Acid and Base Equilibrium

1
Acid and Base Equilibrium

• Chapter 16 Brown LeMay

2
Basic Concepts

• Acids sour or tart taste, electrolytes,
  described by Arrhenius as sub that inc. the H
  conc.
• Bases bitter to the taste, slippery,
  electrolytes, describes by Arrhenius as sub that
  inc. the OH- conc.

3
Dissociation of Water

• Pure water exists almost entirely of water
  molecules. It is essentially a non-electrolyte.
• Water ionizes to a small extent auto-ionization
  
• The equilibrium expression is
• H20(l) lt-gt H(aq) OH-(aq)
• Kw H OH-

4

• Since the H ion does not exist alone in water Kw
  is often expressed
• Kw H3O OH-
• because water conc is constant it does not
  appear in the expression

• The proton in water

5
Values for Kw and H and OH-

• Kw H3O OH- 1.0 x 10-14
• 1.0 x 10-14 X X
• 1.0 x 10-14 X2
• X 1.0 X 10-7 H3O OH-

6
The Bronsted Lowry definition

• holds true for situations not involving water
• Acids donate protons
• Bases accept protons

7

• HCl(g) NH3(g) ?? NH4(g) Cl-(aq)\
• donates H accepts H conj acid conj
  base
• BL-Acid BL-Base
• notice that the reaction doesnt happen in water
  and that the OH- concentration has not increased

8
Conjugate Acids Bases and Amphoteric Substances

• HNO2(aq) H2O(l) ?? NO2-(aq)H3O(aq)
• 
  conjugate
• acid base base acid
• donates - accepts protons
• note HNO2 is considered a Arrenius acid H inc in
  H20 also a BL because donates a Proton
• all Arrenius acids and bases are also BL acids
  and bases however all BL acids and bases may or
  may not be arrenius acids or bases water is not
  an Arrenius base in this example

9

• every acid has a conjugate base formed from the
  removal of a proton from that acid
• every base has a conjugate acid formed from the
  addition of a proton to that base

10
Amphotheric Substances

• NH3(aq)H20(l) ?? NH4(aq) OH-(aq)
• Base Acid Conj Acid Conj Base
• p-acc p-donar
• note water is acting as an acid in this reaction
  and a base in the previous one that makes it a
  amphortic substance

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Strengths of Acid, Bases

• The stronger the acid is the weaker its conjugate
  base (weaker acid ? stronger conj base)
• The stronger the base the weaker its conjugate
  acid (weaker base ? stronger conj acid)
• Stronger acids and bases ionize to a greater
  extent than do weak acids and bases.

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Strong Acids dissociate completely into ions

• HNO3(aq) ? H(aq) NO3-(aq)
• the production of H ions from the acid
  dominates ignore the H donated from the water
  it is insignificant
• The ? equilibrium lies so far to the right
  because HNO3 doesnt reform
• The neg log of the H from the acid determines
  pH.
• Strong bases also dissociate completely and the
  conc of OH- from the base is the only factor
  considered when calculating pH.

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Acids and Bases

• Strong Acids
• HClO4 prechloric
• H2SO4 sulfuric
• HI hydroiodic
• HCl hydochloric
• HBr hydrobromic
• HNO3 nitric

• Strong Bases
• GI hydroxides ex. NaOH,KOH
• G2 Hydroxides Sr(OH)2
• GI Oxides ex.
• Na2O, K2O
• GI,II Amides ex. KNH2,Ca(NH2)2

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The pH scale

• pH is defined as the neg log (base-10) of the
  H ion concentration
• pH -log H
• What is the pH of a neutral solution
• H 1.0 x 10-7
• pH -log 1.0 x 10-7
• pH 7

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Strong acids and the pH scale

• An acidic solution must have a H conc greater
  than 1.0 X 10-7 ex 1.0 X 10-6
• -log 1.0 X 10-6 pH 6
• What is the pH of a basic solution?
• A basic solution is one in which the OH-
• is greater than 1X10-7.

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Calc. pH of strong basic solutions

• Calculate the pH of a sol that has a OH- con.
  Of 1.0 X10-5
• Kw 1x10-14 H OH-
• Kw 1x10-14 H 1.0X10-5
• H 1x10-14 1.0 X 10-9
• 1.0X10-5
• pH -log 1.0 x 10-9 pH 9

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The p Scale

• The negative log of a quantity is labeled p
  (quantity)
• Ex we could reference the quantity of
• OH- directly pOH -logOH-
• From the definition of Kw
• -log Kw (-log H) (-log OH-) -log 1x10-4
• Kw pH pOH 14

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Calc. pH using the p scale

• Ex. OH- conc 1.0X10-5
• -log 1.0X10-5 5 pOH
• pH pOH 14
• pH 5 14
• pH 9

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Weak Acids

• partially ionize in aqueous solution
• mixture of ions and un-ionized acid in sol.
• WA are in equilibrium (H20 is left out because it
  a pure liquid)
• HA(aq) H20(l) ?? H30(aq) A-(aq)
• Ka is the acid dissociation constant
• Ka H30 A- H A-
• HA HA

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Acid Dissociation Constant

• The larger the Ka value the stronger the acid is
  more product is in solution

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Weak Bases

• Weak bases in water react to release a hydroxide
  (OH-) ion and their conjugate acid
• Weak Base(aq) H2O(l) Conjugate Acid(aq)
  OH-(aq)

