Acids and BasesAcids and Bases

Chapter 14

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 H: Acids are H++ donors, bases are donors, bases are proton acceptors.proton acceptors.

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

acid baseacid base

Hydronium IonHydronium Ion

Hydronium ion is a hydrated proton -- Hydronium ion is a hydrated proton --

HH++.H2O.

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

Conjugate Acid/Base PairsConjugate Acid/Base Pairs

HA(HA(aqaq) + H) + H22O(O(ll) ) H H33OO++((aqaq) + A) + A((aqaq))

conj conj conj conjconj conj conj conjacid 1 base 2 acid 2 base 1acid 1 base 2 acid 2 base 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--??

Acid Dissociation Constant (Acid Dissociation Constant (KKaa))

HA(HA(aqaq) + H) + H22O(O(ll) ) H H33OO++((aqaq) + A) + A((aqaq))

Ka values for common acids are found in Table 14.2 on page 663.

Ka3H O A

HA

H A

HA

14_02T

Table 14.2 Values of Ka for Some Common Monoprotic Acids

Formula Name Value of Ka*

HSO4 Hydrogen sulfate ion 1.2 x 10 2

HClO2 Chlorous acid 1.2 x 10 2

HC2H2ClO2 Monochloracetic acid 1.35 x 10 3

HF Hydrofluoric acid 7.2 x 10 4

HNO2 Nitrous acid 4.0 x 10 4

HC2H3O2 Acetic acid 1.8 x 10 5

[Al(H2O)6]3+ Hydrated aluminum(III) ion 1.4 x 10 5

HOCl Hypochlorous acid 3.5 x 10 8

HCN Hydrocyanic acid 6.2 x 10 10

NH4 Ammonium ion 5.6 x 10 10

HOC6H5 Phenol 1.6 x 10 10

Incr

eas

ing

aci

d st

reng

th

*The units of Ka are mol/L but are customarily omitted.

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!

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. (CH(CH33COOH)COOH)

- Yields a much stronger (water is relatively Yields a much stronger (water is relatively strong) conjugate base than water. strong) conjugate base than water. (CH(CH33COOCOO))

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.

14_322

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.

14_323

Relativeacid strength

Relativeconjugatebase strength

Veryweak

Weak

Strong

Verystrong

Veryweak

Weak

Strong

Verystrong

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

Arranging Species According Arranging Species According to Increasing Basic Strengthto Increasing Basic Strength

HH22O, FO, F--, Cl, Cl--, NO, NO22--, & CN, & CN--

Use Table 14.2 on page 663.Use Table 14.2 on page 663.

ClCl-- is weakest since it is conjugate base of is weakest since it is conjugate base of strong acid and weaker than water. Use Kstrong acid and weaker than water. Use Kaa

values to arrange the remaining bases.values to arrange the remaining bases.

ClCl-- < H < H22O < FO < F-- < NO < NO22-- < CN < CN--

Water as an Acid and a BaseWater as an Acid and a Base

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

HH22O + HO + H22O O H H33OO++ + OH + OH

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

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

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-] [OH-] > 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

KKww & & HH

At 60At 60ooC, the value of KC, the value of Kww is 1 x 10 is 1 x 10-13-13 for the for the

dissociation of water:dissociation of water:

2 H2 H22OO(l)(l) <---> H <---> H33OO++(aq) (aq) + OH+ OH--

(aq)(aq)

Is this reaction exothermic or endothermic?Is this reaction exothermic or endothermic?

Endothermic -- KEndothermic -- Kww increased with increased with

temperature.temperature.

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 = 1.00 = 1.00 10 101414 = [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).

pH & [HpH & [H++]]

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

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.

14_324

[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 7pH scale and pH values for common substances. A pH of1 is 100 times more acidic than a pH of 3.

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.

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

Solving Weak Acid Equilibrium Solving Weak Acid Equilibrium ProblemsProblems

- List major species in solution.List major species in solution.

- Choose species that can produce HChoose species that can produce H++ and write and write reactions.reactions.

- Based on K values, decide on dominant Based on K values, decide on dominant equilibrium.equilibrium.

