17 Acids and Bases
A group of chemical properties is related to acidity. According to these
properties, a substance can be called an acid or a base.
The word acid comes from Latin acere meaning sour. A base is an
alkaline, which is derived from Arabic alqali.
Presence of acids and bases have been recognized by ancient people.
Lavoisier named oxygen as the element from which acids are derived.
Liebig (German) proposed that an acid contains hydrogen.
Many chemical reactions are called acid-base reactions; they are not
necessarily neutralizations.
The acid-base concept is interesting, especially the way it evolved.
Understand acid or base, important
properties
of substances.
17 Acid and
Base
1
Answer these questions
What are acids?
What are bases?
Understand important properties of substances
What are acid-base reactions?
What are neutralization reactions?
What are the relationships between acids and bases?
What is the role of the solvent in acid or base solutions?
How does the acid-base concept evolve and why?
What are Arrhenius acid, Bronsted-Lowry acid and Lewis acids?
What is the relationship between conjugate acids and bases?
What does pH mean and how is the pH scale related to acidity?
What are salts?
17 Acid and Base
2
Arrhenius acids and bases
Sevante Arrhenius proposed that substances exists as ions in
solution in his dissertation, which was awarded a fourth class (D)
in 1884. He was unable to find a job in his native Sweden.
He was awarded the Nobel Prize in 1903 for his electrolytic
dissociation theory.
The fundamental concept: (Text PHH: 17-1)
Acid - any substance which delivers hydrogen ion (H+) to the solution.
HA  H+ + A¯
Base - any substance which delivers hydroxide ion (OH¯) to the solution.
BOH  X+ + OH¯
17 Acid and Base
3
Problems with Arrhenius theory
Like Dalton’s atomic theory, Arrhenius theory have problems today:
Acidity did not show in other solvent. What is the solvent role?
Some salts produce acidic or basic solutions, not neutral. Why?
Which is the base, NH3 or NH4OH? Is OH¯ really the only base?
How can H+ be stable? Are proton donated?
Some chemists want students to learn Brønsted-Lowry theory of acids
and bases before learning Arrhenius theory, because the former is
more general.
Question the established theory.
17 Acid and Base
4
Brønsted-Lowry theory of acids and bases
An acid is a substance from which a proton can be removed.
Acids are proton donors.
A base is a substance that can remove a proton from an acid.
Bases are proton acceptors.
Because of strong desire for protons, bases rip protons off acids.
Acid-base reactions are competitions for protons.
HCl + H2O  H3O+ + Cl¯
HNO3 + H2O  H3O+ + NO3¯
HC2H3O2 + H2O  H3O+ + C2H3O2¯
H2O + NH3  NH4+ + OH¯
conjugate acids and bases
17–2
17 Acid and Base
5
Problems with B-L theory
The theory works very nicely in all protic solvent, but fails to explain
acid-base behavior in aprotic solvents and non-solvent situations.
A more general concept on acid and base was proposed by G.N.
Lewis at about the same time Bronsted-Lowry theory was proposed.
Question the established theory.
17 Acid and Base
6
Evolution of the acid-base concept
year
thinker
Acid
Base
acid-base reaction
1884
Arrhenius
ionize
H+
ionize
OH¯
H+ + OH¯ = HOH
1923
BronstedLowry
Proton
Donor
proton
acceptor
HA + B = HB + A
conjugation
1923
Lewis
electrophil nucleophil
17 Acid and Base
E + Nu = E:Nu
7
Lewis acids and bases
Gilbert Newton Lewis (1875-1946) influential
American chemist. His theories include the
Lewis dot structure taught in Chem120 and
covalent bond theories.
Lewis acids are electrophils: H+, Na+, BF3,
Lewis bases are nucleophils: NH3, H2O, PH3
Acid base reactions:
BF3 + :NH3  F3B:NH3
Lewis at his desk.
He generalized an idea.
