Models of Acids and Bases

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Acids, Bases, and Aqueous Equilibria
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Nature of Acids and Bases

• Acids--
• Sour taste, corrosive to metals
• Bases--
• Bitter taste, feel slippery, corrosive to fat
• Dont use these to identify acids/bases in lab!

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Definitions of Acids and Bases

• Arrhenius Concept Acids produce H in solution,
  bases produce OH? ion.
• Brønsted-Lowry Acids are H donors, bases are
  proton acceptors.
• HCl H2O ? Cl? H3O
• acid base

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Conjugate Acid/Base Pairs

• HA(aq) H2O(l) ? H3O(aq) A?(aq)
• conj conj
• acid 1 base 2 acid 2
  base 1
• conjugate base everything that remains of the
  acid molecule after a proton is lost.
• conjugate acid formed when the proton is
  transferred to the base.

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Acid Dissociation Constant (Ka)

• An equilibrium exists in water solutions of acids
  
• HA(aq) H2O(l) ? H3O(aq) A?(aq)
• or HA(aq) ? H (aq) A- (aq)

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Example 14.1

• Give dissociation reactions for these HCl,
  HC2H3O2, NH4, C6H5NH3
• HCl ? H Cl-
• HC2H3O2 ? H C2H3O2-
• NH4 ? H NH3
• C6H5NH3 ? H C6H5NH2

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Acid Strength
Strong Acid

• Its equilibrium position lies far to the right.
  (HNO3, HCl, HBr, HI, HClO4, H2SO4) Ka
  gtgt 1 these are the ONLY strong acids
• Yields a weak conjugate base. (NO3?, or others
  from above acids)
• H2SO4 is only strong in its 1st H

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Acid Strength(continued)
Weak Acid

• Its equilibrium lies far to the left. (CH3COOH,
  and other organic acids) Ka ltlt 1
• Yields a much stronger (it is relatively strong)
  conjugate base than water. (CH3COO?)

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Types of Acids

• Binary Acids Hydrogen bonded to elements other
  than oxygen, which has acid characteristics HCl,
  HCN, H2S
• Oxyacids Hydrogen bonded to a polyatomic ion
  containing oxygenH2CO3, H3PO4
• Organic Acidscontain the carboxyl group,
• OH
• -CO which are all weak acids

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Strong Acids
Weak Acids
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Strong
Weak
Weak
Strong
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Example 14.2

• From the previous slide, arrange these bases from
  weak to stronger H2O, F-, Cl-, NO2-, CN-
• Cl- is from strong acid, as is H2O (from H3O),
  so both are very weak. CN- is from the weakest
  acid and is therefore the strongest. HF is a
  stronger acid than HNO2, so NO2- is stronger than
  F-, so the ranking from weak to strong is
• Cl- lt H2O lt F- lt NO2- lt CN-

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Water as an Acid and a Base

• Water is amphoteric (it can behave either as an
  acid or a base).
• H2O H2O ? H3O OH?
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  conj conj
• acid base acid base
• Kw 1 ? 10?14 at 25C H OH-
• Must always be a balance between H and OH-

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Example 14.3

• Calculate H and OH- in these solutions
• a. 1.0 x 10-5 M OH-
• b. 1.0 x 10-7 M OH-
• c. 10.0 M H
• OH- 1.0 x 10-5 M H Kw / OH- 1 x
  10-14/ 1.0 x 10-5 1.0 x 10-9 M
• OH- 1.0 x 10-7 M H Kw / OH- 1 x
  10-14/ 1.0 x 10-7 1.0 x 10-7 M
• H 10.0 M OH- Kw / H 1 x
  10-14/10.0 1.0 x 10-15 M

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The pH Scale

• There is a more convenient way to indicate H
• pH ? ?logH
• pH in water normally ranges from 0 to 14, but can
  extend to negative or gt14 values
• Kw 1.00 ? 10?14 H OH?
• pKw 14.00 pH pOH
• As pH rises, pOH falls (sum 14.00).

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Example 14.5

• Calculate pH and pOH for each of these
• solutions a. 1.0 x 10-3 M OH- b. 1.0 M H
• -log OH- 3 pOH pH 14 pOH 11
• b. -log H 0 pH pOH 14 pH 14

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Example 14.6 7

• If pH of blood is 7.41, find pOH, H, and OH-
• pOH 14-pH 6.59 H 10-pH 10-7.41 3.9 x
  10-8 M
• OH- 10-6.59 2.6 x 10-7 M
• Calculate pH for 0.10 M HNO3 and 1 x 10-10 M HCl
• Since both are strong acids, they are totally
  dissociated, and H acid strength of the
  major species. pH of HNO3 is therefore
  log(0.10) 1. But the HCl solution is so
  dilute that the water provides most of the H,
  and so the pH7.