- Write equilibrium expression for dominant Write equilibrium expression for dominant equilibrium.equilibrium.

- List initial concentrations in dominant equilibrium.List initial concentrations in dominant equilibrium.

Solving Weak Acid Equilibrium Solving Weak Acid Equilibrium Problems (continued)Problems (continued)

- Define change at equilibrium (as “Define change at equilibrium (as “xx”).”).

- Write equilibrium concentrations in terms of Write equilibrium concentrations in terms of xx..

- Substitute equilibrium concentrations into Substitute equilibrium concentrations into equilibrium expression.equilibrium expression.

- Solve for Solve for xx the “easy way.” the “easy way.” x can be neglected x can be neglected when concentration is 2 powers of 10 (100x) when concentration is 2 powers of 10 (100x) greater than Kgreater than Kaa or K or Kbb..

- Verify assumptions using 5% rule.Verify assumptions using 5% rule.

- Calculate [HCalculate [H++] and pH.] and pH.

pH of Weak Acid SolutionspH of Weak Acid Solutions

Calculate the pH of a 0.100 M HOCl solution.Calculate the pH of a 0.100 M HOCl solution.

Major species: HOCl and HOHMajor species: HOCl and HOH

KKa a HOCl = 3.5 x 10HOCl = 3.5 x 10-8 -8 & K & Kaa HOH = 1.0 x 10 HOH = 1.0 x 10-14-14

HOCl will be only significant source of [HHOCl will be only significant source of [H++].].

KKaa = 3.5 x 10 = 3.5 x 10-8-8 = [H = [H++][OCl][OCl--]/[HOCl]]/[HOCl]

pH of Weak Acid SolutionspH of Weak Acid SolutionsContinuedContinued

ICEICE

[HOCl] [HOCl] [OCl [OCl--] ] [H[H++] ]

Initial (mol/L)Initial (mol/L) 0.100 0.100 0 0 0 0

Change (mol/L)Change (mol/L) - x - x + x + x + x + x

Equil. (mol/L)Equil. (mol/L) 0.100 - x 0.100 - x 0 + x 0 + x 0 + x 0 + x

pH of Weak Acid SolutionspH of Weak Acid SolutionsContinuedContinued

KKaa = 3.5 x 10 = 3.5 x 10-8-8 = [H = [H++][OCl][OCl--]/[HOCl]]/[HOCl]

3.5 x 103.5 x 10-8-8 = [x][x]/[0.100 - x] = [x][x]/[0.100 - x]

KKaa is more than 100 x smaller than concentration, x is more than 100 x smaller than concentration, x

can be neglected in the denominator.can be neglected in the denominator.

KKaa = 3.5 x 10 = 3.5 x 10-8-8 = [x][x]/[0.100] = [x][x]/[0.100]

xx22 = 3.5 x 10 = 3.5 x 10-9-9

xx = 5.9 x 10= 5.9 x 10-5 -5 MM

pH of Weak Acid SolutionspH of Weak Acid SolutionsContinuedContinued

Approximation check:Approximation check:

% dissociation = (x/[HA]% dissociation = (x/[HA]oo) (100%)) (100%)

% dissociation = (x/[HOCl]% dissociation = (x/[HOCl]oo) (100%)) (100%)

% dissociation = (5.9 x 10% dissociation = (5.9 x 10-5-5/0.100)(100%)/0.100)(100%)

% dissociation = 0.059 %% dissociation = 0.059 %

This is much less than 5 % and therefore the This is much less than 5 % and therefore the approximation was valid.approximation was valid.

Percent Dissociation Percent Dissociation (Ionization)(Ionization)

% d isso c ia tio nam o u n t d isso c ia ted ( )

in itia l co n cen tra tio n ( )1 0 0 % )

M

M(

The percent dissociation calculation is exactly the same as theone to check the 5 % approximation. See Sample Exercise 14.10on pages 678 and 679.