17–9
17 Acid and Base
8
HClO4
H2SO4
HI
HBr
HCl
HNO3
H3O+
HSO4¯
H2SO3
H3PO4
HNO2
HF
HC2H3O2
Al(H2O)63+
H2CO3
H2S
ClO4 ¯
HSO4¯
I¯
Br¯
Cl¯
NO3¯
H2O
SO42¯
HSO3 ¯
H2PO4 ¯
NO2¯
F¯
C2H3O2 ¯
Al(OH)(H2O)52+
HCO3 ¯
HS ¯
17–4 & 17–5 Table 17–3
Relative strengths
of acids and bases
17 Acid and Base
The stronger the acid, the
weaker its conjugate base.
HClO
HBrO
NH4+
HCN
HCO3¯
H2O2
HS ¯
H2O
ROH
ClO¯
BrO¯
NH3
CN ¯
CO32 ¯
HO2
S2¯
OH¯
RO¯
9
Skills regarding strength of acids
What are strong acids?
What are strong bases?
Which is the strongest acid, HF, HCl, HBr, or HI?
How about HNO3 and HNO2? H2SO4, H2SO3?
How about HClO4, HClO3, HClO2, HClO, and HCl?
What is the strongest acid in an aqueous solution?
What is leveling effect?
Order a given list according to the strength.
Interpret reactions as due to strength of acids and bases.
Explain strength with related acids and bases.
17 Acid and Base
10
Molecular structure and acid strength
A strong acid loses its proton easily. A strong base holds onto a proton
tightly.
The more polarized is the
H—X
bond in an acid, the easier the molecule releases H+ thus a stronger acid.
The weaker the H —X bond, the easier the molecule releases H+ thus a
stronger acid.
The bond strength and the polarity are related to electronegativity and
electrostatic interactions – size of the ions.
Able to predict acidity from molecular structure.
17–8
17 Acid and Base
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Self-ionization of water
Water molecules autoionize
2 H2O (l) = H3O+ (aq) + OH¯ (aq)
[H3O+] [OH¯]
Kc = ————————
[H2O] (= 1000/18 = 55.6)
Kw = [H2O] Kc = 1e–14 only at 25oC, it’s T dependent.
The ion product, Kw increases as T increases, and its value remains
the same in the presence of acid or base.
The molecule of life, H2O, and its acidity.
17 – 3
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12
Solutions of strong acids and bases
Strong acids and strong bases completely ionize in their solutions.
HCl (aq) + H2O (l)  H3O+ (aq)+ Cl– (aq)
KOH (aq)  K+ (aq)+ OH– (aq)
In a 0.100 M HCl or HNO3 solution,
[H+] = 0.100 M
and
[OH–] = 1e–14 / 0.100 = 1e–13 M at 25oC
In a 0.100 M NaOH or KOH solution,
[OH–] = 0.100 M
and
[H+] = 1e–14 / 0.100 = 1e–13 M at 25oC
In a 0.010 M Ca(OH)2 solution,
[OH–] = 2*0.010 = 0.020 M
[H+] = 1e–14 / 0.020 = 5e–13 M at 25oC
17 Acid and Base
13
Some strong acids and bases
Strong acids
Hydrogen halides
HCl
HBr
Strong Bases
HI
Oxyacids of halogens
HClO3 HBrO3 HIO3
HClO4 HBrO4 HIO4
Hydroxides of alkali metals
NaOH KOH CsOH
Hydroxides of alkali earth
Ca(OH)2 Sr(OH)2
Other oxyacids
H2SO4 HNO3
17 – 4
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Neutralization Reactions
The neutralization reaction between strong acids and strong bases
has the net ionic reaction
H3O+ (aq)+ OH– (aq) = 2 H2O
The anions are bystander ions. They do not participate the reaction.
These ions are
I–, Br–, Cl–, NO3 –, HSO4 –, CClO4 –, Na+, K+, Cs+, Ca2+
etc.