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HOMEWORK!!

• p. 687ff
• 12, 13, 17, 20, 24, 26, 28

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Solving Weak Acid Equilibrium Problems

• List major species in solution.
• Choose species that can produce H and write
  reactions.
• Based on K values, decide on dominant
  equilibrium.
• Write equilibrium expression for dominant
  equilibrium.
• List initial concentrations in dominant
  equilibrium. (I)

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Solving Weak Acid Equilibrium Problems (continued)

• Define change at equilibrium (as x). (C)
• Write equilibrium concentrations in terms of x.
  (E)
• Substitute equilibrium concentrations into
  equilibrium expression.
• Solve for x the easy way.
• Verify assumptions using 5 rule.
• Calculate H and pH.

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Example 14.8

• Calculate the pH of a 0.100 M solution of HOCl
  (Ka3.5 x 10-8)
• HOCl H OCl-
• I 0.1 0 0
• C -x x x
• E 0.1-x x x

x2 3.5 x 10-8(0.1) 3.5 x 10-9 x 5.9 x 10-5 M
H pH -log(5.9 x 10-5) 4.23
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Percent Dissociation (Ionization)
Calculate this from H
This becomes greater as the acid concentration
becomes more diluteExample 14.10
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Example 14.10

• Calculate percent dissociation for a. 1.00 M
  HC2H3O2 and b. 0.100 M HC2H3O2 Ka 1.8 x
  10-5
• Acid H A- b. Acid H
  A-
• I 1.00 0 0 0.100
  0 0
• C -x x x -x
  x x
• E 1-x x x 0.1-x
  x x
• 1.8 x 10-5 x2 / 1 1.8 x 10-5 x2 / 0.1
• x H 4.2 x 10-3 M x 1.3 x 10-3 M
• diss 4.2 x 10-3 / 1.0 diss 1.3 x 10-3
  / 0.100
• 0.42 1.3

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Homework

• p. 689ff
• 37, 39, 43, 49, 52, 54

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Bases

• Strong and weak are used in the same sense
  for bases as for acids.
• strong complete dissociation (hydroxide ion
  supplied to solution) Most common are metal
  hydroxides. Kb is very large.
• NaOH(s) ? Na(aq) OH?(aq)

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Example 14.12

• Calculate pH of 5.0 x 10-2 M NaOH
• Strong base means OH- NaOH 0.05 M
• pOH -log(0.05) 1.30
• pH 14- pOH 12.70

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Bases(continued)

• weak very little dissociation (or reaction with
  water) Usually contain an -NHn group
• H3CNH2(aq) H2O(l) ? H3CNH3(aq) OH?(aq)
• Kb for weak bases is usually very small lt 10-3

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Calculations Involving Weak Bases
Use the same ICE method as with weak acids Notice
that x will equal OH- rather than
H Calculate pOH and from that calculate pH
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Example 14.13

• Calculate the pH of a 15.0 M solution of NH3
• Kb 1.8 x 10-5 NH3 NH4 OH-
• I 15.0 0 0
• C -x x x
• E 15-x x x
• 1.8 x 10-5 x2 / 15 x2 1.8 x 10-5 (15)2.7 x
  10-4
• x 1.6 x 10-2 M OH- pOH 1.80
• pH 14-pOH12.2

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Polyprotic Acids

• . . . can furnish more than one proton (H) to
  the solution. Each one comes off separately.

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Acid-Base Properties of Salts
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Relationship of Ka to Kb

• For acidic/basic salts, Ka or Kb must be
  calculated from the parent acid/base value
• Kb Kw Ka Kw
• Ka Kb
• Example
• Ka of HC2H3O2 1.8 x 10-5
• Kb 1 x 10-14 / 1.8 x 10-5 5.6 x 10-10

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Structure and Acid-Base Properties

• Two factors for acidity in binary compounds
• Bond Polarity (high is good)
• Bond Strength (low is good)

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Oxides

• Acidic Oxides (Acid Anhydrides)
• O?X bond is strong and covalent.
• SO2, NO2, CrO3
• Basic Oxides (Basic Anhydrides)
• O?X bond is ionic.
• K2O, CaO

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Lewis Acids and Bases

• Lewis Acid electron pair acceptor
• Lewis Base electron pair donor

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Group Homework

• p. 691ff
• 59, 62, 66, 71, 72, 77, 80, 84, 88, 94