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Acids and Bases II IB Chemistry Gr.12

Acids and Bases II IB Chemistry Gr.12. Topic 18. Topics. 18.1 Calculations Involving Acids and Bases 18.2 Buffer Solutions 18.3 Salt Hydrolysis 18.4 Acid-Base Titrations 18.5 Indicators. IB Standards. 18.1.1 State the expression for the ionic product constant of water (K w ).

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Acids and Bases II IB Chemistry Gr.12

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  1. Acids and Bases IIIB Chemistry Gr.12 Chem2_Dr. Dura Topic 18

  2. Topics • 18.1 Calculations Involving Acids and Bases • 18.2 Buffer Solutions • 18.3 Salt Hydrolysis • 18.4 Acid-Base Titrations • 18.5 Indicators Chem2_Dr. Dura

  3. IB Standards • 18.1.1 State the expression for the ionic product constant of water (Kw). • 18.1.2 Deduce [H+(aq)] and [OH-(aq)], for water at different temperatures given Kw values. • 18.1.3 Solve problems involving [H+(aq)], [OH-(aq)], pH and pOH. • 18.1.4 State the equation for the reaction for any weak acid or weak base with water, and hence deduce the expressions for Ka and Kb. Chem2_Dr. Dura

  4. IB Standards • 18.1.5 Solve problems involving solutions of weak acids and bases using the expressions: Ka x Kb = Kw pKa + pKb = pKw pH + pOH = pKw • 18.1.6 Identify the relative strengths of acids and bases using values of Ka, Kb, pKa and pKb. Chem2_Dr. Dura

  5. [H+][OH-] Kc = [H2O] H2O (l) H+(aq) + OH-(aq) The Ion Product of Water [H2O] = constant Kc[H2O] = Kw = [H+][OH-] The ion-product constant (Kw) is the product of the molar concentrations of H+ and OH- ions at a particular temperature. Chem2_Dr. Dura Solution Is [H+] = [OH-] neutral At 250C Kw = [H+][OH-] = 1.0 x 10-14 [H+] > [OH-] acidic [H+] < [OH-] basic

  6. Kw is temperature dependent Chem2_Dr. Dura

  7. pH [H+] pH = - log [H+], [H+] = 10-pH pOH = -log [OH-], [OH-] = 10-pOH At 250C Solution is neutral [H+] = [OH-] [H+] = 1 x 10-7 pH = 7 [H+] > 1 x 10-7 pH < 7 acidic [H+] > [OH-] Chem2_Dr. Dura basic [H+] < [OH-] [H+] < 1 x 10-7 pH > 7

  8. At 250C [H+][OH-] = Kw = 1.0 x 10-14 -log [H+] – log [OH-] = 14.00 pH + pOH = 14.00 Chem2_Dr. Dura

  9. Example 1: The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater? pH = -log [H+] = 10-4.82 = 1.5 x 10-5M [H+] = 10-pH Example 2: The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood? Chem2_Dr. Dura pH + pOH = 14.00 pOH = -log [OH-] = -log (2.5 x 10-7) = 6.60 pH = 14.00 – pOH = 14.00 – 6.60 = 7.40

  10. HA (aq) + H2O (l) H3O+(aq) + A-(aq) HA (aq) H+(aq) + A-(aq) [H+][A-] Ka = [HA] weak acid strength Ka Weak Acids (HA) and Acid Ionization Constants Ka is the acid ionization constant Chem2_Dr. Dura

  11. NH3(aq) + H2O (l) NH4+(aq) + OH-(aq) [NH4+][OH-] Kb = [NH3] weak base strength Kb Weak Bases and Base Ionization Constants Kb is the base ionization constant Chem2_Dr. Dura Solve weak base problems like weak acids except solve for [OH-] instead of [H+].

  12. Ionization Constants of Conjugate Acid-Base Pairs • Consider a weak acid (HA) and its conjugate base (A-) in water: HA (aq)  H+(aq) + A-(aq) • Ka = [H+][A-] / [HA] A-(aq) + H2O(l)  HA(aq) + OH-(aq) • Kb = [HA][OH-] /[A-] • Ka x Kb= [H+][OH-] = Kw Chem2_Dr. Dura

  13. IB Standards 18.2.1 Describe the composition of a buffer solution and explain its action. 18.2.2 Solve problems involving the composition and pH of a specified buffer system. 18.3 Deduce whether salts form acidic, alkaline or neutral aqueous solutions. Chem2_Dr. Dura

  14. Buffer Solutions • A buffer solutionis a solution of: • A weak acid or a weak base and • The salt of the weak acid or weak base • Both must be present! Chem2_Dr. Dura

  15. The presence of a common ion suppresses the ionization of a weak acid or a weak base. CH3COONa (s) Na+(aq) + CH3COO-(aq) common ion CH3COOH (aq) H+(aq) + CH3COO-(aq) The common ion effect is the shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance. Example: A buffer consisting of CH3COONa (salt) and CH3COOH (weak acid). Chem2_Dr. Dura

  16. How buffers work Consider an equal molar mixture of CH3COOH and CH3COONa H+(aq) + CH3COO-(aq) CH3COOH (aq) OH-(aq) + CH3COOH (aq) CH3COO-(aq) + H2O (l) • A buffer solution has the ability to resist changes in pH upon the addition of small amounts of either acid or base. Add strong acid Chem2_Dr. Dura Add strong base

  17. NaA (s) Na+(aq) + A-(aq) HA (aq) H+(aq) + A-(aq) Ka [HA] [H+][A-] [H+] = Ka = [HA] [A-] -log [H+] = -log Ka - log [HA] [A-] [A-] -log [H+] = -log Ka + log [HA] [HA] [A-] pH = pKa + log Consider mixture of salt NaA and weak acid HA. Chem2_Dr. Dura pKa = -log Ka

