Acid-Base Equilibrium

1. Acid-Base Equilibrium Dr. Ron Rusay

2. Introduction to Aqueous Acids • Acids: taste sour and cause certain dyes to change color.

3. Introduction to Aqueous Bases • Bases: taste bitter, feel soapy and cause certain dyes to turn color.

4. Models of Acids and Bases • Arrhenius: Acids produce H+ & bases produce OH ion in aqueous solutions . • Brønsted-Lowry: Acids are H+ donors & bases are proton acceptors. • HCl + H2O  Cl + H3O+ acid base

5. Lewis Acids and Bases • Lewis Acid: electron pair acceptor • Lewis Base: electron pair donor • Example:

6. Lewis Acids and Bases

7. The Mg2+ Ion is a Lewis Acid in the Chlorophyll Molecule; Fe2+ Ion is a Lewis Acid in normal Hemoglobin

8. Conjugate Acid/Base Pairs • HA(aq) + H2O(l)  H3O+(aq) + A(aq) conj conj acid 1 base 2 acid 2 base 1 • conjugate acid: formed when the proton is transferred to the base. • conjugate base: everything that remains of the acid molecule after a proton is lost.

9. http://chemconnections.org/general/movies/ConjugateAcidBaseActivity.swfhttp://chemconnections.org/general/movies/ConjugateAcidBaseActivity.swf

10. Strong & Weak Acids: Dissociation Constant (Ka) • HA(aq) + H2O(l)  H3O+(aq) + A(aq) • HA(aq)  H+(aq) + A(aq)

11. Acid Strength Strong Acid: • Equilibrium position lies far to the right. (HNO3); Ka >> 1 • Produces a conjugate base. (NO3) and a conjugate acid which are weaker than the starting acid and base (H2O).

12. Acid Strength Strong Acids:

13. Acid Strength(continued) Weak Acid: • Equilibrium lies far to the left. (CH3COOH); Ka < 1 • Yields a stronger (relatively strong) conjugate base than water. (CH3COO)

15. Weak Acids • Weak acids are only partially ionized in solution. or • Ka is the acid dissociation constant.

16. Percent Ionization • Percent ionization is a way to assess relative acid strengths. • For the reaction • Percent ionization relates the H3O+(aq) equilibrium concentration, [H+]eqm, to the initial HA(aq) concentration, [HA]0.

17. The Extent of Dissociation for Strong and Weak Acids

18. Weak Acids • The higher percent ionization, the stronger the acid. • Percent ionization of a weak acid decreases as the molarity of the solution increases. • For acetic acid, 0.05 M solution is 2.0 % ionized whereas a 0.15 M solution is 1.0 % ionized.

19. Weak Acids Percent Ionization

20. QUESTION Nitric acid, HNO3, is considered to be a strong acid whereas nitrous acid, HNO2, is considered to be a weak acid. Which of the statements here is fully correct? A. Nitric acid has an aqueous equilibrium that lies far to the right and NO3– is considered a weak conjugate base. B. Nitric acid has a stronger conjugate base than nitrous acid. C. The dissociation of nitrous acid compared to an equal concentration of nitric acid produces more H+. D. The equilibrium of nitrous acid lies far to the left and the conjugate base is weaker than the conjugate base of nitric acid.

21. ANSWER • correctly compares equilibrium and conjugate base characteristics. The conjugate base of a strong acid is considered to be weak. The stronger the acid, the more reaction in water. Therefore, a weak acid’s equilibrium is favored to the left.

23. Bases • “Strong” and “weak” are used in the same sense for bases as for acids. • Strong = complete dissociation, Kb >> 1 (concentration of hydroxide ion in solution) NaOH(s) Na+(aq) + OH(aq) • NaOH(s) + H2O(l) Na+(aq) + OH(aq)

24. Bases(continued) • Weak bases have very little dissociation, Kb < 1 ( little ionization with water) • CH3NH2(aq) + H2O(l)  CH3NH3+(aq) + OH(aq) • How conductive is NaOH(aq) vs morphine, C17H19NO3 (aq)?

25. QUESTION Aniline, C6H5NH2, was isolated in the 1800s and began immediate use in the dye industry. What is the formula of the conjugate acid of this base? A. C6H5NH2+ B. C6H5NH3+ C. C6H5NH– D. C6H5NH+

26. ANSWER B) correctly represents the result of aniline accepting a H+ ion as bases typically do. The conjugate acid of a base is represented as the base with the addition of a H+.

