Acids and BasesAcids and Bases

Chapter 15Chapter 15

15.1-15.10 and 15.1215.1-15.10 and 15.12

BrBrØØnstead Acids and Brnstead Acids and BrØØnstead nstead BasesBases

• Recall from chapter 4:Recall from chapter 4:– BrBrØØnstead Acid- nstead Acid- a substance capable a substance capable

of donating a proton.of donating a proton.– BrBrØØnstead Base- nstead Base- a substance capable a substance capable

of accepting a proton.of accepting a proton.– Conjugate Acid-Base Pair- Conjugate Acid-Base Pair- an acid an acid

and its conjugate base or a base and its and its conjugate base or a base and its conjugate acid. conjugate acid.

BrBrØØnstead Acids and Brnstead Acids and BrØØnstead nstead BasesBases

NHNH33 + H + H22O ↔ NHO ↔ NH44++ + OH + OH--

BrBrØØnstead Acids and Brnstead Acids and BrØØnstead nstead BasesBases

The Acid-Base Properties of The Acid-Base Properties of WaterWater

H2O (l) H+ (aq) + OH- (aq)

autoionization of water

O

H

H + O

H

H O

H

H H OH-+[ ] +

H2O + H2O H3O+ + OH-

base

acid

conjugate

acid

conjugate

base

The Ion Product of WaterThe Ion Product of Water

• Hydrogen Ion concentration is keyHydrogen Ion concentration is key

• Ion Product Constant (KIon Product Constant (Kww)- )- the the product of the molar concentrations product of the molar concentrations of Hof H++ and OH and OH-- ions at a particular ions at a particular temperature.temperature.

At 250CKw = [H+][OH-] = 1.0 x 10-14

The Ion Product of WaterThe Ion Product of Water

Solution isSolution is

[H[H++] = [OH] = [OH--]] neutral neutral

[H[H++] > [OH] > [OH--]] acidic acidic

[H[H++] < [OH] < [OH--]] basic basic

The Ion Product of WaterThe Ion Product of Water

What is the concentration of OH- ions in a HCl solution whose hydrogen ion concentration is 1.3M?

Kw = [H+][OH-] = 1.0 x 10-14

[H+]= 1.3M

[OH-] =Kw

[H+]

1 x 10-14

1.3=

[OH-] = 7.7 x 10-15 M

[OH-]

The Ion Product of WaterThe Ion Product of Water

pH a measure of AciditypH a measure of Acidity

• pH- pH- the negative logarithm of the the negative logarithm of the hydronium ion concentration (mol/L). hydronium ion concentration (mol/L).

pH = -log [H+]

pOH = -log [OH-]

pH + pOH = 14.00

pH calculationspH calculations

The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater?

pH = -log [H+]

[H+] = 10-pH

[H+]= 10-4.82

[H+]= 1.5 x 10-5 M

pH CalculationspH Calculations

The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?

pH + pOH = 14.00

pOH = -log [OH-]

pOH = -log (2.5 x 10-7)

pOH = 6.60

pH = 14.00 – pOH = 14.00 – 6.60 = 7.40

pH = 7.40

Strength of Acids and BasesStrength of Acids and Bases

• Strong Acids- Strong Acids- are strong electrolytes are strong electrolytes that are assumed to ionize completely that are assumed to ionize completely in water. in water.

• Weak Acids-Weak Acids- acids that ionize only to acids that ionize only to a limited extent in water. a limited extent in water.

• Strong Bases-Strong Bases- are strong electrolytes are strong electrolytes that ionize completely in water. that ionize completely in water.

• Weak Bases-Weak Bases- weak electrolytes. weak electrolytes.

Strength of Acids and BasesStrength of Acids and Bases

Conjugate Acid-Base PairsConjugate Acid-Base Pairs

Conjugate Acid-Base PairsConjugate Acid-Base Pairs

Conjugate acid-base pairs:

• The conjugate base of a strong acid has no measurable strength.

• H3O+ is the strongest acid that can exist in aqueous solution.

• The OH- ion is the strongest base that can exist in aqueous solution.

Weak Acids and Acid Ionization Weak Acids and Acid Ionization ConstantsConstants

• Acid Ionization Constant (KAcid Ionization Constant (Kaa)- )- the the equilibrium constant for the equilibrium constant for the ionization of an acid. ionization of an acid. – The larger the kThe larger the kaa, the larger the stronger , the larger the stronger

the acid. the acid. HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

HA (aq) H+ (aq) + A- (aq)

Ka =[H+][A-][HA]

Weak Acids and Acid Ionization Weak Acids and Acid Ionization ConstantsConstants

What is the pH of a 0.5 M HF solution (at 250C)?

HF (aq) H+ (aq) + F- (aq)

Ka =[H+][F-][HF]

= 7.1 x 10-4

ICEICE

Weak Acids and Acid Ionization Weak Acids and Acid Ionization ConstantsConstants

HF (aq) H+ (aq) + F- (aq)

Initial (M)

Change (M)

Equilibrium (M)

0.50 0.00 0.00

-x +x +x

0.50 - x x x

Weak Acids and Acid Ionization Weak Acids and Acid Ionization ConstantsConstants

Ka =x2

0.50 - x= 7.1 x 10-4

Ka << 1

0.50 – x 0.50

Ka x2

0.50= 7.1 x 10-4

x2 = 3.55 x 10-4

x = 0.019 M

[H+] = [F-] = 0.019 M

[HF] = 0.50 – x = 0.48 M

pH = -log [H+] = 1.72

When can we use the When can we use the Approximation Method?Approximation Method?

