Aqueous Equilibria: Acids and Bases

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Aqueous Equilibria Acids and Bases

• Chemistry 4th Edition
• McMurry/Fay

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AcidBase Concepts 01
Arrhenius Acid Substance which dissociates in
water to form hydrogen ions (H) in solution
HA (aq) ? H (aq) A (aq)
Ex HCl(aq) H2O(l) ? H3O(aq) Cl(aq)
Arrhenius Base Substance that dissociates in
water, or reacts with water, to form hydroxide
ions (OH) MOH (aq) ? M (aq)
OH (aq) Ex KOH(aq) ?
K(aq) OH(aq)
OLDdefinitions
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AcidBase Concepts 02
NEWdefinitions

• Brønsted-Lowry Acid Substance that can donate H
• Brønsted-Lowry Base Substance that can accept
  H (often has a lone pair of electrons)
• Compounds whose formulas differ only by a
  proton are said to be conjugate acidbase pairs.

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BrønstedLowry Definition
Give products, and identify the BrønstedLowry
acid, base, and conjugate acid/base. HBr
H2O ? Br H3O NH2
CH3OH ? CH3O NH3 HNO3
PH3 ? NO3 PH4 H2O
tBuO ? tBuOH HO
conj.base
conj.acid
..
base
acid
base
..
conj. acid
conj.base
acid
..
base
acid
base
acid
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AcidBase Concepts 03
Example
hydrochloric acidHCl (aq)
Strong Acid
no HA
(strong electrolyte)
Weak Acid
acetic acidCH3CO2H
water or alcohol H2O CH3CH2OH
Very Weak Acid
no H3O A-
(non-electrolyte)
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AcidBase Concepts 04
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Know these Acids and Bases!
Strong Acids HCl Hydrochloric
Acid HNO3 Nitric Acid H2SO4 Sulfuric Acid
Strong Bases NaOH Sodium Hydroxide
KOH Potassium Hydroxide Ca(OH)2 Calcium
Hydroxide
Weak Acids HF Hydrofluoric
Acid CH3CO2H Acetic Acid
Weak Bases NH3 Ammonia
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Dissociation of Water 01

• Water act as both an acid and as a base 2
  H2O(l) H3O(aq) OH(aq)
• This is called the auto-ionization of water.
  
• H2O(l) H2O(l) ? H3O(aq) OH(aq)

What is the position of this equilibrium?
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H2O(l) H(aq) OH(aq)

• Write the equilibrium expression for this
  reaction( called the ion product constant for
  water )
• Kc H OH 1.0 x 1014
• The Kc for water called Kw. What is the
  equilibrium constant value? (Are products or
  reactants favored?)
• Kw is very small, so this reaction mixture
  will be nearly ALL starting material.

at RT only
Kw
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Dissociation of Water 02

• H2O(l) H(aq) OH(aq)
• By the balanced equation, we see H OH.
  So if H OH 1.0 x 1014 (at
  RT)
• then H 1.0 x 10-7
• log H -7
• -log H 7
• We will call the negative log of the H
  concentration the pH of the aqueous solution.
• The pH of pure water is 7, which is the neutral
  point.

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pH A Measure of Acidity 01

• The pH of a solution is the negative log of the
  H concentration
• pH log H
• Neutral solutions H 1.0 x 107 M, pH
  7.00 OH- 1.0 x 10-7 M also
• Acidic solutions H gt 1.0 x 107 M,
  pH lt 7.00 Example 0.001 M HCl H 1.0
  x 10-3 pH 3
• Basic solutions H lt 1.0 x 107 M, pH gt
  7.00

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Acid/Base Concepts pH Scale
Fig on p. 621 of text
Remember that pH is a log scale. If pH of coffee
is 5 and pH of vinegar is 3,then vinegar is 100
times more acidic than coffee.
pH 5 H 10-5
pH 3 H 10-3
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pH Calculations 03

• 1. The concentration of aqueous nitric acid
  (HNO3) at RT is 0.045M. What is the pH of
  this solution?
• pH -log H -log (0.045) 1.35
• 2. A basic solution has a pH of 10.5. What is
  H?
• H inv log (-10.5) 3.16 x 10-11
  M

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pH Calculations 03

• 3. In a solution where the H 4.8 x 10-12M,
  find a) the OH conc. b) the pH
• (Hint use Kw which is an equilibrium
  constant.)
• H x OH 1.0 x 10-14
• OH 1.0 x 10-14 / H
• 1.0 x 10-14 / 4.8 x 10-12
• 2.08 x 10-3 M
• pH -log(4.8 x 10-12) 11.3

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p OH Base Concentration 01

• pH negative log of H pH log H
• Define
• pOH negative log of OH pOH log OH
• Since H x OH- 1.0 x 10-14
  Kw
• then pH pOH 14

If pH is 3.4, what is the pOH? pH pOH
14 pOH 11.6 If pH is 12.7, what is
pOH? 12.7 pOH 14 pOH 1.3
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Dissociation of Water 03

• The concentration of OH ions in a certain
  household ammonia cleaning solution is 0.0025 M.
  Calculate pH.

