Chapter 14: Acids and Bases

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Chapter 14 Acids and Bases

• General Properties
• Acid sour taste, turns blue litmus red
• Base bitter taste, turns red litmus blue
• Arrhenius Definitions (ca. 1890)
• Acid substance which forms H in water
• Base substance which forms OH in water
• Oldest and most limited definition.
• Note H(aq), hydrogen ion, may also be written
  as H3O(aq), hydronium ion.

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Figure 14.1The Reaction of HCI and H2O

• H2O HCl H3O
  Cl

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• Bronsted-Lowry Definition (1923)
• Acid proton (H) donor
• Base proton acceptor
• Conjugate acid-base pair two species which
  differ by one H.
• ex. HCl / Cl, H2SO4 / HSO4, HSO4 / SO42
• HCl H2O ? H3O Cl
• Acid 1 Base 2 Acid 2
  Base 1

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• H2O can act as either an acid or a base. Such a
  substance (H donor or acceptor) is called an
  amphoteric or amphiprotic substance.
• In the previous equation, water is a base.
• H2O(l) NH3(aq) ? NH4(aq) OH(aq)
• Here, water acts as an acid.
• Polyprotic Acid can donate more than one H.
• ex. H2SO4, H3PO4

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• The Self-Ionization of Water
• H2O(l) ? H(aq) OH(aq)
• Or 2 H2O(l) ? H3O(aq) OH(aq)
• This equilibrium exists in every aqueous
  solution.
• Now, lets write the mass-action expression

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• However, water is the solvent and its
  concentration is about 55.6 M in dilute aqueous
  solutions. This concentration is so large that
  it effectively remains constant.
• Define a new equilibrium constant, Kw Kc
  H2O
• This is called the ion product for water.
• Now, Kw H OH 1.00 x 1014 at 25C
• This expression is true in every aqueous solution.

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• In pure water, H OH 1.00 x 107 M.
• This is called a neutral solution.
• When H gt 1.00 x 107 M and OH lt 1.00 x 107
  M, the solution is acidic.
• When H lt 1.00 x 107 M and OH gt 1.00 x 107
  M, the solution is basic.
• These small values can be awkward to deal with,
  so the pH scale was devised by Sorenson to
  measure relative acidity or basicity.

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• pH log10 H
• Exs. When H 1.00 x 104 M, pH 4.000
• When H 6.25 x 105 M, pH 4.204
• We must also be able to convert from pH to H.
• Ex. When pH 4.60, H 104.60 2.5 x
  105 M
• Similarly, pOH log10 OH
• Note that pH pOH pKw 14.00

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Figure 14.8The pH Scale and pH Values of Some
Common Substances
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• The pH Square