Acids and Bases

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Acids and Bases

• Chapter 16

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Homework pages 532 535 1-7, 9 14, 16, 17,
20 - 34, 37, 40, 42, 44, 45, 47, 52 56, 58,
59, 61 - 63, 66, 68, 71, 72 74, 82.
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Acids
Have a sour taste. Vinegar owes its taste to
acetic acid. Citrus fruits contain citric acid.
React with certain metals to produce hydrogen gas.
React with carbonates and bicarbonates to produce
carbon dioxide gas
Bases
Have a bitter taste.
Feel slippery. Many soaps contain bases.
4.3
4
Arrhenius acid is a substance that produces H
(H3O) in water
Arrhenius base is a substance that produces OH-
in water
4.3
5
A Brønsted acid is a proton donor A Brønsted base
is a proton acceptor
acid
base
acid
base
conjugate base
conjugate acid
acid
base
15.1
6
The conjugate base of an acid is the species
formed when one proton is removed. The conjugate
acid of a base is the species formed when one
proton is added. A conjugate acid-base pair
differ by a proton. acid base
conjugate base conjugate acid What is
conjugate base of HF, H2SO4, NH3? What is
conjugate acid of O2-, SO42-, NH3?
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The Conjugate Pairs in SomeAcid-Base Reactions
Reaction 1 HF H2O
F H3O Reaction
2 HCOOH CN
HCOO HCN Reaction 3 NH4
CO32 NH3
HCO3 Reaction 4 H2PO4
OH HPO42
H2O Reaction 5 H2SO4 N2H5
HSO4
N2H62 Reaction 6 HPO42
SO32 PO43 HSO3
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Identifying Conjugate Acid-Base Pairs

• Problem The following chemical reactions are
  important for industrial
• processes. Identify the conjugate acid-base
  pairs.
• HSO4-(aq) CN-(aq) SO42-(aq)
  HCN(aq)
• (b) ClO-(aq) H2O(l)
  HClO(aq) OH-(aq)
• (c) S2-(aq) H2O(aq) HS-(aq)
  OH-(aq)
• (d) HS-(aq) H2O(aq) H2S(aq)
  OH-(aq)

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Acid-Base Properties of Water
autoionization of water
conjugate acid
base
conjugate base
acid
15.2
10
The Ion Product of Water
H2O constant
KcH2O Kw HOH-
The ion-product constant (Kw) is the product of
the molar concentrations of H and OH- ions at a
particular temperature.
Solution Is
H OH-
neutral
At 250C Kw HOH- 1.0 x 10-14
H gt OH-
acidic
H lt OH-
basic
15.2
11
What is the concentration of OH- ions in a HCl
solution whose hydrogen ion concentration is 1.3
M?
Kw HOH- 1.0 x 10-14
H 1.3 M
7.7 x 10-15 M
What is the H in 0.035M NaOH?
15.2
12
pH A Measure of Acidity
pH -log H
Solution Is
At 250C
H OH-
neutral
H 1 x 10-7
pH 7
H gt OH-
acidic
H gt 1 x 10-7
pH lt 7
H lt OH-
basic
H lt 1 x 10-7
pH gt 7
15.3
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pOH -log OH-
HOH- Kw 1.0 x 10-14
-log H log OH- 14.00
pH pOH 14.00
15.3
14
The pH of rainwater collected in a certain region
of the northeastern United States on a particular
day was 4.82. What is the H ion concentration
of the rainwater?
pH -log H
H 10-pH
10-4.82
1.5 x 10-5 M
pH pOH 14.00
pOH -log OH-
-log (2.5 x 10-7)
6.60
pH 14.00 pOH 14.00 6.60 7.40
15.3
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Calculating H3O, pH, OH-, and pOH
Problem A chemist dilutes concentrated
hydrochloric acid to make two solutions (a) 3.0
M and (b) 0.0024 M. Calculate the H3O, pH,
OH-, and pOH of the two solutions at
25C. What is the H3O, OH-, and pOH of a
solution with pH 3.67? With pH 8.05?


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Strong Electrolyte 100 dissociation
Weak Electrolyte not completely dissociated
Strong Acids are strong electrolytes
15.4
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Weak Acids are weak electrolytes
Strong Bases are strong electrolytes
15.4
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Weak Bases are weak electrolytes (NH3)

• Conjugate acid-base pairs
• The conjugate base of a strong acid has no
  measurable strength.
• H3O is the strongest acid that can exist in
  aqueous solution.
• The OH- ion is the strongest base that can exist
  in aqueous solution.

