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Chapter 19 - Reactions of Acids and Bases

Chapter 19 - Reactions of Acids and Bases. Water is amphoteric - can act as either an acid or base. H 2 O H + + OH - (acting as acid) H 2 O + H + H 3 O + (acting as base) Water forms an equilibrium with itself H 2 O + H 2 O H 3 O + + OH -

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Chapter 19 - Reactions of Acids and Bases

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  1. Chapter 19 - Reactions of Acids and Bases

  2. Water is amphoteric - can act as either an acid or base • H2O H+ + OH- (acting as acid) • H2O + H+ H3O+ (acting as base) • Water forms an equilibrium with itself • H2O + H2O H3O+ + OH- [H3O+] [OH-] Keq = ---------------- [H2O]2

  3. Keq [H2O]2 = [H3O+] [OH-] • H2O is a constant (the amount does not change much on percent basis) • Kw = Keq [H2O]2 = [H3O+] [OH-] • Kw = [H3O+] [OH-] = 1.0 x 10-14 • Kw is the ion product constant for water • Kw = x2 = 1.0 x 10-14 • x = 1.0 x 10-7

  4. If [H3O+] > 1.0 x 10-7 the solution is acidic • If [H3O+] < 1.0 x 10-7 the solution is basic • See sample problem #1 • Assignment – practice problems 1 & 2 on page 628

  5. pH scale – a more compact way to express [H3O+] • pH = -log [H3O+] • If pH < 7 the solution is acidic • If pH > 7 the solution is basic • Example problem: What is the pH if [H3O+] = 7.3 x 10-5 M? pH = -log (7.3 x 10-5 ) = 4.14 Do practice problems 3 & 4 on page 630

  6. More on pH • Measuring pH – use acid-base indicators (will change color with pH) Strong acids dissociate completely. What is the pH of a .1 M solution of HCl? What is the pH of a 1 M solution of HCl? What is the pH of a 6 M solution of HCl? One is not the lowest pH we can have.

  7. Buffers – resist change in pH by absorbing or releasing H+ ions. • The most common buffers are mixtures of weak acids and their conjugate bases. • Example buffers: HC2H3O2 + NaC2H3O2 Acetic acid and its conjugate base the C2H3O2- ion NH4Cl and NH3 (acid and conj. Base)

  8. How a buffer works show diagram

  9. The buffering capacity is limited by the amount of HC2H3O2 and NaC2H3O2. If we run out of either the buffering capacity is exceeded. • Equal amount of acetic acid and acetate ion stablize the pH near 4.7 • We can derive this from the Ka of acetic acid, which is 1.8 x 10-5

  10. [H3O+] [C2H3O2-] Ka = ---------------- = 1.8 x 10-5 [HC2H3O2] If [C2H3O2-] and [HC2H3O2] are equal they cancel out of the equation Ka = [H3O+] = 1.8 x 10-5 M pH = -log 1.8 x 10-5 = 4.74

  11. Experimental Determination of acid concentration • The concentration of a very strong acid can be determined by pH. • The concentration of a weak acid is more accurately determined by an acid/base titration. • An acid-base titration is a carefully controlled neutralization reaction. (see page 637)

  12. Acid – Base Titration • Equivalence point – when we have added just enough base to neutralize all of the acid. • End point – when we are at the color change point of a titration • Phenolphthalein indicator – changes color over a pH range of 8 – 10 (clear to red)

  13. Do example problem on page 638 together in class • Assignment – problems 5 & 6 on page 638 • Questions page 646-647 (1-19, 32, 33) • Problem bank 37, 39, 41, 43, 45, 47, 49, 51, 53, 54, 56, 58, 60 (answers on page 945)

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