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Acid-Base Titrations

Acid-Base Titrations. Introduction to Acids and Bases. Chapter 8. There are three (3) definitions of acids and bases: 1. Arrhenius, 2. Bronsted -Lowry and 3. Lewis. There are three (3) definitions of acids and bases: 1. Arrhenius, 2. Bronsted -Lowry and 3. Lewis. base. acid. acid. base.

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Acid-Base Titrations

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  1. Acid-Base Titrations

  2. Introduction to Acids and Bases Chapter 8

  3. There are three (3) definitions of acids and bases: 1. Arrhenius, 2. Bronsted-Lowry and 3. Lewis.

  4. There are three (3) definitions of acids and bases: 1. Arrhenius, 2. Bronsted-Lowry and 3. Lewis. base acid acid base conjugatebase conjugateacid base acid Conjugate acid-base pairs differ by only one proton.

  5. There are three (3) definitions of acids and bases: 1. Arrhenius, 2. Bronsted-Lowry and 3. Lewis. Arrhenius Bases: LiOH, Mg(OH)2 Arrhenius Acids: HCl, H2SO4 Bronsted Acids: H2O, HS-, H2PO4- Bronsted Bases: H2O, NH3, HS- Lewis Acids: Any species that accepts electron pairs. (H+ CO2, Mn+) Lewis Bases: Any species with a lone pair

  6. Acids and Bases undergo neutralization reactions. NaOH + HCl NaCl + H2O Na+ + OH– + H+ + Cl– Na+ + Cl– + H2O H+ + OH–  H2O

  7. Acids and Base strengths are defined by how much H3O+ or OH- is produced by a given concentration Strong Acid Weak Acid

  8. Ionized acid concentration at equilibrium x 100% Initial concentration of acid Acids and Base strengths are defined by how much H3O+ or OH- is produced by a given concentration percent ionization = For a monoprotic acid HA [H+]eq x 100% Percent ionization = [HA]0= initial concentration [HA]0

  9. Weak acids and bases are in equilibrium with their original species. CH3COOH + H2O H3O+ +CH3COO– Kc << 1

  10. Weak acids and bases are in equilibrium with their original species. NH3 + H2O NH4+ +OH– Kc << 1

  11. Ka and Kb tells us something about the relative acid/base strengths

  12. Ka and Kb tells us something about the relative acid/base strengths

  13. Water can act both as an acid and a base (AUTOIONIZATION) Theion-product constant (Kw) is the product of the concentration of H3O+ and OH– ions at a particular temperature At 250C Kw = [H3O+][OH-] = 1.0 x 10-14

  14. The p-scale conveniently handles a wide range of concentrations

  15. Solving weak acid ionization problems: • Identify the major species that can affect the pH. • In most cases, you can ignore the autoionization of water. [H3O+] from water is negligible in comparison to [H3O+] from the weak acid. • Ignore [OH-] because it is determined by [H+]. • Use ICE to express the equilibrium concentrations in terms of single unknown x. • Write Kain terms of equilibrium concentrations. Solve for x by the approximation method. If approximation is not valid, solve for x exactly. • Calculate concentrations of all species and/or pH of the solution.

  16. Exercises • What is the pH of a 0.5 M HF solution (at 25°C, Ka = 7.1 x 10-4)? • What is the pH of a 0.05 M HF solution? • What is the pH of a 0.122 M monoprotic acid whose Ka is 5.7 x 10-4?

  17. Complete the following table:

  18. Complete the following table:

  19. Exercises • What is the pH, [H3O+], [OH-] of 7.52 x 10-4 M CsOH? • What is the pOH, [H3O+], [OH-] of 1.59 x10-3 M HClO4? • What is the [H3O+], [OH-] and pOH in a solution with a pH of 2.77 • What is the [H3O+], [OH-] and pH in a solution with a pOH of 11.27

  20. Exercises • Chloroacetic acid has a pKa of 2.87. What are [H3O+], pH, [ClCH2COOH], [ClCH2COO-] in 1.05 M [ClCH2COOH] • A 0.735 M of weak acid is 12.5% dissociated. Calculate [H3O+], pH, [OH-], pOH of solution. Calculate Ka of acid

  21. Buffers Chapter 9

  22. Buffers solutions are solutions that resist changes in pH

  23. Buffers contain appreciable amounts of a weak acid and its conjugate base HA/ A– NH4Cl/NH3 H3PO4/NaH2PO4 NH4SH/Na2S HCOOH/HCOOK HBr/KBr H3IO3/Li2HIO3 NaOH/Na2O

  24. Buffers contain appreciable amounts of a weak acid and its conjugate base What do we mean by appreciable? • A 50-mL solution of 0.25 M Acetic acid. • A 50-mL solution of 0.25 M Sodium Acetate • A solution containing 0.125 M Acetic acid and 0.125 M Acetate HA / A– ** If we take the ratio of base to acid or acid to base, it should be within 10% of each other + base + acid

