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Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry , 2007 (John Wiley)      ISBN: 9 78047081 0866. CHEM1002 [Part 2]. Dr Michela Simone Weeks 8 – 13 Office Hours: Monday 3-5, Friday 4-5

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  1. Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille,Chemistry, 2007 (John Wiley)     ISBN: 9 78047081 0866

  2. CHEM1002 [Part 2] Dr Michela Simone Weeks 8 – 13 Office Hours: Monday 3-5, Friday 4-5 Room: 412A (or 416) Phone: 93512830 e-mail:michela.simone@sydney.edu.au

  3. Acids & Bases • Lecture 3: • Salts of Acids and Bases • Buffer systems • Blackman Chapter 11, Sections 11.3-11.6 • Lecture 4: • Titrations • Blackman Chapter 11, Section 11.7 Reproduced from ‘The Extraordinary Chemistry of Ordinary Things, C.H. Snyder, Wiley, 2002(Page 245)

  4. Strong acids completed dissociates in solution: HA + H2O  A-(aq) + H3O+(aq) pH = -log10([strong acid]initial) If strong base is added, it reacts with the H3O+ so [H3O+(aq)] is reduced: H3O+(aq) + OH-(aq)  H2O(l) pH = -log10([strong acid]remaining) Equivalence point: when the amount of added base = initial amount of acid: [H+(aq)] = 10-7.0 M pH = 7.00 After the equivalence point, any added base increases [OH-(aq)]: pH = 14.00 - pOH = 14.00 - (-log10([excess base])) Strong Acid/Strong Base Titration

  5. Initial pH is pH = 14 - pOH = 14 -(-log10([strong base]initial) At the equivalence point, [H+(aq)] = 10-7.0M pH = 7.00 (at 25 °C) After the equivalence point, pH = -log10([excess acid]) Strong Base/Strong Acid Titration

  6. Strong Base/Strong Acid Titration Add HCl(g) in 0.10 mol amounts to 1.0 L of 0.5 M NaOH(aq)

  7. Add NaOH(s) in 0.10 mol amounts to 1.0 L of 0.5 M acetic acid Initial pH: initially the solution contains just acetic acid and its pH must be calculated using the procedure outlined in slide 7 of lecture 2. pKa = 4.76. Weak Acid/Strong Base Titration Ka =

  8. Add NaOH(s) in 0.10 mol amounts to 1.0 L of 0.50 M acetic acid Acidic region: in the acidic region, OH- has reacted with some of the acetic acid to make acetate. The solution then contains both acetic acid and its conjugate base and the Henderson-Hasselbalch equation can be used to calculate the pH. Weak Acid/Strong Base Titration • CH3COOH(aq) + OH-(aq)  CH3COO-(aq) + H2O(aq) cf lecture 3

  9. Add NaOH(s) in 0.10 mol amounts to 1.0 L of 0.50 M acetic acid Equivalence point: all of the acetic acid has reacted with OH- to form CH3COO-. At this point, the solution contains a weak base so is basic. The pOH must be calculated. Weak Acid/Strong Base Titration Kb = cf lecture 2

  10. Add NaOH(s) in 0.10 mol amounts to 1.0 L of 0.50 M acetic acid Alkaline region: after the equivalence point, the major source of OH- is that from the excess NaOH. Very little is contributed from the acetate ion. After the equivalence point, the calculations are then exactly the same as for the strong acid/strong base titration. When 0.60 mol of NaOH(s) is added, 0.50 mol react with the CH3COOH leaving a 0.10 M solution of OH-: Weak Acid/Strong Base Titration

  11. Initial pH is higher than for strong acid / strong base A weak acid is present initially Weak Acid/Strong Base Titration • In acidic region, weak acid and conjugate base present • Buffering region with slow change in pH • At the 1/2 equivalence point, pH = pKa • At equivalence point, conjugate base present • pH > 7 • In alkaline region, excess strong base present • Curve is the same as for strong acid/strong base titration

  12. Weak Acid/Strong Base Titrations

  13. Weak Base/Strong Acid Titration

  14. Equivalence Point: When number of moles of added base = original number of moles of acid Strong acid/strong base pH = 7 Weak acid/strong base pH > 7 Strong acid/weak base pH < 7 End Point: When a colour change in the indicator is observed Choose an indicator that changes colour close to the equivalence point Titrations

  15. weak acid base Indicators • each form has a different colour • The pH at which acid  base depends on the pKaof the indicator pH 3.2 pH 4.4 pH 4.8 pH 5.4 pH 6.0 pH 7.6 pH 8.2 pH 10.0 methyl orange methyl purple bromothymol blue phenolphthalein

  16. 12 10 8 pH 6 4 2 10 20 30 40 50 Amount of solution added (mL) Practice Examples

  17. Summary: Acids & Bases 4 • Learning Outcomes - you should now be able to: • Complete the worksheet • Understand strong acid/strong base, strong base/strong acid, weak acid/strong base and weak base/strong acid titrations • Be able to extract pKa from the half equivalence point • Be able to work at the pH at the equivalence point • Answer Review Problems 11.38-11.44 in Blackman • Next lecture: • Periodic Trends

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