When can I use the approximation Method?

When x is less than 5% of the value from which it is subtracted.

x = 0.019 0.019 M0.50 M

x 100% = 3.8%

Less than 5%

Approximation ok.

The Quadratic EquationThe Quadratic Equation

• If approximation method does not If approximation method does not work, use the quadratic equation.work, use the quadratic equation.– Write the ionization expression in terms Write the ionization expression in terms

of x.of x.-b ± b2 – 4ac

2ax =

Solving Weak Acid Ionization Solving Weak Acid Ionization ProblemsProblems

• Solving weak acid ionization problems:• Identify the major species that can affect the pH.

– In most cases, you can ignore the autoionization of water.

– Ignore [OH-] because it is determined by [H+].

• Use ICE to express the equilibrium concentrations in terms of single unknown x.

• Write Ka in terms of equilibrium concentrations. Solve for x by the approximation method. If approximation is not valid, solve for x exactly.

• Calculate concentrations of all species and/or pH of the solution.

Percent IonizationPercent Ionization

• Percent Ionization- Percent Ionization- measure of the measure of the strength of an acid.strength of an acid.

percent ionization = Ionized acid concentration at equilibrium

Initial concentration of acidx 100%

For a monoprotic acid HA:

Percent ionization = [H+]

[HA]0

x 100%

Percent IonizationPercent Ionization

Weak Bases and Base Weak Bases and Base Ionization ConstantsIonization Constants

• Base Ionization Constant- Base Ionization Constant- the the equilibrium constant for the equilibrium constant for the ionization of a base.ionization of a base.

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Kb =[NH4

+][OH-][NH3]

Weak Bases and Base Weak Bases and Base Ionization ConstantsIonization Constants

Relationship between Relationship between Ionization Constants and their Ionization Constants and their

Conjugate BasesConjugate Bases

KaKb = Kw

Weak Acid and Its Conjugate Base

Ka = Kw

Kb

Kb = Kw

Ka

Diprotic and Polyprotic Diprotic and Polyprotic AcidsAcids

• An ionization constant expression An ionization constant expression can be written for each ionizable can be written for each ionizable proton.proton.

• Two or more expressions may be Two or more expressions may be necessary to calculate the necessary to calculate the concentrations of all species present concentrations of all species present in the acid solution.in the acid solution.

Diprotic and Polyprotic Diprotic and Polyprotic AcidsAcids

Diprotic and Polyprotic Diprotic and Polyprotic AcidsAcids

Molecular Structure and Molecular Structure and Strength of AcidsStrength of Acids

• Strength of an acid depends on:Strength of an acid depends on:– Structure of the acidStructure of the acid– Strength of the bondStrength of the bond– Polarity of bondPolarity of bond

H X H+ + X-

The stronger the bond

The weaker the acid

Molecular Structure and Molecular Structure and Strength of AcidsStrength of Acids

Hydrohalic AcidsHydrohalic Acids

• Order of relative acid strength:Order of relative acid strength:

HF << HCl < HBr < HI

H X H+ + X-

Acid-Base Properties of Acid-Base Properties of SaltsSalts

• Salt- Salt- an ionic compound formed by the an ionic compound formed by the reaction between an acid and a base. reaction between an acid and a base. They are string electrolytes that They are string electrolytes that completely dissociate into ions in water.completely dissociate into ions in water.

• Salt Hydrolysis- Salt Hydrolysis- the reaction of an the reaction of an anion or a cation of a salt, or both, in anion or a cation of a salt, or both, in water. Usually affects the pH of a water. Usually affects the pH of a solution. solution.

Salts that produce Neutral Salts that produce Neutral SolutionsSolutions

Neutral Solutions:Salts containing an alkali metal or alkaline earth metal ion (except Be2+) and the conjugate base of a strong acid (e.g. Cl-, Br-, and NO3

-).

NaCl (s) Na+ (aq) + Cl- (aq)H2O

Salts that produce Basic Salts that produce Basic SolutionsSolutions

Basic Solutions:Salts derived from a strong base and a weak acid.

NaCH3COOH (s) Na+ (aq) + CH3COO- (aq)H2O

CH3COO- (aq) + H2O (l) CH3COOH (aq) + OH- (aq)

Salts that produce Acidic Salts that produce Acidic SolutionsSolutions

Acid Solutions:Salts derived from a strong acid and a weak base.

NH4Cl (s) NH4+ (aq) + Cl- (aq)

H2O

NH4+ (aq) NH3 (aq) + H+ (aq)

Salts with small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+) and the conjugate base of a strong acid.

Al(H2O)6 (aq) Al(OH)(H2O)5 (aq) + H+ (aq)3+ 2+

Lewis Acids and BasesLewis Acids and Bases

Lewis Acid- a substance that can accept a pair of electrons.

Lewis Base- a substance that can donate a pair of electrons.

H+ + OH-••

•••• H O H

••••

acid base

H+ + N H••

H

H

N H

H

H

H+

acid

base

Lewis Acids and BasesLewis Acids and Bases

B(OH)B(OH)33(aq)(aq) + H + H22OO(l)(l) ↔↔ B(OH B(OH44))--(aq)(aq) + H + H++

(aq)(aq)

COCO22(g)(g) + H + H22OO(l)(l) ↔↔ H H22COCO33(aq)(aq)