27. Acid-Base Strengths Strong Acid: Strong Base: Weak Acid: Weak Base:

28. Water as an Acid and a Base Self-ionization

29. Water as an Acid and a Base • Water is amphoteric (it can behave either as an acid or a base). • H2O + H2O  H3O+ + OH conj conj acid 1 base 2 acid 2 base 1 • Kw = 1  1014 at 25°C

30. Water as an Acid and a Base Self-ionization http://chemconnections.org/general/movies/KwActivity.swf

31. The pH Scale • pH log[H+]  log[H3O+] • pH in water ranges from 0 to 14. Kw = 1.00  1014 = [H+] [OH] pKw = 14.00 = pH + pOH • As pH rises, pOH falls (sum = 14.00). • There are no theoretical limits on the values of pH or pOH. (e.g. pH of 2.0 M HCl is -0.301)

33. The pH Values of Some Familiar Aqueous Solutions KW [OH-] = [H3O+] [H3O+] [OH-] [H3O+]> [OH-] [H3O+]< [OH-] acidic solution neutral solution basic solution [H3O+] = [OH-]

34. QUESTION In a solution of water at a particular temperature the [H+] may be 1.2  10–6 M. What is the [OH–] in the same solution? Is the solution acidic, basic, or neutral? A. 1.2  10–20 M; acidic B. 1.2  10–20 M; basic C. 8.3  10–9 M; basic D. 8.3  10–9 M; acidic

35. ANSWER D. correctly shows the OH– molarity and classifies the solution as acidic. Kw = [H+][OH–] = 1.0  10–14 at 25°C. The H+ molarity is approximately 1,000 times greater than the OH–concentration. Solutions with higher H+ concentrations than OH– are acidic.

36. http://chemconnections.org/general/movies/pHEstimation.swf pH Estimation

37. QUESTION An environmental chemist obtains a sample of rainwater near a large industrial city. The [H+] was determined to be 3.5  10–6 M. What is the pH, pOH, and [OH–] of the solution? A. pH = 5.46 ; pOH = 8.54; [OH–] = 7.0  10–6 M B. pH = 5.46 ; pOH = 8.54; [OH–] = 2.9  10–9 M C. pH = 12.56 ; pOH =1.44 ; [OH–] = 3.6  10–2 M D. pH = 8.54; pOH = 5.46; [OH–] = 2.9  10–9 M

38. ANSWER B. provides all three correct responses. The expression pH = –log[H+] can be used to find the pH then: 14.00 = pH + pOH can be used to obtain the pOH. Finally, [OH–] = 10–pOH.

39. Name: ____________________ Partner (if any): ____________________ The pH Scale

40. The pH Scale 1.3 x10 -12 2.1 11.9 Acid 4.6 9.4 2.8 x10 -5 Acid Base 1.78 x10 -6 5.75 5.62 x10 - 9 Base 5.00 x10 - 9 2.00 x10 - 6 8.30

41. Indicators

42. Acid-Base Indicators

43. Titrations: Indicators & (pH) Curves • pH Curve is a plot of pH of the solution being analyzed as a function of the amount of titrant added. • Equivalence (stoichiometric) point: Enough titrant has been added to react exactly with the solution being analyzed. An indicator provides a visible color change to determine an (end point) volume of titrant.

46. QUESTION Most acid-base indicators are weak acids. In a titration of 0.50 M acetic acid (at 25°C, Ka = 1.8  10–5) with KOH, which indicator would best indicate the pH at the equivalence point? The approximate Ka for each choice is provided. A. Bromophenol blue; Ka ~ 1  10–4 B. Methyl red; Ka ~ 1  10–5 C. Bromothymol blue; Ka ~ 1  10–7 D. Alizarin yellow; Ka ~ 1  10–10

47. ANSWER D. provides the best choice although there may also be better choices available than these four. The equivalence point pH should be as close as possible to the pKa of the indicator. As acetic acid is a fairly weak acid and NaOH is a strong base, the pH at the equivalence point will be above 7. The only choice above 7 in the list was Alizarin yellow. Without a more detailed calculation, this would be the best choice.

48. Methods for Measuring the pH of an Aqueous Solution (a) pH paper (b) Electrodes of a pH meter

49. QUESTION The acid-base indicator bromocresol purple has an interesting yellow-to-purple color change. If the approximate Ka of this indicator is 1.0  10–6, what would be the ratio of purple [A–] to yellow [HA] at a pH of 4.0? A. 100:1 B. 1:100 C. 1:1 D. This choice indicates that I don’t know.

50. ANSWER B. shows the [A–]/[HA] ratio at pH 4.0 for bromocresol purple. The pH can be converted to [H+] and divided into the Ka value to reveal the [A–]/[HA] ratio at pH 4.0. Ka/[H+] = [A–]/[HA].