15.4
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15.4
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Strong Acid
Weak Acid
15.4
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What is the pH of a 2 x 10-3 M HNO3 solution?
HNO3 is a strong acid 100 dissociation.
0.0 M
0.0 M
Start
0.002 M
0.002 M
0.002 M
0.0 M
End
pH -log H -log H3O -log(0.002) 2.7
Ba(OH)2 is a strong base 100 dissociation.
0.0 M
0.0 M
Start
0.018 M
0.018 M
0.036 M
0.0 M
End
pH 14.00 pOH 14.00 log(0.036) 12.56
15.4
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Weak Acids (HA) and Acid Ionization Constants
Ka is the acid ionization constant
15.5
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15.5
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What is the pH of a 0.5 M HF solution (at 250C)?
Initial (M)
0.50
0.00
0.00
Change (M)
-x
x
x
Equilibrium (M)
0.50 - x
x
x
0.50 x ? 0.50
Ka ltlt 1
x2 3.55 x 10-4
x 0.019 M
H F- 0.019 M
pH -log H 1.72
HF 0.50 x 0.48 M
15.5
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When can I use the approximation?
0.50 x ? 0.50
Ka ltlt 1
When x is less than 5 of the value from which it
is subtracted.
Less than 5 Approximation ok.
x 0.019
What is the pH of a 0.05 M HF solution (at 250C)?
x 0.006 M
More than 5 Approximation not ok.
Must solve for x exactly using quadratic equation
or method of successive approximation.
15.5
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Finding the Ka of a Weak Acid from the pH of its
Solution and Vice Versa
Problem The weak acid hypochlorous acid is
formed in bleach solutions. If the pH of a 0.12
M solution of HClO is 4.19, what is the value of
the Ka of this weak acid? What is the
ionization? Ka 3.5 x 10-8
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• Solving weak acid ionization problems
• Identify the major species that can affect the
  pH.
• In most cases, you can ignore the autoionization
  of water.
• Ignore OH- because it is determined by H.
• Use ICE to express the equilibrium concentrations
  in terms of single unknown x.
• Write Ka in terms of equilibrium concentrations.
  Solve for x by the approximation method. If
  approximation is not valid, solve for x exactly.
• Calculate concentrations of all species and/or pH
  of the solution.

15.5
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What is the pH of a 0.122 M monoprotic acid whose
Ka is 5.7 x 10-4?
Initial (M)
0.122
0.00
0.00
Change (M)
-x
x
x
Equilibrium (M)
0.122 - x
x
x
0.122 x ? 0.122
Ka ltlt 1
x2 6.95 x 10-5
x 0.0083 M
More than 5 Approximation not ok.
15.5
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x2 0.00057x 6.95 x 10-5 0
ax2 bx c 0
x 0.0081
x - 0.0081
H x 0.0081 M
pH -logH 2.09
15.5
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percent ionization
For a monoprotic acid HA
HA0 initial concentration
15.5
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The Stepwise Dissociation of Phosphoric Acid
Phosphoric acid is a weak acid, and normally only
loses one proton in solution, but it will lose
all three when reacted with a strong base with
heat. The ionization constants are given for
comparison.
H3PO4 (aq) H2O(l)
H2PO4-(aq) H3O(aq)
H2PO4-(aq) H2O(l)
HPO42-(aq) H3O(aq)
HPO42-(aq) H2O(l) PO43-(aq)
H3O(aq)
H3PO4 (aq) 3 H2O(l) PO43-(aq)
3 H3O(aq)
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15.8
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Terms to know diprotic and polyprotic Calculate
the concentration of all species in 0.10 M H2S.
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Weak Bases and Base Ionization Constants
Kb is the base ionization constant
15.6
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The base-dissociation constant Kb refers to
the equilibrium that occurs when a weak base is
added to water. Ammonia and amines are the
most common molecules that act as weak
bases. Most anions can act as weak bases.
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15.6
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Determining pH from Kb and Initial B
Problem Ammonia is commonly used cleaning agent
in households and is a weak base, with a Kb of
1.8 x 10-5. What is the pH of a 1.5 M
NH3 solution?
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Ionization Constants of Conjugate Acid-Base Pairs
Ka
Kb
Kw
KaKb Kw
Weak Acid and Its Conjugate Base
15.7
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All anions can act as weak bases except those
that are the conjugate bases of the strong
acids.
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Determining the pH of a Salt Solution
Problem Sodium cyanide in water can produce a
basic solution. What is the pH of a 0.25 M
solution of NaCN? Ka for HCN is 4.9 x 10-10
Plan We have to find the pH of a
solution of the cyanide ion, CN -, which acts
as a base in water
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The stronger an acid, the weaker is its
conjugate base and vice versa. An acid-base
reaction proceeds to the greater extent in the
direction to form the weaker acid and base. If
the weaker acid and base are on the right Kc gt 1
Sample Problem H2PO4-(aq) NH3(aq)
NH4(aq) HPO42-(aq) H2O(l) HS-(aq)
OH-(aq) H2S(aq)
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The reaction of an ion with water is
frequently called hydrolysis. Salt solutions are
usually acidic or basic because of hydrolysis of
the cation or anion. Cations all cations act as
acids except those in groups 1 and 2. Anions A
few are acids HSO4- Those that are conjugate
bases of strong acids are neutral. (Cl-, NO3-,
) All others are bases.
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Acid-Base Properties of Salts
Acid Solutions
It is easy to see how a salt like NH4Cl can
produce an acidic solution.
How can salts Al(NO3)3, CrCl3, or FeBr3 produce
acidic solutions?
15.10
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Acid Hydrolysis of Al3
15.10
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Problem 16.62 Which of these salts will undergo
hydrolysis? KF, NaNO3, NH4I, KCN, C6H5COONa, RbI,
Na2CO3, CaCl2, HCOOK, AlCl3. Are the solutions
acidic, basic or neutral? Write hydrolysis
equation.
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Definition of An Acid
Arrhenius acid is a substance that produces H
(H3O) in water
A Brønsted acid is a proton donor
A Lewis acid is a substance that can accept a
pair of electrons
A Lewis base is a substance that can donate a
pair of electrons
H
acid
base
acid
base
15.12
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Lewis Acids and Bases

acid
base
No protons donated or accepted!
15.12
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Problem 16.74 Describe this equation according
to the Lewis theory of acids and bases. AlCl3
Cl- ? AlCl4-