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Acid-Base Theories and Definitions

Explore the Arrhenius and Bronsted-Lowry theories of acids and bases, including the concepts of conjugate acids and bases, amphoteric substances, and the ionization of water.

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Acid-Base Theories and Definitions

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  1. Chapter 16 Acids & Bases

  2. Acid-Base Theory • How would you describe an acid or a base? • Similar questions about acids and bases have been asked ever since their discovery in the 1200’s • Because of these curiosities, several theories were generated in attempts to clarify what makes an acid an acid (or a base a base)

  3. Acid-Base Theory - Arrhenius • Arrhenius defined the terms acid and base as follows: • An acid is a substance that produces H+ ions when dissolved in water (now described as hydronium rather than H+). • A base is a substance that produces OH- ions when dissolved in water

  4. Hydronium Ion (H3O+) or (H+(aq))

  5. Acid-Base Theory - Arrhenius • We can demonstrate these definitions as follows: H2O HNO3(l) H+(aq) + NO3-(aq) H2O Ca(OH)2(s) Ca2+(aq) + 2OH-(aq)

  6. Acid-Base Theory - Arrhenius • Are there any errors, or oversights, in the Arrhenius Theory? • Consider ammonia, NH3, which is a well known base: • What happens when ammonia is placed in water? H2O NH3(g) ????

  7. Brnsted-Lowry Theory • Bronsted and Lowry came up with an alternative definition for acids and bases

  8. Bronsted Acids & Bronsted Bases • A bronsted acid is a substance from which a proton (H+) can be removed (proton donor). • A bronsted baseis a substance that can remove a proton from an acid (proton acceptor).

  9. Conjugate Acids & Bases • A conjugate base is formed when an acid releases a proton (H+). • A conjugate acid is formed when a base accepts a proton (H+).

  10. Acids and Conjugate Bases (Page 491)

  11. Bases and Conjugate Acids (Page 491)

  12. Bronsted Acid – Base Reactions • An acid and a base that are related by the gain and loss of a proton are called a conjugate acid-base pair. • Every acid has associated with it a conjugate base. • Likewise, every base has associated with it a conjugate acid.

  13. Conjugate Acid – Base Pairs

  14. Conjugate Acid – Base Pairs

  15. Water is amphoteric

  16. Amphoteric • A substance that may either gain or lose a proton is amphoteric. • In other words it can act like either a Bronsted acid or a Bronsted base. • Amphoteric substances include hydrogen containing anions, hydroxides of some metals (especially those near the metal/nonmetal dividing line).

  17. What ion do you think is present in the solution below? • For every ion in a solid or solution there must be an ion of opposite charge, a counter ion, in order for the solid or solution to be electrically neutral. Cu2+

  18. Hydronium ion is the conjugate acid of water and hydroxide ion is the conjugate base of water. Self – ionization (autoionization) of water Water can react with itself in a process known as self – ionization (autoionization).

  19. Self – ionization of water • Occurs to a very slight extent (≈2 in every billion water molecules self – ionize at 25°C). Write the equilibrium expression for this reaction.

  20. Ion Product of water Kw = [H3O+][OH-] Kw = [H3O+][OH-] = 1.0 x 10-14 (at 25°C)° Kw = [H3O+][OH-] = 1.0 x 10-12 (at 100°C)° Is this reaction exothermic or endothermic? Support your answer.

  21. How does adding and acid (i.e. HCl) to water affect [H3O+], [OH-] and the Kw? • How does adding and base (i.e. NaOH) to water affect [H3O+], [OH-] and the Kw?

