13. Ka for HC2H3O2 = 1.8 x 10−5 0.110 +x 0.090 x

15. Ka for HC2H3O2 = 1.8 x 10−5 x = 1.47 x 10−5

16. 0.090 x x 0.110 + x x = 1.47 x 10−5

18. Ka for HC2H3O2 = 1.8 x 10−5

19. Example: What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + H2O  C2H3O2 + H3O+

20. HC2H3O2 + H2O  C2H3O2 + H3O+ Ka for HC2H3O2 = 1.8 x 10−5 Tro: Chemistry: A Molecular Approach, 2/e

21. HC2H3O2 + H2O  C2H3O2 + H3O+ pKa for HC2H3O2 = 4.745

22. Compare effect on pH of adding 0.010 molNaOH to a 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 buffer in 1.00 L to adding 0.010 molNaOH to 1.00 L of pure water HC2H3O2 + H2O  C2H3O2 + H3O+ pKa for HC2H3O2 = 4.745

23. adding 0.010 mol NaOH to 1.00 L of pure water

24. Clicker question: What is the pH of a buffer that has 0.140 moles HF (pKa = 3.15) and 0.071 moles KF in 1.00 L of solution when 0.020 moles of HCl is added? (The “x is small” approximation is valid)

25. What is the pH of a buffer that has 0.140 moles HF and 0.071 moles KF in 1.00 L of solution when 0.020 moles of HCl is added?

26. F− + H3O+  HF + H2O

27. F− + H3O+  HF + H2O

28. F− + H3O+  HF + H2O −0.020 −0.020 +0.020

29. HF + H2O  F + H3O+

30. Henderson-Hasselbalch Equation for Basic Buffers • chemical equation of a basic buffer is written with a weak base as a reactant and its conjugate acid as a product

31. B: + H2O  H:B+ + OH− chemical eqtn of basic buffer must be looked at like an acid reaction this does not affect [ ]’s, just the way we are looking at the rxtn

32. Relationship between pKa and pKb • relationship btwn Ka of a weak acid and Kb of its conjugate base, • also a relationship btwn pKa of a weak acid and pKb of its conjugate base

33. What is pH of a buffer that is 0.50 M NH3 (pKb = 4.75) and 0.20 M NH4Cl? NH3 + H2O  NH4+ + OH−

34. Henderson-Hasselbalch Equation for Basic Buffers • chemeqtn of basic buffer is written with weak base as a reactant and its conjugate acid as a product • B: + H2O  H:B+ + OH−

35. Henderson-Hasselbalch Equation for Basic Buffers • We can rewrite Henderson-Hasselbalch eqtn for chem eqtn of basic buffer in terms of __________

36. What is pH of a buffer that is 0.50 M NH3 (pKb = 4.75) and 0.20 M NH4Cl? NH3 + H2O  NH4+ + OH−

37. Polyprotic Acids & Bases Polyprotic acids contain > one ionizable p+ and the protons are lost in a stepwise manner. Fully protonated species is always the strongest acid. Why?

38. Strong acids have very weak conjugate bases Strong bases have very weak conjugate acids

39. Titrations of Polyprotic Acids or Bases When strong base is added to soltn of a polyprotic acid – neutralization rxtn occurs in stages. Most acidic group titrated 1st, followed by next most acidic group.

40. Titrations of Polyprotic Acids or Bases If pKa values are separated by at least 3 pKa units, then overall titration curve shows well-resolved “steps” corresponding to titration of each p+.

43. Indicators Most acid-base titrations are not monitored by recording pH as a function of amount of strong acid or base soltn used as a titrant

44. Indicators Instead, acid-base indicator is used, Compounds that D color at a particular pH

45. If carefully selected, undergo dramatic color D at pH corresponding to ________________ of titration

46. Indicators Acid-base indicators are typically weak acids or bases. Color D’scorrespond to _____________________________ of the indicator itself.

47. Indicators Chemistry of indicators general equation: Protonated form = conjugate base (deprotonated) =

48. Indicators ionization constant for deprotonation of indicator Hn: Kin = [H+] [n–] / [Hn] pKindetermines pH at which indicator D’s color

49. Properties of Good Indicators • Color change must be easily detected • Color change must be rapid • Indicator must not react with substance being titrated • Indicator should have pKin within

50. Choosing a Good Indicator for Acid/Base Titration For titrations of strong acids and strong bases (and vice versa), any indicator with a pKin