1. Use Kw to find the H concentration, then
calc. pH. - or -
1. H x OH- 1.0 x 10-14 Kw
H 1x10-14/(0.0025) 4.0 x 10-12
pH -log(4.0x10-12) 11.40
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Dissociation of Water 03

• The concentration of OH ions in a certain
  household ammonia cleaning solution is 0.0025 M.
  Calculate pH.

1. Use Kw to find the H concentration, then
calc. pH. - or -2.
Calculate pOH, then find pH (pH pOH 14)
2. pOH -log(0.0025) 2.60 pH
pOH 14 pH 14 pOH 11.40
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Acid / Base Review

• Strong acids and bases ionize completely in
  water
• HA H2O A- (aq) H3O (aq)
  at equilib 0 100
• Weak acids and bases ionize only slightly in
  water
• HA H2O A- (aq) H3O
  (aq) at equilib 98.5 1.5
  99.96 0.04
• Solutions of weak acids and bases contain ionized
  AND non-ionized species.

or
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Acid / Base Review

• Write balanced equations for the dissociation of
  each of the following BrønstedLowry acids.
• (a) H2SO4
• H2SO4 (aq) H2O(l) HSO4
  (aq) H3O (aq)
• (b) HSO4
• HSO4 SO42 H
  
• (c) H4N
• H4N (aq) H2O(l) H3N (aq)
  H3O (aq)

better
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Acid / Base Review

• Write balanced equations for the reaction of each
  of the following BrønstedLowry bases.
• (a) NaOH
• NaOH (aq) H Na (aq)
  H2O (l)
• (b) NH3
• NH3 (aq) H2O(l) H4N (aq) OH
  (aq)

strong base
weak base
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Strength of Acids and Bases 02

• If an acid is strong (ex HCl), its conjugate
  base (Cl) has no measurable base strength.
• A weak acid (ex HF) has a conjugate base (F)
  that is a weak base.
• H3O is the strongest acid that can exist in
  aqueous solution.
• OH ion is the strongest base that can exist in
  aqueous solution.

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Indicators Color Depends on pH
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Acid Dissociation Constants 01

• Acid Dissociation Constant the equilibrium
  constant for the ionization of an acid. HA(aq)
  H2O(l) H3O(aq) A(aq)
• Or simply HA(aq) H(aq) A(aq)

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Base Dissociation Constants 01

• Base Dissociation Constant (Kb) the equilibrium
  constant for the reaction of a base w/ a
  proton.NH3(aq) H2O(l) NH4(aq)
  OH(aq)

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Weak Acid Dissociation Constants
ACID Ka
CONJ. BASE Kb
7.1 x 10 4 4.5 x 10 4 3.0 x 10 4 1.7 x 10
4 8.0 x 10 5 6.5 x 10 5 1.8 x 10 5 4.9 x 10
10 1.3 x 10 10
HF HNO2 C9H8O4 (aspirin) HCO2H (formic) C6H8O6
(ascorbic) C6H5CO2H (benzoic) CH3CO2H
(acetic) HCN C6H5OH (phenol)
F NO2 C9H7O4 HCO2 C6H7O6 C6H5CO2
CH3CO2 CN C6H5O
1.4 x 10 11 2.2 x 10 11 3.3 x 10 11 5.9 x 10
11 1.3 x 10 10 1.5 x 10 10 5.6 x 10 10 2.0 x
10 5 7.7 x 10 5
WeakeRbase
STRONGER Acid
WeakeRacid
STRONGER Conj. base
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Strength of Acids and Bases 04

• Stronger acid stronger base ?
• weaker acid weaker base
• Predict the direction of the following
• HNO2(aq) CN(aq) HCN(aq) NO2(aq)
• HF(aq) NH3(aq) F(aq) NH4(aq)

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Percent Dissociation 10

• Concentration Dependence

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Base Ionization Constants 02
BASE Kb
CONJ. ACID Ka
5.6 x 10 4 4.4 x 10 4 4.1 x 10 4 1.8 x 10
5 1.7 x 10 9 3.8 x 10 10 1.5 x 10 14
C2H5NH2 (ethylamine) CH3NH2 (methylamine) C8H10N4O
2 (caffeine) NH3 (ammonia) C5H5N
(pyridine) C6H5NH2 (aniline) NH2CONH2 (urea)
C2H5NH3 CH3NH3 C8H11N4O2 NH4 C5H6N C6H5NH3 N
H2CONH3
1.8 x 10 11 2.3 x 10 11 2.4 x 10 11 5.6 x 10
10 5.9 x 10 6 2.6 x 10 5 0.67
Note that the positive charge sits on the
nitrogen.