When dried, the ions left behind in neutralization reactions form salts.
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The pH scale
Sorensen introduced the pH scale in 1909 using the symbol pH.
The p is from the German word potenz, power of (10).
pH = – log [H+];
pOH = – log [OH–];
pK = – log [K];
[H+] = 10 –pH
[OH–] = 10 –pOH
K = 10 –pK
pH = 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.8
[H+] = 0.79 0.63 0.50 0.40 0.32 0.25 0.20 0.16
(not linear; need not copy, figure out yourself)
0.9
0.13
For aqueous solution;
Kw = [H+] [OH–]
- log Kw = pH + pOH = 14 only at 25oC
17–3, p–672
17 Acid and Base
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pH meter and pH electrodes
The pH meter is based on the principle to be
discussed in electrochemistry.
This topic is also related to the equilibrium
constant K and Gibbs free energy, G.
17 Acid and Base
17
Neutrality
In a neutral solution, [H+] = [OH–]. (Different from Fig. 17-5)
In an acidic solution, [H+] > [OH–], and a basic solution, [H+] < [OH–].
In a saturated CaO (same as Ca(OH)2) solution, [OH–] = 0.025.
Calculate pH, [H+], [Ca2+], and pOH at 25oC.
Solution:
pOH = – log 0.025 = 1.60
pH = 14.00 – 1.60 = 12.40
only applicable at 25oC
[H+] = 10–12.40 = 4.0e–13 M Note that 4.0e-13*0.025 = 1e-14
[Ca2+] = [OH–] / 2= 0.013 M
17 Acid and Base
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Answer these questions – review
What are acid-base reactions?
What are neutralization reactions?
What is the role of the solvent in acidity?
What does pH mean and how is the pH scale related to acidity?
What are salts?
17 Acid and Base
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Acid-Base Equilibria
Strong acids and bases completely ionize in their aqueous solutions.
HCl  H+ + Cl–
KOH  K+ + OH–
Weak acids and bases ionize but not completely in their solutions.
HCH3COO  H+ + CH3COO– acetic acid, vinegar spirit
HNic  H+ + Nic–
HNic = Niacine, C5NH5COOH, a water-soluble vitamin required by the body for
health, growth and reproduction; part of the vitamin B complex. It was first prepared in
pure form by oxidizing nicotine using conc. HNO3. Nicotine is a major chemical in
tobacco .
N
CH3
N
<= Nicotine
N
Niacine =>
Vitamine B3
17 Acid and Base
COOH
20
Caffeine
Caffeine C8H10N4O2 is a weak
base, (pH of 1% soln 6.9)
taste bitter smell like tea, a
cardiac stimulant, (boost of
energy), mild diuretic, addictive,
operates using the same
mechanisms that amphetamines,
cocaine and heroin use to
stimulate the brain.
Crystals are hexagonal prisms by
sublimation, mp 238°C. Sublimes
178°. Fast sublimation is
obtained at 160-165° under
1 mm Hg pressure.
Many organic bases are
interesting drugs, and their
chemistry is fascinating.
17 Acid and Base
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Ionization of weak acids
Ionization of acetic acid, HCH3COO, HA
HA = H+ + A–
same as HA + H2O = H3O+ + A–
[H+] [A–]
Ka = —————
[HA]
A– = CH3COO– (acetate)
Ka acid ionization constant, important for an acid
Ka equilibrium constant with solvent effect
The pKa is defined similar to the pH,
pKa = – log Ka
Ka = 10 –pKa
See Table 17-3 on page 678
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Some weak acids and bases
Common Weak Acids
Formic acid
Acetic acid
Trichloroacetic acid
Hydrofluoric
Hydrocyanic
Hydrogen sulfide
Water
HCOOH
CH3COOH
CCl3COOH
HF
HCN
H 2S
H 2O
Conjugate acids
of weak bases
NH4+
Common Weak Bases
ammonia
trimethyl ammonia
pyridine
ammonium hydroxide
water
HS- ion
conjugate bases of
weak acids e.g.:
17 Acid and Base
NH3
N(CH3)3
C5H5N
NH4OH
H2O
HSHCOO-
23
Determine Ka and percent ionization
Nicotinic acid, HNic, is a monoprotic acid. A solution containing
0.012 M HNic, has a pH of 3.39. What is its Ka? What is the percent
of ionization?