  18. [conjugate base] pH = pKa + log [acid] Henderson-Hasselbalch equation pH of buffers can be determined from Henderson-Hasselbalch Equation. Chem2_Dr. Dura

  19. Exercise: Which of the following are buffer systems? (a) KF/HF (b) KBr/HBr, (c) Na2CO3/NaHCO3 (a) KF is a weak acid and F- is its conjugate base buffer solution (b) HBr is a strong acid not a buffer solution Chem2_Dr. Dura (c) CO32- is a weak base and HCO3- is its conjugate acid buffer solution

  20. NH4+(aq) H+(aq) + NH3(aq) pH = 9.25 + log [0.25] [0.30] 0.028 0.025 [NH3] pH = pKa + log [NH4+] [0.36] [0.28] 0.10 0.10 NH4+(aq) + OH-(aq) H2O (l) + NH3(aq) pH = 9.25 + log [NH4+] = [NH3] = Problem: Calculate the pH of the 0.30 M NH3/0.36 M NH4Cl buffer system. What is the pH after the addition of 20.0 mL of 0.050 M NaOH to 80.0 mL of the buffer solution? pKa= 9.25 = 9.17 0.024 start (moles) 0.029 0.001 Chem2_Dr. Dura end (moles) 0.028 0.0 0.025 final volume = 80.0 mL + 20.0 mL = 100 mL = 9.20

  21. H2O NaCl (s)Na+ (aq) + Cl- (aq) H2O NaCH3COOH (s)Na+ (aq) + CH3COO- (aq) CH3COO-(aq) + H2O (l) CH3COOH (aq) + OH-(aq) Acid-Base Properties of Salts Neutral Solutions: Salts containing an alkali metal or alkaline earth metal ion (except Be2+) and the conjugate base of a strong acid (e.g. Cl-, Br-, and NO3-). Basic Solutions: Salts derived from a strong base and a weak acid. Chem2_Dr. Dura

  22. H2O NH4Cl (s)NH4+ (aq) + Cl- (aq) NH4+(aq) NH3(aq) + H+(aq) 3+ 2+ Al(H2O)6(aq) Al(OH)(H2O)5(aq) + H+(aq) Acid-Base Properties of Salts Acid Solutions: Salts derived from a strong acid and a weak base. Salts with small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+) and the conjugate base of a strong acid. Chem2_Dr. Dura

  23. Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: • Kb for the anion > Ka for the cation, solution will be basic • Kb for the anion < Ka for the cation, solution will be acidic • Kb for the anion Ka for the cation, solution will be neutral Chem2_Dr. Dura

  24. IB Standards • 18.4.1Sketch the general shapes of graphs of pH and volume for titrations of strong and weak acids and bases and explain their important features. • 18.5.1 Describe qualitatively the action of acid-base indicator. • 18.5.2 State and explain how the pH range of an acid-base indicator relates to its pKa value. • 18.5.3 Identify an appropriate indicator for a titration given the equivalence point of titration and the pH range of the indicator. Chem2_Dr. Dura

  25. Titrations In a titration a solution of accurately known concentration is added gradually to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point – the point at which the reaction is complete Indicator – substance that changes color at (or near) the equivalence point Chem2_Dr. Dura Slowly add base to unknown acid UNTIL The indicator changes color (pink)

  26. NaOH (aq) + HCl (aq) H2O (l) + NaCl (aq) OH-(aq) + H+(aq) H2O (l) Strong Acid-Strong Base Titrations Chem2_Dr. Dura

  27. CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l) CH3COOH (aq) + OH-(aq) CH3COO-(aq) + H2O (l) CH3COO-(aq) + H2O (l) OH-(aq) + CH3COOH (aq) Weak Acid-Strong Base Titrations At equivalence point (pH > 7): Chem2_Dr. Dura

  28. HCl (aq) + NH3(aq) NH4Cl (aq) H+(aq) + NH3(aq) NH4Cl (aq) NH4+(aq) + H2O (l) NH3(aq) + H+(aq) Strong Acid-Weak Base Titrations At equivalence point (pH < 7): Chem2_Dr. Dura

  29. HIn (aq) H+ (aq) + In- (aq) [HIn] [HIn]  10  10 [In-] [In-] Acid-Base Indicators Color of acid (HIn) predominates Color of conjugate base (In-) predominates Chem2_Dr. Dura

  30. The titration curve of a strong acid with a strong base. Chem2_Dr. Dura

  31. Which indicator(s) would you use for a titration of HNO2 with KOH ? Weak acid titrated with strong base. At equivalence point, will have conjugate base of weak acid. At equivalence point, pH > 7 Use cresol red or phenolphthalein Chem2_Dr. Dura

  32. H2O NH4Cl (s)NH4+ (aq) + Cl- (aq) NH4+(aq) NH3(aq) + H+(aq) 3+ 2+ Al(H2O)6(aq) Al(OH)(H2O)5(aq) + H+(aq) Acid-Base Properties of Salts Acid Solutions: Salts derived from a strong acid and a weak base. Salts with small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+) and the conjugate base of a strong acid. Chem2_Dr. Dura

  33. Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: • Kb for the anion > Ka for the cation, solution will be basic • Kb for the anion < Ka for the cation, solution will be acidic • Kb for the anion Ka for the cation, solution will be neutral Chem2_Dr. Dura

  34. NaHCO3 (aq) + HCl (aq) NaCl (aq) + H2O (l) + CO2 (g) Mg(OH)2 (s) + 2HCl (aq) MgCl2 (aq) + 2H2O (l) Chemistry In Action: Antacids and the Stomach pH Balance Chem2_Dr. Dura

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