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.

17 Acid and Base 7

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.

17 Acid and Base 11

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.

17 Acid and Base 16

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

17 Acid and Base 17

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.

17 Acid and Base 22

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

17 Acid and Base 23

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

17 Acid and Base 29

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

17 Acid and Base 33

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