% Dissociation Calculations% Dissociation CalculationsIn a 0.100 M lactic acid solution (HCIn a 0.100 M lactic acid solution (HC33HH55OO33), ),

lactic acid is 3.7 % dissociated. Calculate lactic acid is 3.7 % dissociated. Calculate the Kthe Kaa for this acid. for this acid.

Major species: HCMajor species: HC33HH55OO3 3 & HOH & HOH

HCHC33HH55OO3(aq) 3(aq) <---> H<---> H++(aq)(aq)+ C+ C33HH55OO33

--(aq)(aq)

KKaa = [H = [H++][C][C33HH55OO33--]/ [HC]/ [HC33HH55OO33]]

% Dissociation Calculations% Dissociation CalculationsContinuedContinued

ICEICE

[HC[HC33HH55OO33] ] [C [C33HH55OO33--] [H] [H++] ]

Initial (M)Initial (M) 0.10 0.10 0 0 0 0

Change (M) - 3.7 x 10Change (M) - 3.7 x 10-3 -3 + 3.7 x 10 + 3.7 x 10-3 -3 + 3.7 x 10+ 3.7 x 10-3 -3

Equil. (M)Equil. (M) 0.10 + 3.7 x 100.10 + 3.7 x 10-3 -3 + 3.7 x 10+ 3.7 x 10-3-3

% Dissociation Calculations% Dissociation CalculationsContinuedContinued

KKaa = [H = [H++][C][C33HH55OO33--]/ [HC]/ [HC33HH55OO33]]

KKaa = [3.7 x 10 = [3.7 x 10-3-3]]22/ [0.10]/ [0.10]

KKaa = 1.4 x 10 = 1.4 x 10-4-4

14_325

Acid concentration

Percent dissociation

H+ concentration

More concentrated More dilute

The effect of dilution on the % dissociation and [H+] of a weak acid solution.

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 with (or reaction with water)water)

HH33CNHCNH22((aqaq) + H) + H22O(O(ll) ) H H33CNHCNH33++((aqaq) + OH) + OH((aqaq))

See Table 14.3 on page 685 for KSee Table 14.3 on page 685 for Kbb values of common values of common bases.bases.

KKbb calculations are identical to K calculations are identical to Ka a calculations.calculations.

Polyprotic AcidsPolyprotic Acids

. . . can furnish more than one proton (H. . . can furnish more than one proton (H++) to ) to the solution.the solution.

H CO H HCO ( )

HCO H CO ( )

2 3 3 a

3 32

a

1

2

K

KSee Table 14.4 on page 689 for Ka values for commonpolyprotic acids.Know Sample Exercises 14.15 & 14.16 on pages 689-692.

Acid-Base Properties of SaltsAcid-Base Properties of Salts

Cation AnionAcidic

or Basic Exampleneutral neutral neutral NaCl

neutral conj base ofweak acid

basic NaF

conj acid ofweak base

neutral acidic NH4Cl

conj acid ofweak base

conj base ofweak acid

depends onKa & Kb

values

Al2(SO4)3

Structure and Acid-Base Structure and Acid-Base PropertiesProperties

Two factors for acidity in binary compounds:Two factors for acidity in binary compounds:

- Bond Polarity Bond Polarity (high is good)(high is good)

- Bond Strength Bond Strength (low is good)(low is good)

14_326 Cl O H

Electron density

Cl O H

Electron density

O

Cl O H

Electron density

O

O

Cl O H

Electron density

O

OO

The effect of the number of attached oxygen on the H-O bondin a series of chlorine oxyacids.

OxidesOxides

Acidic Oxides (Acid Anhydrides):Acidic Oxides (Acid Anhydrides):

- OOX bond is strong and covalent.X bond is strong and covalent.

SOSO22, NO, NO22, CrO, CrO33

Basic Oxides (Basic Anhydrides):Basic Oxides (Basic Anhydrides):

- OOX bond is ionic.X bond is ionic.

KK22O, CaOO, CaO

Lewis Acids and BasesLewis Acids and Bases

Lewis AcidLewis Acid: electron pair : electron pair acceptoracceptor

Lewis BaseLewis Base: electron pair : electron pair donordonor

Al3+ + 6H

OH

Al OH

H6

3+