Chapter 19 Reactions of Acids and Bases

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Chapter 19 - Reactions of Acids and Bases
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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 ----------------
• H2O2

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• Keq H2O2 H3O OH-
• H2O is a constant (the amount does not change
  much on percent basis)
• Kw Keq H2O2 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

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• If H3O gt 1.0 x 10-7 the solution is acidic
• If H3O lt 1.0 x 10-7 the solution is basic
• See sample problem 1
• Assignment practice problems 1 2 on page 628

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pH scale a more compact way to express H3O

• pH -log H3O
• If pH lt 7 the solution is acidic
• If pH gt 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

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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.

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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)

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How a buffer works

• show diagram

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• 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

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• 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

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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)

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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)

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• 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)