1 / 88

Chapter 16 Acid – Base Equilibria

Chapter 16 Acid – Base Equilibria. Contents and Concepts. Solutions of a Weak Acid or Base 1. Acid-Ionization Equilibria 2. Polyprotic Acids 3. Base-Ionization Equilibria 4. Acid – Base Properties of Salt Solutions Solutions of a Weak Acid or Base with Another Solute 5. Common-Ion Effect

jerry-henry
Télécharger la présentation

Chapter 16 Acid – Base Equilibria

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 16 Acid–Base Equilibria

  2. Contents and Concepts • Solutions of a Weak Acid or Base • 1. Acid-Ionization Equilibria • 2. Polyprotic Acids • 3. Base-Ionization Equilibria • 4. Acid–Base Properties of Salt Solutions • Solutions of a Weak Acid or Base with Another Solute • 5. Common-Ion Effect • 6. Buffers • 7. Acid–Base Titration Curves

  3. Learning Objectives • Solutions of a Weak Acid or Base • Acid-Ionization Equilibria a. Write the chemical equation for a weak acid undergoing acid ionization in aqueous solution. b. Define acid-ionization constant and degree of ionization. c. Determine Kafrom the solution pH. d. Calculate concentrations of species in a weak acid solution using Ka(approximation method).

  4. 1. Acid-Ionization Equilibria (cont) a. State the assumption that allows for using approximations when solving problems. b. Calculate concentrations of species in a weak acid solution using Ka(quadratic formula). • 2. Polyprotic Acids a. State the general trend in the ionization constants of a polyprotic acid. b. Calculate concentrations of species in a solution of a diprotic acid.

  5. 3. Base-Ionization Equilibria a. Write the chemical equation for a weak base undergoing ionization in aqueous solution. b. Define base-ionization constant. c. Calculate concentrations of species in a weak base solution using Kb.

  6. 4. Acid–Base Properties of Salt Solutions a. Write the hydrolysis reaction of an ion to form an acidic solution. b. Write the hydrolysis reaction of an ion to form a basic solution. c. Predict whether a salt solution is acidic, basic, or neutral. d. Obtain Ka from Kb or Kb from Ka. e. Calculating concentrations of species in a salt solution.

  7. Solutions of a Weak Acid or Base with Another Solute • 5. Common-Ion Effect a. Explain the common-ion effect. b. Calculate the common-ion effect on acid ionization (effect of a strong acid). c. Calculate the common-ion effect on acid ionization (effect of a conjugate base).

  8. 6. Buffers a. Define buffer and buffer capacity. b. Describe the pH change of a buffer solution with the addition of acid or base. c. Calculate the pH of a buffer from given volumes of solution. d. Calculate the pH of a buffer when a strong acid or a strong base is added. e. Learn the Henderson–Hasselbalch equation. f. State when the Henderson–Hasselbalch equation can be applied

  9. 7. Acid–Base Titration Curves a. Define equivalence point. b. Describe the curve for the titration of a strong acid by a strong base. c. Calculate the pH of a solution of a strong acid and a strong base. d. Describe the curve for the titration of a weak acid by a strong base. e. Calculate the pH at the equivalence point in the titration of a weak acid by a strong base.

  10. 7. Acid–Base Titration Curves (cont) f. Describe the curve for the titration of a weak base by a strong acid. g. Calculate the pH of a solution at several points of a titration of a weak base by a strong acid.

  11. The simplest acid–base equilibria are those in which a weak acid or a weak base reacts with water. • We can write an acid equilibrium reaction for the generic acid, HA. • HA(aq) + H2O(l) H3O+(aq) + A-(aq)

  12. Acetic acid is a weak acid. It reacts with water as follows: • HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2-(aq) • Here A from the previous slide = C2H3O2-(aq)

  13. The equilibrium constant for the reaction of a weak acid with water is called the acid-ionization constant (or acid-dissociation constant), Ka. • Liquid water is not included in the equilibrium constant expression.

  14. In order of decreasing Ka: Highest pH Ka 6.8 × 10-4 HCN HF HC2H3O2 4.5 × 10-4 HNO2 HNO2 HC2H3O2 1.7 × 10-5 HCN HF 4.9 × 10-10 Lowest pH

  15. Calculations with Ka • Given the value of Ka and the initial concentration of HA, you can calculate the equilibrium concentration of all species. • Given the value of Ka and the initial concentration of HA, you can calculate the degree of ionization and the percent ionization. • Given the pH of the final solution and the initial concentration of HA, you can find the value of Ka and the percent ionization.

  16. We can be given pH, percent or degree ionization, initial concentration, and Ka. • From pH, we can find [H3O+]. • From percent or degree ionization, we can find Ka. • Using what is given, we can find the other quantities.

  17. Sore-throat medications sometimes contain the weak acid phenol, HC6H5O. A 0.10 M solution of phenol has a pH of 5.43 at 25°C. • a. What is the acid-ionization constant, Ka, for phenol at 25°C? • b. What is the degree of ionization?

  18. We were told that pH = 5.43. That allows us to find [H3O+] = 3.7 × 10-6M = x = [C6H5O-]. • Now we find [HC6H5O] = 0.10 – x = 0.10 M.