Solution:
HNic = H+ + Nic–
0.012-x x
x
x = [H+] = 10–3.39 = 4.1e-4
[HNic] = 0.012 – 0.00041 = 0.012
(4.1e-4)2
Ka = ————— = 1.4e-5
0.012
Degree of ionization = 0.00041 / 0.012 = 0.034 = 3.4%
17 Acid and Base
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Application of Ka
The Ka of nicotinic acid, HNic, is 1.4e-5. A solution containing 0.22
M HNic. What is its pH? What is the degree of ionization?
Solution:
HNic = H+ + Nic–
0.22-x
x
x
x2
Ka = ———— = 1.4e-5
0.22 – x
(use approximation, small indeed)
x =  (0.22*1.4e-5) = 0.0018
pH = – log (0.0018) = 2.76
Degree of ionization = 0.0018 / 0.22 = 0.0079 = 0.79%
compare with 3.4% when the solution was 0.012 M
17 Acid and Base
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pH of a weak acid
The pH of C M acid HA (Ka) solution.
Method:
HA =
C-x
H+ + A–
x
x
The pOH of C M base BOH (Kb)
solution.
Method:
BOH = B+ + OH–
C-y
y
y
x2
Ka = ————
C–x
x 2 + Ka x – C Ka = 0
– Ka + Ka2 + 4 C Ka
x = ———————————
2
pH = – log x
y2
Kb = ————
C–y
y 2 + Kb x – C Kb = 0
– Kb + Kb2 + 4 C Kb
y = ———————————
2
pOH = -log y
17 Acid and Base
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Using the quadratic formula
The Ka of nicotinic acid, HNic, is 1.4e-5. A solution containing
0.00100 M HNic. What is its pH? What is the degree of ionization?
Solution:
HNic = H+ + Nic–
0.001-x x
x
x2
Ka = —————— = 1.4e-5
0.00100 – x
x 2 + Ka x – C Ka = 0
– Ka + Ka2 + 4 C Ka
x = ———————————
2
x2 + 1.4e-5 x – 1.4e-8 = 0
–1.4e–5 +  (1.4e–5)2 + 4*1.4e-8
x = —————————————————— = 0.000111 M
2
pH = – log (0.000111) = 3.95
Degree of ionization = 0.000111/ 0.001 = 0.111 = 11.1%
17 Acid and Base
Deg.’f ioniz’n
0.22
0.012
0.001
0.79%
3.4 %
11.1 %
27
Degree of or percent ionization
% ionization
The degree or percent of ionization of a weak
acid always decreases as its concentration
increases, as shown from the table given earlier.
Deg.’f ioniz’n
0.22
0.012
0.001
0.79%
3.4 %
11.1 %
Concentration of acid
17 Acid and Base
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Polyprotic acids
Polyprotic acids such as sulfuric and carbonic acids have more than
one hydrogen to donate.
H2SO4 = H+ + HSO4–
HSO4– = H+ + SO42–
Ka1 very large completely ionized
Ka2 = 0.012
H2CO3 = H+ + HCO3–
HCO3– = H+ + CO32–
Ka1 = 4.3e-7
Ka2 = 4.8e-11
C
Ascorbic acid (vitamin C)
is a diprotic acid, abundant
in citrus fruit.
