Acids and Bases

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Acids and Bases

• Chapter 16

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Topics of Discussion

• Acid-Base Concepts
• Arrhenius Concept
• Bronsted-Lowry Concept
• Lewis Concept
• Acid and Base Strengths
• Relative Strength of Acids and Bases
• Molecular Structure and Acid Strength
• Self-Ionization of Water and pH
• Self-Ionization of Water
• Solutions of Strong Acid or Base
• The pH of Solution

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Acid-Base Concepts

• Acids sour taste, turns litmus blue to red
• Bases bitter taste, turns litmus red to blue
• Arrhenius Concept
• Acids increase ?H? in aqueous solution.
• Bases increase ?OH-? in aqueous solution.
• Strong acid/base completely ionizes in aqueous
  solution to produce H3O/OH- and an ion.
• HClO4(aq) H2O ? H3O(aq) ClO4-(aq)
• SA HClO4, HClO3, H2SO4, HI, HBr, HCl and HNO3
• SB LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2
• Neutralization of a strong acid by a strong base
  yield the same ?H0 per mole of water formed.
• H3O Cl- Na OH- ? Na Cl- H2O

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Acid-Base Concepts-II

• Bronsted-Lowry Concept
• An acid donates a proton in a proton-transfer
  reaction.
• A base accepts a proton in a proton-transfer
  reaction.
• In any reversible acid-base reaction, both
  forward and reverse reactions involve proton
  transfer.
• NH3(aq) H2O ? NH4(aq) OH-(aq)
• base acid acid
  base
• The conjugate acid-base pair consists of two
  species that differ by the loss or gain of a
  proton.
• NH3-NH4, H2O-OH- conjugate acid-base pair.

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Acid-Base Concepts-III

• Amphiprotic species can donate or accept proton.
• Lewis Concept
• An acid can form a covalent bond by accepting an
  electron pair.
• A base can form a covalent bond by donating an
  electron pair.
• H NH3 ? NH4
• The formation of complex ions is an acid-base
  reaction according to Lewis concept.
• Al3 6H2O ? ?Al(H2O)6? 3

Base
Acid
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Example (Lewis Acid)

• Identify the Lewis acid and base. Write chemical
  equation using electron dot formulas.

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Acid Base Strengths

• The stronger acids lose their protons more
  easily.
• The stronger bases hold on to protons more
  strongly.
• HCl(aq) H2O ? H3O(aq) Cl-(aq)
• Since HCl is strong acid it completely ionizes in
  water. HCl (in fact any strong acid) is stronger
  than H3O. The same reaction shows H2O is
  stronger base than Cl-.

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Example (Acid-Base Strength)

• Only about 1 of acetic acid molecules ionize in
  0.1 M HC2H3 O2(aq).
• HC2H3 O2(aq) H2O ? H3O(aq) C2H3 O2- (aq)
• HC2H3 O2 (a weak acid) is weaker than H3O. The
  strongest acids have the weakest conjugate bases.

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Acid Base Strengths-II

• The direction for an acid-base reaction always
  favors the weaker acid and weaker base.
• Determine the direction of following reaction
• H2S(aq) C2H3 O2- (aq) ? HC2H3O2(aq) HS-(aq)
• The reaction goes from right to left. Reactants
  are favored

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Molecular Structure and Acid Strength

• The strength of an acid depends on how easily the
  proton is removed from an H-X bond.
• The more polarized the bond, the more easily the
  proton is removed and the greater the acid
  strength.
• The larger the atom X, the weaker is the bond and
  the greater the acid strength.
• In going down a column, size of an atom
  increases, the bond strength decreases, acid
  strength increases.
• HFltHClltHBrltHI
• Going across a row, the electronegativity
  increases, the bond polarity increases, acid
  strength increases. H2OltHF

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Strength of Oxoacids

• Oxoacids contain in addition to H and O one other
  element. H-O-Y- (acidic H is always bonded to O)
• The strength of oxoacids depends on bond polarity
• For oxoacids of the same structure, differing
  only in Y, the acid strength increases with EN of
  Y.
• HIOltHBrOltHClO, H-O-Y
• For oxoacids of (HO)m-YOn, the acid strength
  increases with n, the number of O atoms bonded to
  Y
• HClOltHClO2ltHClO3lt HClO4, H2SO3lt H2SO4
• The acid strength of polyprotic acids and its
  anions decreases with increasing negative charge.
• HSO4-lt H2SO4, HPO42-lt H2PO4-lt H3PO4

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Ionization of water

• H2O H2O ? H3O OH-
• H3OOH- KC H2O2 KW
• KW is ion product of water
• Kw 1.0x10-14 (at 25 0C)

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The pH concept

• pH is a logarithmic system to express very small
  numbers
• pH -logH
• H 10-pH
• In general pX -logX
• pKW -logKW -log(1x10-14)
• -(-14) 14

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Equilibrium constant is temperature dependent
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pH pOH 14
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pH calculations

• A carbonate beverage was found to have a hydrogen
  ion concentration of 3.67x10-5 mol/L. What is the
  pH and pOH of this beverage?
• Solution
• pH -log(3.67x10-5)
• 3.44
• pOH 14 - 3.44 10.56

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Measuring pH

• Acid-Base Indicators

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pH meter
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A glass electrode
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pH of dilute solutions of strong acids and bases

• Self ionization of water
• H or OH- contribution from acid or base,
  respectively.
• If strong acid or base concentration is greater
  than 1x10-6 M, ignore ionization of water
• H total Hsolute Hwater

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Example

• What is the pH of 0.00035 M Ba(OH)2?
• Solution
• Ba(OH)2 ? Ba2 2OH-
• OH- 2(0.00035) 0.00070 M
• pOH 3.15
• pH 10.85

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Example

• What is the pH of 1.5x10-7 M HCl solution?
• Answer
• H HHCl Hwater
• Cl- OH-water
• 1.5x10-7 KW/H
• H2 - 1.5x10-7H -1x10-14 0
• H 2x10-7
• 6.7