  19. Finally, we write the expression for Ka and substitute the concentrations we now know. • HC6H5O + H2O H3O+ + C6H5O- • [H3O+] = [C6H5O-] = 3.7 × 10-6M • [HC6H5O] = 0.10 M

  20. The degree of ionization is the ratio of ionized concentration to original concentration: Percent ionization is the degree of ionization × 100%: Percent ionization = 3.7 x 10-3% or 0.0037%

  21. Simplifying Assumption for Acid and Base Ionizations • The equilibrium concentration of the acid is most often ([HA]0 – x). • If x is much, much less than [HA]0, we can assume that subtracting x makes no difference to [HA]: • ([HA]0 – x) = [HA]0 • This is a valid assumption when the ratio of [HA]0 to Ka is > 103. If it is not valid, you must use the quadratic equation to solve the problem.

  22. Para-hydroxybenzoic acid is used to make certain dyes. What are the concentrations of this acid, of hydronium ion, and of para-hydroxybenzoate ion in a 0.200 M aqueous solution at 25°C? What is the pH of the solution and the degree of ionization of the acid? The Ka of this acid is 2.6 × 10-5. We will use the generic formula HA for para-hydroxybenzoic acid and the following equilibrium: HA + H2O H3O+ + A-

  23. Polyprotic Acids • A polyprotic acid has more than one acidic proton—for example, H2SO4, H2SO3, H2CO3, H3PO4. • These acids have successive ionization reactions with Ka1, Ka2, . . . • The next example illustrates how to do calculations for a polyprotic acid.

  24. Tartaric acid, H2C4H4O6, is a diprotic acid used in food products. What is the pH of a 0.10 M solution? What is the concentration of the C4H4O62 ion in the same solution? • Ka1 = 9.2  104; Ka2 = 4.3  105. First, we will use the first acid-ionization equilibrium to find [H+] and [HC4H4O6-]. In these calculations, we will use the generic formula H2A for the acid. Next, we will use the second acid-ionization equilibrium to find [C4H4O62-].

  25. Base-Ionization Equilibrium • The simplest acid–base equilibria are those in which a weak acid or a weak base reacts with water. • We can write a base equilibrium reaction for the generic base, B. • B(aq) + H2O(l) HB+(aq) + OH-(aq)

  26. Ammonia is a weak base. It reacts with water as follows: • NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) • Here the generic B from the previous slide is NH3(aq).

  27. The equilibrium constant for the reaction of a weak base with water is called the base-ionization constant, Kb. • Liquid water is not included in the equilibrium constant expression.

  28. Writing Kb Reactions • The bases in Table 16.2 are nitrogen bases; that is, the proton they accept adds to a nitrogen atom. Next we’ll practice writing the Kb reactions. • Ammonium becomes ammonium ion: • NH3+ H2O  NH4+ + OH- • Ethylamine becomes ethyl ammonium ion: • C2H5NH2 + H2O  C2H5NH3+ + OH-

  29. Dimethylamine becomes dimethylammonium ion: • (CH3)2NH2 + H2O  (CH3)2NH3+ + OH- • Pyridine becomes pyridinium ion: • C5H5N+ H2O  C5H5NH+ + OH- • Hydrazine becomes hydrazinium ion: • N2H4+ H2O  N2H5+ + OH-

  30. We can be given pH, initial concentration, and Kb. • From the pH, we can find first [H3O+] and then [OH-]. Using what is given, we can find the other quantities. • We can also use a simplifying assumption: When [B]0 / Kb > 103, the expression ([B]0 – x) = [B]0.

  31. Aniline, C6H5NH, is used in the manufacture of some perfumes. What is the pH of a 0.035 M solution of aniline at 25°C? • Kb= 4.2 × 10-10 at 25°C. We will construct an ICE chart and solve for x.

  32. We are told that Kb = 4.2 × 10-10. That allows us to substitute into the Kb expression to solve for x.

  33. C6H5N + H2O C6H5NH+ + OH- • [B] = 0.200 – x and [B+] = [OH-] = x

  34. The question asks for the pH:

  35. Salt Solutions • We will look at the cation and the anion separately, and then combine the result to determine whether the solution is acidic, basic, or neutral.

  36. The conjugate acid of a strong base is very weak and does not react with water. It is therefore considered to be neutral. • Na+ + H2O  No Reaction (NR) • The conjugate base of a strong acid is very weak and does not react with water. It is therefore considered to be neutral. • Cl- + H2O  NR

  37. The conjugate acid of a weak base reacts with water to form an acidic solution: • NH4+ + H2O <==> NH3 + H3O+ • The conjugate base of a weak acid reacts with water to form a basic solution: • F- + H2O <==> HF + OH-

  38. We classify each salt by examining its cation and its anion, and then combining the result. • NaBr • Na+ is the conjugate acid of NaOH, a strong base. It does not react with water, so it is neutral. • Br- is the conjugate base of HBr, a strong acid. It does not react with water, so is neutral. • The cation is neutral; the anion is neutral. • NaBr is neutral.

  39. NaC2H3O2 • Na+ is the conjugate acid of NaOH, a strong base. It does not react with water, so it is neutral. • C2H3O2- is the conjugate base of HC2H3O2-, a weak acid. It reacts with water to give a basic solution. • The cation is neutral; the anion is basic. • NaC2H3O2 is basic.

  40. NH4Cl • NH4+ is the conjugate acid of NH3, a weak base. It reacts with water to give an acidic solution. • Cl- is the conjugate base of HCl, a strong acid. It does not react with water, so it is neutral. • The cation is acidic; the anion is neutral. • NH4Cl is acidic.

More Related