Acids and BasesAcids and Bases

Chapter 15

Acids in IndustryAcids in IndustrySulfuric acid, HSulfuric acid, H22SOSO44, is the chemical , is the chemical

manufactured in greatest quantity in the U.S.manufactured in greatest quantity in the U.S.

Eighty billion pounds of sulfuric acid are used Eighty billion pounds of sulfuric acid are used each year to manufacture:each year to manufacture:

-fertilizers-fertilizers -pharmaceuticals-pharmaceuticals

-detergents-detergents -storage batteries-storage batteries

-plastics-plastics -metals-metals

-petroleum-petroleum

Properties of Acids Properties of Acids

Acids:Acids:

• taste sour (citrus fruits & vinegar)taste sour (citrus fruits & vinegar)

• affect indicators (e.g. turn blue litmus red)affect indicators (e.g. turn blue litmus red)

• produce Hproduce H++ ions in aqueous solution ions in aqueous solution

• corrosive to metalscorrosive to metals

• pH < 7pH < 7

Classifying AcidsClassifying AcidsOrganic acids contain a carboxyl group or -Organic acids contain a carboxyl group or -

COOH -- HCCOOH -- HC22HH33OO2 2 & citric acid.& citric acid.

Inorganic acids -- HCl, HInorganic acids -- HCl, H22SOSO44, HNO, HNO33..

Oxyacids -- acid proton attached to oxygen -- Oxyacids -- acid proton attached to oxygen -- HH33POPO44. .

Monoprotic -- HCl & HCMonoprotic -- HCl & HC22HH33OO22

Diprotic -- HDiprotic -- H22SOSO44

Triprotic -- HTriprotic -- H33POPO44

Properties of BasesProperties of BasesBases:Bases:

• taste bittertaste bitter

• feel slipperyfeel slippery

• affect indicators (e.g. turn red litmus blue)affect indicators (e.g. turn red litmus blue)

• produce OHproduce OH- - ions in aqueous solutionions in aqueous solution

• pH > 7pH > 7

• causticcaustic

Models of Acids and BasesModels of Acids and Bases

Arrhenius ConceptArrhenius Concept: Acids produce H: Acids produce H++ in in solution, bases produce OHsolution, bases produce OH ion. ion.

Brønsted-LowryBrønsted-Lowry: Acids are proton (H: Acids are proton (H++) ) donors, bases are proton (Hdonors, bases are proton (H++) acceptors.) acceptors.

HCl + HHCl + H22O O Cl Cl + H + H33OO++

acid baseacid base

Bronsted-Lowry ModelBronsted-Lowry Model

The Bronsted-Lowry Model is not limited to The Bronsted-Lowry Model is not limited to aqueous solutions like the Arrhenius aqueous solutions like the Arrhenius Model.Model.

NHNH3(g)3(g) + HCl + HCl(g)(g) ----> NH ----> NH44ClCl(s)(s)

This is an acid-base reaction according to This is an acid-base reaction according to Bronsted-Lowry, but not according to Bronsted-Lowry, but not according to Arrhenius!Arrhenius!

Hydronium IonHydronium Ion

Hydronium (HHydronium (H33OO++)) ion is a hydrated proton -- ion is a hydrated proton --

HH+ + . H2O.

The H+ ion is simply a proton. It has a very high charge density, so it is strongly attracted to the very electronegative oxygen of the polar water molecule.

Conjugate Acid/Base PairsConjugate Acid/Base Pairs

HAHA((aqaq)) + + HH22OO((ll)) HH33OO++((aqaq)) + + AA

((aqaq))

conj conj conj conj conjconj conjconjacid 1acid 1 base 2base 2 acid 2acid 2 base 1base 1

conjugate baseconjugate base: everything that remains of : everything that remains of the acid molecule after a proton is lost.the acid molecule after a proton is lost.

conjugate acidconjugate acid: formed when the proton is : formed when the proton is transferred to the base.transferred to the base.

Which is the stronger base--HWhich is the stronger base--H22O or AO or A--??