  25. Buffers work because there are weak acids and weak bases present to counter-act small amounts of acid/bases.

  26. The effectivity (and pH) of the buffer is dependent on the ratio between the weak acid and its conjugate base Henderson-Hasselbach Equation:

  27. The effectivity (and pH) of the buffer is dependent on the ratio between the weak acid and its conjugate base • A 0.25 M Acetic acid buffer with pH 4.74 • A 0.25 M Acetic acid buffer with pH 5.10 • A 0.25 M Acetic acid buffer with pH 4.40 ** If we take the ratio of base to acid or acid to base, it should be within 10% of each other BUFFER RANGE

  28. The effectivity (and pH) of the buffer is dependent on the ratio between the weak acid and its conjugate base • A buffer for pH 10.00 • A buffer for pH 4.00 • A buffer for pH 7.00 The closer the pH of the buffer to the pKa the better

  29. The effectivity (and pH) of the buffer is also dependent on the total amount of weak acid and conjugate base. • A 1.00 M Acetic acid buffer with pH 4.74 • A 0.30 M Acetic acid buffer with pH 4.74 • A 0.10 M Acetic acid buffer with pH 4.74 • A 0.030 M Acetic acid buffer with pH 4.74

  30. Buffer Capacity is the measure of the ability of a buffer to resist pH Changes + appreciable amounts (High Concentration)… GOOD BUFFER RANGE

  31. Buffers work through a phenomenon known as the common ion effect What is the common ion effect? This effect occurs when a reactant containing a given ion is added to an equilibrium mixture that already contains that ion, and the position of equilibrium shifts away from forming more of it

  32. Preparation of Buffers Choose the conjugate acid-base pair Calculate the ratio of the buffer component concentrations Determine the buffer concentration Mix the solution and adjust pH

  33. Buffers can be prepared using a weak acid and the salt of its conjugate base (or a weak base and the salt of its conjugate acid). Example 1 Preparing a pH 10.00 carbonate buffer. How many grams of Na2CO3 must one add to 1.5 L of freshly prepared 0.20 M NaHCO3 to make the buffer? Ka of HCO3- is 4.7 x 10-11 Example 2 Prepare a 50-mL of 0.12 M Acetic acid buffer with equal concentrations of acetic acid and acetate from 3.00 M acetic acid stock solution and sodium acetate salt

  34. Buffers can also be prepared by using a weak acid (or weak base) then add a strong base (or acid) to desired pH. 5.0 g of CH3COONa is dissolved in 100. mL of water. How many mL of 0.50 MHCl should be added to form a buffer with pH 4.90? Will diluting the final mixture to 500 mL affect the pH of the buffer? What is affected by dilution?

  35. Acid-Base Titrations Chapter 10

  36. In an acid-base reaction, the key parameter that changes in the system is pH. Example: A 40.00 mL sample of 0.1000 M HCl solution was titrated with 0.1000 M NaOH solution. Calculate the pH when the following volume of the NaOH is added. 0.00 10.00 20.00 30.00 35.00 39.00 40.00 41.00 45.00 50.00

  37. In an acid-base reaction, pH is monitored through colored indicators.

  38. Acid-base indicators are usually weak acids (HIn) which have different color than its conjugate base (In-). basic change occurs over ~2 pH units acidic

  39. Acid-base indicators are usually weak acids (HIn) which have different color than its conjugate base (In-).

  40. TITRATION OF A STRONG ACID BY A STRONG BASE 40 mL of 0.1000 M HCl

  41. TITRATION OF A WEAK ACID BY A STRONG BASE REGION 1: Before addition (weak acid) REGION 2: Before Equivalence point (buffer) REGION 3: At Equivalence point (weak base) REGION 4: After Equivalence point (strong base)

  42. TITRATION OF A WEAK ACID BY A STRONG BASE EXAMPLE in Book: Titration of 50.00 mL of 0.0200 M MES (pKa = 6.27) with 0.1000 M NaOH REGION 1: Before addition (weak acid) REGION 2: Before Equivalence point (buffer) REGION 3: At Equivalence point (weak base) REGION 4: After Equivalence point (strong base)

  43. TITRATION OF A WEAK BASE BY A STRONG ACID REGION 1: Before addition (weak base) REGION 2: Before Equivalence point (buffer) REGION 3: At Equivalence point (weak acid) REGION 4: After Equivalence point (strong acid)

  44. TITRATION OF A WEAK BASE BY A STRONG ACID Example in Book: Titration of 25.00 mL of 0.08364 M pyridine (Kb = 1.6 x 10-9) with 0.1067 M HCl. REGION 1: Before addition (weak base) REGION 2: Before Equivalence point (buffer) REGION 3: At Equivalence point (weak acid) REGION 4: After Equivalence point (strong acid)

  45. TITRATION OF A POLYPROTIC SYSTEMS

  46. TITRATION OF A POLYPROTIC SYSTEMS

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