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Acids & Bases CHAPTER 16 (& part of CHAPTER 17)

Acids & Bases CHAPTER 16 (& part of CHAPTER 17) Chemistry: The Molecular Nature of Matter, 6 th edition By Jesperson , Brady, & Hyslop. CHAPTER 16: Acids & Bases. Learning Objectives: Define Brønsted -Lowry Acid/Base Define Lewis Acid/Base Evaluate the strength of acids/bases

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Acids & Bases CHAPTER 16 (& part of CHAPTER 17)

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  1. Acids & Bases CHAPTER 16 (& part of CHAPTER 17) Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop

  2. CHAPTER 16: Acids & Bases • Learning Objectives: • Define Brønsted-Lowry Acid/Base • Define Lewis Acid/Base • Evaluate the strength of acids/bases • Strong vs weak acids/bases • Periodic trends • Conjugate acids/bases • Identify likely compounds that will form acids and bases from the periodic table • Acidic metal ions • Acid/Base equilibrium: • pH, pOH • Ka, Kb, pKa, pKb • Kw of water Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  3. CHAPTER 16: Acids & Bases Lecture Road Map: Brønsted-Lowry Acids/Bases Trends in acid strength Lewis Acids & Bases Acidity of hydrated metal ions Acid/Base equilibrium Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  4. CHAPTER 16 Acids & Bases Acid/Base Equilibrium Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  5. Acid/Base Equilibrium Weak Acids & Bases • Incompletely ionized • Molecules and ions exist in equilibrium • HA = any weak acid; B = any weak base HA(aq) + H2O A–(aq) + H3O+(aq) B (aq) + H2O B H+(aq) + OH–(aq) CH3COOH(aq) + H2O CH3COO–(aq) + H3O+(aq) HSO3–(aq) + H2O SO32–(aq) + H3O+(aq) NH4+(aq) + H2O NH3 (aq) + H3O+ (aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  6. Acid/Base Equilibrium Weak Acids & Bases • Often simplify as • HA (aq) A –(aq) + H+(aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  7. Acid/Base Equilibrium Weak Acids & Bases Acid + Water Conjugate Base + Hydronium Ion Or generally HA(aq) + H2O A–(aq) + H3O+(aq) • But [H2O] = constant (55.6 M ) so rewrite as • Where Ka = acid ionization constant Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  8. Acid/Base Equilibrium Weak Acids & Bases Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  9. Acid/Base Equilibrium Weak Acids & Bases CH3COO–(aq) + H2O CH3COOH(aq) + OH–(aq) NH3(aq) + H2O NH4+(aq) + OH–(aq) • Or generally B (aq) + H2O B H+(aq) + OH–(aq) But [H2O] = constant so rewrite as Where Kb = base ionization constant Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  10. Acid/Base Equilibrium Weak Acids & Bases Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  11. Acid/Base Equilibrium pH • Lots of weak acids and bases • How can we quantify their relative strengths? • Need reference • Choose H2O • Water under right voltage • Slight conductivity • Where does conductivity come from? Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  12. Acid/Base Equilibrium pH • Trace ionization  self-ionization of water • H2O + H2O H3O+(aq) + OH–(aq) acidbaseacidbase • Equilibrium law is: • But [H2O]pure = = 55.6 M [H2O] = constant even for dilute solutions Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  13. Acid/Base Equilibrium pH H2O + H2O H3O+(aq) + OH–(aq) • Since [H2O] = constant, equilibrium law is • K w = is called the ion product of water • Often omit second H2O molecule and write • H2O H+(aq) + OH–(aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  14. Acid/Base Equilibrium Kw H2O H+(aq) + OH–(aq) • For pure H2O at 25 °C • [H+] = [OH–] = 1.0 × 10–7M • Kw= (1.0 × 10–7)(1.0 × 10–7) = 1.0 × 10–14 • See Table 17.1 for K wat various temperatures • H2O auto-ionization occurs in all solutions • When other ions present • [H+] is usually NOT equal to [OH–] • But Kw= [H+][OH–] = 1.0 × 10–14 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  15. Acid/Base Equilibrium Definition of Acidic & Basic • In aqueous solution, • Product of [H3O+] and [OH–] equals K w • [H3O+] and [OH–] may not actually equal each other • Solutions are classified on the relative concentrations of [H3O+] and [OH–] Solution Classification Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  16. Acid/Base Equilibrium Weak Acids & Bases: Example Ex. 1 In a sample of blood at 25 °C, [H+] = 4.6  10–8M. Find [OH–] and determine if the solution is acidic, basic or neutral. • So 2.2 × 10–7M > 4.6 × 10–8M • [OH–] > [H3O+] so the solution is basic Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  17. Acid/Base Equilibrium pH • Arrhenius (of kinetics fame) • Sought an easy way to write the very small numbers associated with [H+] and [OH–] • Developed the “p” notation where p stands for the –log mathematical operation • Result is a simple number • pH is defined as: • Define pOH as: • Define pKw as: • Take anti-log to obtain [H+], [OH–] or Kw Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  18. Acid/Base Equilibrium General Properties of Logarithms Using Logarithms • Start with • Taking –log of both sides of eqn. gives • So at 25 °C: Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  19. Acid/Base Equilibrium Definition of Acidic and Basic • As pHincreases, [H+] decreases; pOH decreases, and [OH–] increases • As pH decreases, [H+] increases; pOHincreases, and [OH–] decreases Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  20. Acid/Base Equilibrium Measuring pH • pH meter • Most accurate • Calibrate with solutions of known pH before use • Electrode sensitive to [H+] • Accurate to  0.01 pH unit • Acid-base indicator • Dyes, change color depending on [H+] in solution • Used in pH paper and titrations • Give pH to  1 pH unit • Litmus paper • RedpH  4.7 acidic • Blue pH  4.7 basic • Strictly acidic vs. basic Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  21. Acid/Base Equilibrium Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  22. Acid/Base Equilibrium Example pH Calculations Calculate pH and pOH of blood in Ex.1. We found [H+] = 4.6 × 10–8M [OH–] = 2.2 × 10–7 M pH = –log(4.6 × 10–8) = 7.34 pOH = –log(2.2 × 10–7) = 6.66 14.00 = pKw Or pOH = 14.00 – pH = 14.00 – 7.34 = 6.66 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  23. Acid/Base Equilibrium Example pH Calculations What is the pH of NaOH solution at 25 °C in which the OH– concentration is 0.0026 M? [OH–] = 0.0026 M pOH = –log(0.0026) = 2.59 pH = 14.00 – pOH = 14.00 – 2.59 = 11.41 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  24. Acid/Base Equilibrium Strong Acids • Assume 100% dissociated in solution • Good ~ if dilute • Makes calculating [H+] and [OH] easier • 1 mole H+ for every 1 mole HX • So [H+] = CHX for strong acids • Thus, if 0.040 M HClO4 • [H+] = 0.040 M • And pH = –log (0.040) = 1.40 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  25. Acid/Base Equilibrium Strong Bases • 1 mole OH– for every 1 mole M OH • [OH–] = CMOH for strong bases • 2 mole OH– for 1 mole M(OH)2 • [OH–] = for strong bases Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  26. Acid/Base Equilibrium Effect of Auto ionization of Water with Strong Bases • The auto-ionization of H2O will always add to [H+] and [OH–] of an acid or base. Does this have an effect on the last answer? • The previous problem had 0.00022 M [OH–] from the Ca(OH)2 but the [H+] must have come from water. If it came from water an equal amount of [OH–] comes from water and the total [OH–] is • [OH–]total = [OH–]from Ca(OH)2 + [OH–]from H2O • [OH–]total= 0.00022 M + 4.6 × 10–11 M = 0.00022 M (when properly rounded) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  27. Acid/Base Equilibrium Example pH Calculations • So [H+] from H2O must also be 5.0  10–13 M • [H+]total = 0.020 M + (5.0  10–13M) = 0.020 M (when properly rounded) • So we see that [H+]from H2O will be negligible except in very dilute solutions of acids and bases Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  28. Acid/Base Equilibrium Looking at Weak Acids Again What is the pKa of HF if Ka = 3.5 × 10–4? HF(aq) + H2O F–(aq) + H3O+(aq) or HF(aq) F–(aq) + H+(aq) = 3.5 × 10–4 pKa= –log Ka = –log(3.5 × 10–4) = 3.46 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  29. Acid/Base Equilibrium Conjugate Acid-Base Pairs 1. Consider ionization reaction of generic acid and water HA(aq) + H2O A–(aq) + H3O+(aq) 2. Consider reaction of a salt containing anion of this acid (its conjugate base) with water A–(aq) + H2O HA(aq) + OH–(aq) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  30. Acid/Base Equilibrium Conjugate Acid-Base Pairs HA(aq) + H2O A–(aq) + H3O+(aq) A–(aq) + H2O HA(aq) + OH–(aq) 2H2O H3O+(aq) + OH–(aq) For any conjugate acid base pair: (at 25 °C) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

  31. Acid/Base Equilibrium More About Logarithms • Then taking –logof both sides of equation gives: • Earlier we learned the inverse relationship of conjugate acid-base strengths, now we have numbers to illustrate this. • The stronger the conjugate acid, the weaker the conjugate base. So (at 25 °C) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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