Conjugate Acid-Base PairsConjugate Acid-Base Pairs

Conjugate AcidConjugate Acid Substance Conjugate Base Substance Conjugate Base HOH HOH

NHNH33

HSOHSO44--

HH22POPO44--

HPOHPO442-2-

H3O+

NH4+

H2SO4

H3PO4

H2PO41-

OH-

NH2-

SO42-

HPO42-

PO43-

Conjugate Acid-Base PairsConjugate Acid-Base Pairs

Which of the following represent conjugate acid-Which of the following represent conjugate acid-base pairs?base pairs?

a) HF, Fa) HF, F-- e)e) OHOH--, HNO, HNO33

b) NHb) NH44++, NH, NH33 f) Hf) H22O, HO, H33OO++

c) HCl, Hc) HCl, H22OO g) H g) H22SOSO44, SO, SO442-2-

d) HCd) HC22HH33OO22, C, C22HH33OO22-- h) HClOh) HClO44, ClO, ClO44

--

Conjugate BasesConjugate Bases

Write the conjugate base for each of the Write the conjugate base for each of the following:following:

a) HClOa) HClO44

b) Hb) H33POPO44

c) CHc) CH33NHNH33++

ClO4-

H2PO4-

CH3NH2

Acid StrengthAcid Strength

- Its equilibrium position lies far to the Its equilibrium position lies far to the right. right. (HNO(HNO33))

- Yields a weak conjugate base. Yields a weak conjugate base. (NO(NO33))

Strong Acid:Strong Acid:

Acid StrengthAcid Strength(continued)(continued)

- Its equilibrium lies far to the left. Its equilibrium lies far to the left. (HC(HC22HH33OO22))

- Yields a much stronger (water is relatively Yields a much stronger (water is relatively strong) conjugate base than water. strong) conjugate base than water. (C(C22HH33OO22

--))

Weak Acid:Weak Acid:

14_1577

(a) (b)

H+H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

A-

A-

A-

A-

A-

A-

A-

A- A-

A-

A-

A- B-

HBHB

HBHB

HB HB

HB

HBHBHB

HB

A strong acid is nearly 100 % ionized, while a weak acid is only slightly ionized.

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HA

(a)

H+ A–

HA

(b)

H+ A–

Before dissociationAfter dissociation,at equilibrium

HA

Diagram a represents a strong acid, while b represents a weakacid which remains mostly in the molecular form.

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Relativeacid strength

Relativeconjugatebase strength

Veryweak

Weak

Strong

Verystrong

Veryweak

Weak

Strong

Verystrong

The relationship of acid strength and conjugate basestrength for acid-base reactions.

BasesBases

Bases are often called Bases are often called alkalisalkalis because they because they often contain alkali or alkaline earth metals.often contain alkali or alkaline earth metals.

““StrongStrong” and “” and “weakweak” are used in the same ” are used in the same sense for bases as for acids.sense for bases as for acids.

strongstrong = = complete dissociation complete dissociation (hydroxide ion (hydroxide ion supplied to solution)supplied to solution)

NaOH(NaOH(ss) ) Na Na++((aqaq) + OH) + OH((aqaq))

BasesBases(continued)(continued)

weakweak = very = very little dissociation little dissociation (or reaction (or reaction with water)with water)

NHNH33((aqaq) + HOH() + HOH(ll) ) NH NH44++((aqaq) + OH) + OH((aqaq))

Water as an Acid and a BaseWater as an Acid and a BaseWater is Water is amphotericamphoteric (it can behave either as an (it can behave either as an acid or a base).acid or a base).

HH22OO + H + H22O <---> HO <---> H33OO++ + + OHOH

conj conj conjconj acid 1acid 1 base 2 acid 2 base 2 acid 2 base 1base 1

KKww = 1 = 1 10 101414 M M22 at 25°C at 25°C

KKww = [H = [H++][OH][OH--]]

Only about two molecules in a billion ionize!!Only about two molecules in a billion ionize!!

Ion product Constant, KIon product Constant, Kww

Kw is called the ion-product constant or dissociation constant.

neutral solution [H+] = [OH-] = 1.0 x 10 -7 M

acidic solution [H+] > [OH-] [H+] > 1.0 x 10-7 M

basic solution [H+] < [OH-] [H+] < 1.0 x 10-7 M

No matter what the concentration of H+ or OH- in an aqueous solution, the product, Kw, will remain the same.

[H[H++] & [OH] & [OH--] Calculations] Calculations

Calculate the [HCalculate the [H++] for a 1.0 x 10] for a 1.0 x 10-5 -5 M OHM OH--..

KKww = [H = [H++][OH][OH--]]

[H[H++] = K] = Kww/[OH/[OH--]]

[H[H++] = 1.0 x 10] = 1.0 x 10-14-14 M M22/1.0 x 10/1.0 x 10-5-5 M M

[H[H++] = ] = 1.0 x 10-9 M

[H[H++] & [OH] & [OH--] Calculations] CalculationsContinuedContinued

Calculate the [OHCalculate the [OH--] for a 10.0 M H] for a 10.0 M H++..

