17 Acid and Base 1
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 2
Answer these questions
What are acids? What are bases?
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?
Understand important properties of substances
17 Acid and Base 3
Arrhenius acids and bases
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¯
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.
17 Acid and Base 4
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 5
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 bases17–2
17 Acid and Base 6
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.
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Evolution of the acid-base concept
year thinker Acid Base acid-base reaction
1884 Arrhenius ionize ionize H+ + OH¯ = HOH H+ OH¯
1923 Bronsted- Proton proton HA + B = HB + A Lowry Donor acceptor conjugation
1923 Lewis electrophil nucleophil E + Nu = E:Nu
17 Acid and Base 8
Lewis acids and basesGilbert 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 9
Relative strengths of acids and bases
HClO4 ClO4 ¯
H2SO4 HSO4¯ HI I¯
HBr Br¯ HCl Cl¯
HNO3 NO3¯H3O+ H2O
HSO4¯ SO42¯
H2SO3 HSO3 ¯
H3PO4 H2PO4 ¯
HNO2 NO2¯
HF F ¯ HC2H3O2 C2H3O2 ¯
Al(H2O)63+ Al(OH)(H2O)5
2+
H2CO3 HCO3 ¯
H2S HS ¯
HClO ClO¯ HBrO BrO¯
NH4+ NH3
HCN CN ¯ HCO3¯ CO3
2 ¯
H2O2 HO2
HS ¯ S2¯ H2O OH¯ ROH RO¯
17–4 & 17–5 Table 17–3
The stronger the acid, the weaker its conjugate base.
17 Acid and Base 10
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.
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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.
17–8
Able to predict acidity from molecular structure.
17 Acid and Base 12
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.
17 – 3
The molecule of life, H2O, and its acidity.
17 Acid and Base 13
Solutions of strong acids and basesStrong 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 14
Some strong acids and bases
Strong acids
Hydrogen halides HCl HBr HI
Oxyacids of halogens HClO3 HBrO3 HIO3
HClO4 HBrO4 HIO4
Other oxyacidsH2SO4 HNO3
Strong Bases
Hydroxides of alkali metalsNaOH KOH CsOH
Hydroxides of alkali earthCa(OH)2 Sr(OH)2
17 – 4
17 Acid and Base 15
Neutralization ReactionsThe 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+]; [H+] = 10 –pH
pOH = – log [OH–]; [OH–] = 10 –pOH
pK = – log [K]; K = 10 –pK
pH = 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 [H+] = 0.79 0.63 0.50 0.40 0.32 0.25 0.20 0.16 0.13
(not linear; need not copy, figure out yourself)
For aqueous solution;
Kw = [H+] [OH–]- log Kw = pH + pOH = 14 only at 25oC
17–3, p–672
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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 18
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 = 1.60pH = 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
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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 20
Acid-Base EquilibriaStrong 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 NCH3
N COOH
<= Nicotine
Niacine =>Vitamine B3
17 Acid and Base 21
CaffeineCaffeine 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.
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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–] A– = CH3COO– (acetate) Ka = ————— Ka acid ionization constant, important for an acid
[HA] 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 HCOOHAcetic acid CH3COOH
Trichloroacetic acid CCl3COOHHydrofluoric HF
Hydrocyanic HCN Hydrogen sulfide H2S
Water H2O
Conjugate acids of weak bases NH4
+
Common Weak Bases ammonia NH3
trimethyl ammonia N(CH3)3
pyridine C5H5N
ammonium hydroxide NH4OH
water H2O
HS- ion HS-
conjugate bases of weak acids e.g.: HCOO-
17 Acid and Base 24
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 25
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
x 2
Ka = ———— = 1.4e-50.22 – 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 26
pH of a weak acidThe pH of C M acid HA (Ka) solution.
Method: HA = H+ + A–
C-x x x
x 2
Ka = ————C – x
x 2 + Ka x – C Ka = 0
– Ka + Ka2 + 4 C Ka
x = ———————————2
pH = – log x
The pOH of C M base BOH (Kb) solution.