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• A common weak base is ammonia
• NH3(aq) H2O(l) ?? NH4(aq) OH-(aq)
• Since H2O is a pure liquid it is not expressed in
  the equilibrium Kb expression
• Kb NH4OH- (base dissociation
• NH3 constant)
• Kb always refers to the equilibrium in which a
  base reacts with H2O to form the conjugate acid
  and OH-

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Lewis Acids and Bases

• Review - An Arrhenius acid reacts in water to
  release a proton - base reacts in water to
  release a hydroxide ion
• In the Bronstead-Lowry description of acids and
  bases acid reacts to donate a proton - a base
  accepts a proton

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• G.N.Lewis defination -
• Lewis acid is defined as an electron-pair
  acceptor
• Lewis base is defined as an electron-pair donor

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• In the example with ammonia, the ammonia is
  acting as a Lewis base (donates a pair of
  electrons), and the proton is a Lewis acid
  (accepts a pair of electrons)

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• Lewis is consistent with the description by
  Arrhenius, and with the definition by
  Bronstead-Lowry. However, the Lewis description,
  a base is not restricted in donating its
  electrons to a proton, it can donate them to any
  molecule that can accept them.

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Calculating the pH of a Weak Acid

• What is the pH of an aq sol that is 0.0030M
  pyruvic acid HC2H3P3? Ka 1.4x10-4 at 25oC
• HC2H3P3 ?? H C2H3P3-
• I 0.0030 0 0
• C -X X X
• E 0.0030-X X X

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• Ka H C2H3P3- pluggin in the values
• HC2H3P3 from the
  table
• 1.4x10-4 X2
• (0.0030-X)
• 1.4x10-4 (0.0030) X2
• 4.2x10-7-1.4x10-4x X2
• X2 1.4x10-4x-4.2x10-7 0 a quadratic
• ignore the neg sol x 5.82 x10-4
• pH -log 5.82 x10-4 pH 3.24

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Learning Check

• What is the pH at 25oC of a solution made by
  dissolving a 5.00 grain tablet of aspirin
  (acetylsalicylic acid) in 0.500 liters of water?
  The tablet contains 0.325g of the acid HC9H7O4.
  Ka 3.3x10-4 mm 180.2g/l
• H 9.4x10-4 pH 3.03

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Buffers

• A solution that resists changes in pH when a
  limited amount of an acid or base is added to it.
• Buffers contain either a weak acid and its conj.
  base or a weak base and its conj. acid.

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Examples

• Ex. Weak acid and conj. base equal molar
• amounts
• Strong Acid added
• H A- ? HA
• conj. base weak acid
• the conj base interacts with the H ions
• from the strong acid changing them to a
• weak acid

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• Strong base added
• OH HA ? HOH A-
• weak acid conj base
• the weak acid interacts with the OH- ion from
  the base to form water and the conj. base
• If the concentration of A- and HA are large
  and the amount of H or OH- is small the
• solution will be buffered or the change in pH
  will be minimized.

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• Buffering capacity the amount of acid or base a
  buffer can react with before a significant change
  in pH occurs
• Ratio of acid to conj base unless the ratio is
  close to 1 ( between 110 and 101) will be too
  low to be useful.

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Calculating the pH of a buffer

• Note a solution of 0.10 M acedic acid and its
  conj base 0.20 M acetate from sodium acetate is a
  buffer solution pH 5.07
• Ex. Calc. the pH of a buffer by mixing 60.0 ml of
  0.100 M NH3 with 40.0ml of 0.100 M NH4Cl.

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• 1St cal the conc. of each species
• M moles/liters
• mol of NH3 0.10M X/0.060 l 0.0060mol
• mol of NH4 0.10M X/0.040 l 0.0040mol
• NH3 0.0060 mol/ 0.100 l 0.060 M
• NH4 0.0040 mol/ 0.100 l 0.040 M
• NH3 H2O ?? NH4 OH-
• I 0.060M O.040 0
• C -X X X
• E 0.060-x 0.040x X

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• Kb NH4 OH- ( 0.040X)X
• 1.8X10-5 NH3 (0.060-X)
• Ignore X 1.8X10-5 0.040X X 2.7X10-5
• 0.060
• -log (2.7x10-5) 4.6 pOH pH 9.4
• Or using Henderson - Hasselbalch equation
• pOH pKb log conj acid 4.74 log (
  0.04
• B
  0.06)
• 4.6 pOH pH 9.4

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What is the pH of a buffer prepared by adding
30.0ml of .15M HC2H3O2 to 70ml of .2M NaC2H3O2?

• HC2H3O2 .15M x/.03 .0045/.01 .045
• C2H3O2 .20M x/.07 .0140/.01 .140
• ka 1.7x10-5
• HHeq pH pKa log conj base
• acid
• pH 4.77 log(.140 5.3
• .045)

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Adding an acid or a base to a buffer

• Calc the ph of 75ml of the buffer solution
  of(0.1M HC2H3O2 and 0.2M NaC2H3O2)
• to which 9.5 ml of 0.10M HCl has been added.
  Compare the change to that of adding HCl to pure
  water.
• H C2H3O2 - ?? HC2H3O2
• H 0.10M n/.0095l 0.00095 moles
• C2H3O2 - 0.2M n/.075 0.0150 moles
• HC2H3O2 0.10M n/.075 0.0075 moles

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• Neutralization Reaction
• C2H3O2 C2H3O2 moles H moles
• 0.0150n - .00095n 0.014
• HC2H3O2 Orginial Conc. Conc Contributed by
  reaction
• 0.075 moles 0.00095 0.0085mol
• C2H3O2 0.014mol/0.085l 0.16M
• HC2H3O2 0.0085/0.085 0.10M
• pH 4.76 log (.16/.10) 4.96