KKww = [H = [H++][OH][OH--]]

[OH[OH--] = K] = Kww/[H/[H++]]

[OH[OH-]-] = 1.0 x 10 = 1.0 x 10-14-14 M M22/10.0 M/10.0 M

[OH[OH--] = ] = 1.0 x 10-15 M

[H[H++] & [OH] & [OH--] Calculations] Calculations

Calculate the [HCalculate the [H++] for a 2.0 x 10] for a 2.0 x 10-2 -2 M OHM OH--..

KKww = [H = [H++][OH][OH--]]

[H[H++] = K] = Kww/[OH/[OH--]]

[H[H++] = 1.0 x 10] = 1.0 x 10-14-14 M M22/2.0 x 10/2.0 x 10-2-2 M M

[H[H++] = 5] = 5.0 x 10-13 M

The pH ScaleThe pH Scale

pH = pH = log[Hlog[H++]]

pH in water usually ranges from 0 to 14.pH in water usually ranges from 0 to 14.

KKww = 1.0 = 1.0 10 1014 14 MM22 = [H = [H++] [OH] [OH]]

ppKKww = = 14.00 = pH + pOH14.00 = pH + pOH

As pH rises, pOH falls (sum = 14.00).As pH rises, pOH falls (sum = 14.00).

Figure 15.5: Indicator Figure 15.5: Indicator paper being used to paper being used to measure the pH of a measure the pH of a solutionsolution

Figure 15.4: A pH meterFigure 15.4: A pH meter

OH -

OH-

OH-H3O+H3O+

H3O+

1x 10-14 1 x 10-7 1 x 100

1 x 100 1 x 10-7 1 x 10-14

pH = 0 pH = 7 pH = 14

pOH = 14 pOH = 7 pOH = 0

LogarithmsLogarithms-log 1.00 x 10-log 1.00 x 10-7 -7 = 7.000= 7.000

7.0007.000

characteristiccharacteristic mantissamantissa

The number of significant digits in 1.00 x 10The number of significant digits in 1.00 x 10 -7-7 is three, therefore, the log has three is three, therefore, the log has three decimal places. The mantissa represents decimal places. The mantissa represents the log of 1.00 and the characteristic the log of 1.00 and the characteristic represents the exponent 7.represents the exponent 7.

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[H+] pH

10–14 14

10–13 13

10–12 12

10–11 11

10–10 10

10–9 9

10–8 8

10–6 6

10–5 5

10–4 4

10–3 3

10–2 2

10–1 1

1 0

Acidic

Neutral

Basic

1 M NaOH

Ammonia (Householdcleaner)

BloodPure water

Milk

VinegarLemon juice

Stomach acid

1 M HCl

10–7 7

pH scale and pH values for common substances. A pH of1 is 100 times more acidic than a pH of 3.

pH & Significant FigurespH & Significant Figures

loglog

# Significant Figures -------> # decimal places <-------

inv log

pH = - log [HpH = - log [H++]] [H[H++] = inv log (-pH)] = inv log (-pH)

[H[H++] = 1.0 x 10] = 1.0 x 10-5-5 M M pH = 5.00pH = 5.00

pH CalculationspH Calculations

Calculate the pH value for the following Calculate the pH value for the following solution at 25 solution at 25 ooC.C.

[H[H++] = 1.0 x 10] = 1.0 x 10-9 -9 MM pH = - log [HpH = - log [H++]]

pH = - log [1.0 x 10pH = - log [1.0 x 10--

99]]

pH = 9.00pH = 9.00

pH CalculationspH Calculations

Calculate the pH for the following solution at 25 Calculate the pH for the following solution at 25 ooC.C.

[OH[OH--] = 1.0 x 10] = 1.0 x 10-6-6MM pH + pOH = 14.00 pH + pOH = 14.00

pOH = - log [OHpOH = - log [OH--]] pH = 14.00 - pOH pH = 14.00 - pOH

pOH = - log [1.0 x 10pOH = - log [1.0 x 10-6-6]] pH = 14.00 - 6.00pH = 14.00 - 6.00

pOH = 6.00pOH = 6.00 pH = 8.00 pH = 8.00

pH CalculationspH Calculations

What is the pOH, [HWhat is the pOH, [H++], & [OH], & [OH--] for human ] for human blood with a pH of 7.41?blood with a pH of 7.41?

pH + pOH = 14.00pH + pOH = 14.00

pOH = 14.00 - pHpOH = 14.00 - pH

pOH = 14.00 - 7.41pOH = 14.00 - 7.41

pOH = pOH = 6.596.59

pH CalculationspH CalculationsContinuedContinued

What is the pOH, [HWhat is the pOH, [H++], & [OH], & [OH--] ] for human blood with a pH of for human blood with a pH of 7.41?7.41?