Method:BOH = B+ + OH–
C-y y y
y 2
Kb = ————C – y
y 2 + Kb x – C Kb = 0
– Kb + Kb2 + 4 C Kb
y = ———————————2
pOH = -log y
17 Acid and Base 27
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 x2 + 1.4e-5 x – 1.4e-8 = 0
0.00100 – x
–1.4e–5 + (1.4e–5)2 + 4*1.4e-8x = —————————————————— = 0.000111 M
2pH = – log (0.000111) = 3.95
Degree of ionization = 0.000111/ 0.001 = 0.111 = 11.1%
Deg.’f ioniz’n
0.22 0.79%0.012 3.4 %0.001 11.1 %
x 2 + Ka x – C Ka = 0
– Ka + Ka2 + 4 C Ka
x = ——————————— 2
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Degree of or percent ionization
Deg.’f ioniz’n
0.22 0.79%0.012 3.4 %0.001 11.1 %
The degree or percent of ionization of a weak acid always decreases as its concentration increases, as shown from the table given earlier.
Concentration of acid
% ionization
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Polyprotic acidsPolyprotic acids such as sulfuric and carbonic acids have more than one hydrogen to donate.
H2SO4 = H+ + HSO4– Ka1 very large completely ionized
HSO4–
= H+ + SO42– Ka2 = 0.012
H2CO3 = H+ + HCO3– Ka1 = 4.3e-7
HCO3–
= H+ + CO32– Ka2 = 4.8e-11
Ascorbic acid (vitamin C) is a diprotic acid, abundant in citrus fruit.
Others:H2S, H2SO3, H3PO4, H2C2O4 (oxalic acid) …
C
17 Acid and Base 30
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 31
Species concentrations of diprotic acids
Evaluate concentrations of species in a 0.10 M H2SO4 solution.
Solution:H2SO4 = H+ + HSO4
– completely ionized
(0.1–0.1) 0.10+y 0.10-y
HSO4–
= H+ + SO42– Ka2 = 0.012
0.10–y 0.10+y y Assume y = [SO42–]
(0.10+y) y————— = 0.012(0.10-y)
[SO42–] = y = 0.01M
[H+] = 0.10 + 0.01 = 0.11 M; [HSO4
–] = 0.10-0.01 = 0.09 M
Y2 + 0.112 y – 0.0012 = 0
- 0.112+0.1122 + + 4*0.0012y = —————————————— = 0.0098
2
If concentration’f H2SO4 = 1.0 M, what doUdo?
17 Acid and Base 32
Species concentrations of weak diprotic acids
Evaluate concentrations of species in a 0.10 M H2S solution.
Solution:H2S = H+ + HS– Ka1 = 1.02e-7(0.10–x) x+y x-y Assume x = [HS–]
HS– = H+ + S2– Ka2 = 1.0e-13
x–y x+y y Assume y = [S2–]
(x+y) (x-y) (x+y) y————— = 1.02e-7 ———— = 1.0e-13(0.10-x) (x-y)
[H2S] = 0.10 – x = 0.10 M[HS–] = [H+] = x y = 1.0e–4 M; [S2–] = y = 1.0e-13 M
0.1>> x >> y: x+ y = x-y = x
x = 0.1*1.02e-7 = 1.00e-4y = 1e-13
See Example 16.4
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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– Ka2 = 1.0e-13
H2O = H+ + OH- Kw = 1e-14
H2O
H2O H2S
HS- H+
H+ H+
H2S
H2O H2O H2S
OH-
OH- OH- S2-
OH-
S2-
S2-
H+
H2S(g)
HS-
17 Acid and Base 34
Base-ionization constant
For acid, HA
HA = H+ + A–
[H+] [A–]Ka = —————
[HA] The pKa is defined similar to the pH,
pKa = – log Ka Ka = 10 –pKa
From slide 3
For a base BOH,
BOH = B+ + OH–
[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
June 18
17 Acid and Base 35
Common weak bases
Substance Formula Kb
Ammonia NH3 1.8e-5 aniline C6H5NH2 4.2e-10
dimethylamine (CH3)2NH 5.1e-4ethylamine C2H5NH2 4.7e-4Hydrozine N2H4 1.7e-6
Hydroxylamine H2NOH 1.1e-8methylamine CH3NH2 4.4e-4
Pyridine C5H5N 1.4e-9Urea NH2CONH2 1.5e-14
no need to copy Table 16.2
Many drugs are salts of weak bases, such as Advil Pseudoephedrine HCl
Buckley's mixtureDextromethorphan hydrobromide
DristanTraxodone HCl