Others:
H2S, H2SO3, H3PO4, H2C2O4 (oxalic acid) …
17 Acid and Base
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Aspartame nutrasweet
A dipeptide methyl ester :
L-aspartyl-L-phenylalanine methyl ester
C14H18N2O5 (molar mass = 294.31)
Aspartame has two ionizable protons
1965. Jim Schlatter synthesized
it and discovered it sweet leaving
no bitter after tast.
17 Acid and Base
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Species concentrations of diprotic acids
Evaluate concentrations of species in a 0.10 M H2SO4 solution.
Solution:
H2SO4 = H+ +
(0.1–0.1) 0.10+y
HSO4–
0.10-y
completely ionized
SO42–
y
Ka2 = 0.012
Assume y = [SO42–]
HSO4– = H+ +
0.10–y
0.10+y
(0.10+y) y
————— = 0.012
(0.10-y)
Y2 + 0.112 y – 0.0012 = 0
[SO42–] = y = 0.01M
[H+] = 0.10 + 0.01 = 0.11 M;
[HSO4–] = 0.10-0.01 = 0.09 M
- 0.112+0.1122 + + 4*0.0012
y = —————————————— = 0.0098
2
If concentration’f H2SO4 = 1.0 M,
what doUdo?
17 Acid and Base
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Species concentrations of weak diprotic acids
Evaluate concentrations of species in a 0.10 M H2S solution.
Solution:
H2S = H+ +
(0.10–x) x+y
HS–
x–y
= H+
x+y
HS–
x-y
+
S2–
y
(x+y) (x-y)
————— = 1.02e-7
(0.10-x)
[H2S] = 0.10 – x = 0.10 M
[HS–] = [H+] = x  y = 1.0e–4 M;
[S2–] = y = 1.0e-13 M
See Example 16.4
Ka1 = 1.02e-7
Assume x = [HS–]
Ka2 = 1.0e-13
Assume y = [S2–]
(x+y) y
———— = 1.0e-13
(x-y)
0.1>> x >> y:
x+ y = x-y = x
x = 0.1*1.02e-7 = 1.00e-4
y = 1e-13
17 Acid and Base
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Beware of Misconceptions
These equations show dynamic equilibria at the molecular level in a
system. These equations are not separate reactions, but they
indicate possible combination and dissociation in both directions.
H2S = H+ +
HS– Ka1 = 1.02e-7
HS– = H+ +
S2–
H2O = H+ +
OH- Kw = 1e-14
Ka2 = 1.0e-13
H2S(g)
HS
H
S
2
OH
H2O
+
+
H
+
H
H
OHH2O H S
H
O
+
2
2
H2S
H
H2O
2- S2S
OH
OH
2S
HS-
17 Acid and Base
33
Base-ionization constant
For a base BOH,
For acid, HA
BOH = B+ + OH–
HA = H+ + A–
[B+] [OH–]
Kb = ———————
[BOH]
The pKb is defined similar to pKa
pKb = – log Kb,
Kb =
10–pKb
Whatever you have learned for
weak acids apply to weak bases
[H+] [A–]
Ka = —————
[HA]
The pKa is defined similar to the pH,
pKa = – log Ka
Ka = 10 –pKa
From slide 3
June 18
17 Acid and Base
34
Common weak bases
Substance
Ammonia
aniline
dimethylamine
ethylamine
Hydrozine
Hydroxylamine
methylamine
Pyridine
Urea
Formula
NH3
C6H5NH2
(CH3)2NH
C2H5NH2
N2H4
H2NOH
CH3NH2
C5H5N
NH2CONH2
Kb
1.8e-5
4.2e-10
5.1e-4
4.7e-4
1.7e-6
1.1e-8
4.4e-4
1.4e-9
1.5e-14
no need to copy Table 16.2
17 Acid and Base
Many drugs are salts of
weak bases, such as
Advil
Pseudoephedrine HCl
Buckley's mixture
Dextromethorphan
hydrobromide
Dristan
Traxodone HCl
35