pH = - log [HpH = - log [H++] ]

[H[H++] = antilog (-pH)] = antilog (-pH)

[H[H++] = antilog (-7.41)] = antilog (-7.41)

[H[H++] = 3.9 x 10] = 3.9 x 10-8 -8 MM

Note: The number of significant figures in the antilog is equal to the number of decimal places in the pH.

pH CalculationspH CalculationsContinuedContinued

What is the pOH, [HWhat is the pOH, [H++], & [OH], & [OH--] ] for human blood with a pH of for human blood with a pH of 7.41?7.41?

pOH = - log [OHpOH = - log [OH--] ]

[OH[OH--] = antilog (-pOH)] = antilog (-pOH)

[OH[OH--] = antilog (-6.59)] = antilog (-6.59)

[OH[OH--] = 2.6 x 10] = 2.6 x 10-7 -7 MM

Note: The number of significant figures in the antilog is equal to the number of decimal places in the pOH.

pH of Strong Acid SolutionspH of Strong Acid SolutionsCalculate the pH of a 0.10 M HNOCalculate the pH of a 0.10 M HNO33 solution. solution.

Major species are: HMajor species are: H++, NO, NO33--, and H, and H22OO

Sources of HSources of H+ + are from HNOare from HNO33 and H and H22O -- O --

amount from water is insignificant.amount from water is insignificant.

[H[H++] = 0.10 M pH = - log [H] = 0.10 M pH = - log [H++]]

pH = - log [0.10]pH = - log [0.10]

pH = pH = 1.001.00

Note: The number ofsignificant figures inthe [H+] is the same asthe decimal places inthe pH.

A Buffered SolutionA Buffered Solution. . . resists change in its pH when either H. . . resists change in its pH when either H++ or or OHOH are added. are added.

1.0 L of 0.50 M HC1.0 L of 0.50 M HC22HH33OO22

+ 0.50 M Na C+ 0.50 M Na C22HH33OO22

pH = 4.74pH = 4.74

Adding 0.010 mol Adding 0.010 mol solid NaOH solid NaOH raises the raises the pHpH of the solution to of the solution to 4.764.76, a very minor change., a very minor change.

Preparation of Buffered SolutionsPreparation of Buffered SolutionsBuffered solution can be made from:Buffered solution can be made from:

1.1. a weak acid and its salt (e.g. HCa weak acid and its salt (e.g. HC22HH33OO22 & &

NaCNaC22HH33OO22).).

2. a weak base and its salt (e.g. NH2. a weak base and its salt (e.g. NH33 & NH & NH44Cl).Cl).

Other examples of buffered pairs are:Other examples of buffered pairs are:

HH22COCO3 3 & NaHCO& NaHCO33 HH33POPO44 & NaH & NaH22POPO44

NaHNaH22POPO4 4 & Na& Na22HPOHPO44 NaNa22HPOHPO44 & Na & Na33POPO44

Characteristics of a BufferCharacteristics of a Buffer

1. The solution contains a weak acid HA and its 1. The solution contains a weak acid HA and its conjugate base Aconjugate base A--..

2. The buffer resists changes in pH by reacting 2. The buffer resists changes in pH by reacting with any added Hwith any added H++ or OH or OH-- so that these ions do so that these ions do not accumulate.not accumulate.

3. Any added H3. Any added H++ reacts with the base A reacts with the base A--..

4. Any added OH4. Any added OH-- reacts with the weak acid HA. reacts with the weak acid HA.

Buffered SolutionsBuffered SolutionsUsed when need to maintain a certain pH in the system.Used when need to maintain a certain pH in the system.

BloodBlood

Buffers work by reacting with added HBuffers work by reacting with added H+1+1 or OH or OH-1-1 ions so ions so they do not accumulate and change the pH.they do not accumulate and change the pH.

Buffers will only work as long as there is sufficient Buffers will only work as long as there is sufficient weak acid and conjugate base molecules present.weak acid and conjugate base molecules present.

Buffering MechanismBuffering Mechanism

HCHC22HH33OO2(aq)2(aq) <---> H <---> H++(aq)(aq) + C + C22HH33OO22

--(aq)(aq)

The buffering materials dissolved in the The buffering materials dissolved in the solution prevent added Hsolution prevent added H++ or OH or OH- - from from

building up in solution.building up in solution.

Buffering CapacityBuffering Capacity

. . . represents the amount of H. . . represents the amount of H++ or OHor OH the buffer can absorb the buffer can absorb without a significant change without a significant change in pH.in pH.