  22. Weak Acids • Weak acids are much less than 100% ionized in water. • One of the best known is acetic acid (CH3CO2H)

  23. A

  24. Strong Bases • Strong Base:100% dissociated in water. NaOH (aq) → Na+ (aq) + OH- (aq)

  25. Weak Bases • Weak base:less than 100% ionized in water • One of the best known weak bases is ammonia NH3 (aq) + H2O (l) ↔ NH4+ (aq) + OH- (aq)

  26. CH3CO2H is added to H2O • Write the reaction that occurs. • Write the equilibrium expression for the reaction.

  27. Equilibrium Constants for Weak Acids • A Weak acid has a Ka • The higher the Ka the stronger the acid

  28. NH3 is added to H2O • Write the reaction that occurs. • Write the equilibrium expression for the reaction.

  29. Equilibrium Constants for Weak Bases • A Weak base has a Kb • The higher the Kb the stronger the base

  30. Appendix F and G in the front of your notebook contain many Ka and Kb values. • This is not a complete listing of weak acids and weak bases however it does seem that there are more weak acids.

  31. We can use Ka and Kb values to rank the strength of weak acids and weak bases.

  32. Which weak acid below is the strongest?

  33. The higher the Ka the stronger the acid.

  34. Why does a higher Ka value indicate that an acid is stronger? For two acids of the same molarity the one with the higher Ka is stronger because it dissociates more. This increases the concentration of the products (more specifically hydronium).

  35. Which weak base below is the strongest?

  36. Why does a higher Kb value indicate that an base is stronger? For two bases of the same molarity the one with the higher Kb is stronger because it dissociates more. This increases the concentration of the products (more specifically hydroxide).

  37. Relative Strengths of Conjugate Pairs

  38. Ka x Kb = Kw = 1 x 10-14 • This equation is used to determine the Ka or Kb value for an acid or base within a conjugate pair. • Teacher Example: Determine the Kb of acetate ion. • (See Appendix F) Ka CH3CO2H = 1.8 x 10-5 • 1.8 x 10-5 x Kb = 1 x 10-14 • Kb CH3CO2- = 5.6 x 10-10

  39. What is the Kb of PO43-? • Ka HPO42- = 3.6 x 10-13 • For any conjugate pair: Ka x Kb = Kw = 1 x 10-14 • Kb for PO43- = 2.8 x 10-2

  40. For the conjugate pair: HPO42- &PO43- • See Appendix F and G in the front of your notebook • Ka HPO42- = 3.6 x 10-13 Ka x Kb = Kw = 1 x 10-14 • Kb for PO43- = 2.8 x 10-2

  41. Conjugate Pair Acid – Base Strength Comparison CH3CO2H and CH3CO2- • Ka CH3CO2H = 1.8 x 10-5 • Kb CH3CO2- = 5.6 x 10-10 HPO42- andPO43- • Ka HPO42- = 3.6 x 10-13 • Kb for PO43- = 2.8 x 10-2

  42. Is NH4+ a stronger acid than HCN? Support your answer.

  43. Acid Base Neutralization Acid + Base→ Salt + Water • These reactions go to completion. (→)

  44. Acid Base Neutralization Acid+Base→ Salt +Water HCl + NaOH → NaCl + H2O 2HNO3 + Ba(OH)2 → Ba(NO3)2 + 2H2O • salt(an ionic compound containing the cation from a base and the anion from an acid)

  45. Acid – Base Neutralization • Assume stoichiometric amounts of acid and base. • A strong acid and a strong base form a neutral solution. • A strong acid and a weak base form an acidic solution. • A weak acid and a strong base form a basic solution. • A weak acid and a weak base can form an acidic, basic, or neutral solution. • The salt formed in the neutralization reaction is responsible for the observations listed above.

  46. Neutralization Reaction • A neutralization reaction occurs when an acid and a base react to form water and a salt(an ionic compound containing the cation from a base and the anion from an acid) NaOH + HCl → NaCl + H2O KOH + CH3CO2H → KCH3CO2 + H2O Is the solution that results from each reaction acidic, basic or neutral?

  47. Neutralization Reaction NaOH + HCl → NaCl + H2O (neutral) KOH + CH3CO2H → KCH3CO2 + H2O (basic) Why are the solutions that result from neutralization reactions acidic, basic or neutral? NaCl is a neutral salt and KCH3CO2 is a basic salt

  48. Acidic, Basic, and Neutral Salts * Cations from groups I and II * * * All anions other than those from strong acids or HSO4- and H2PO4- * * All cations other than those from strong bases

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