Chapter 17 – Chapter 17 – Chemistry of Acids Chemistry of Acids

and Basesand Bases

Objectives:1. Know the Bronsted-Lowry and Lewis theories of

acids and bases.2. Apply chemical equilibrium to acids and bases.3. Predict reactions between acids and bases.4. Relate structure and bonding to acid-base

properties.

How do we know ions are How do we know ions are present in aqueous solutions?present in aqueous solutions?

The solutions The solutions conduct electricity!!

The The substances that provide ions in an aqueous solution are calledare called ELECTROLYTESELECTROLYTES..

HCl, CuClHCl, CuCl22, and NaCl are, and NaCl are__________electrolytes. __________electrolytes.

What is a Strong What is a Strong Electrolyte?Electrolyte?

They They ionizeionize or or dissociatedissociate completely completely (or nearly so) into (or nearly so) into ____________________..

HCl, CuClHCl, CuCl22, and NaCl are , and NaCl are strong strong electrolytes.electrolytes.

HCl (aq) H+(aq) + Cl-(aq)

What is a Weak Electrolyte?What is a Weak Electrolyte?

Acetic acid ionizes only to a small extent, so it is a

weak electrolyte.weak electrolyte.CH3CO2H(aq)

CH3CO2-(aq) +

H+(aq)

What is an Acid?What is an Acid?

An acid - - -> HAn acid - - -> H++ (in (in water)water)

Some Some strongstrong acidsacids(completely ionized) are:(completely ionized) are:HClHCl hydrochlorichydrochloric

HH22SOSO44 sulfuricsulfuric

HClOHClO44 perchloricperchloric

HNOHNO33 nitricnitric

HNOHNO33

PROTONBecause a hydrogen atom is one proton and one electron, a hydrogen ion (H+) is simply a proton.

What is an Acid?What is an Acid?

An acid ---> HAn acid ---> H++ in water in waterAn acid ---> HAn acid ---> H++ in water in water

HCl(aq) H+(aq) + Cl-(aq)

What is the Hydronium Ion?What is the Hydronium Ion?

HCl

H2O

Cl-

H3O+

hydronium ion

HCl(aq) H+(aq) + Cl-(aq)

Bronsted – Lowry Theory:An acid donates a proton

What is a Weak Acid?What is a Weak Acid?

WEAK ACIDS (only partially WEAK ACIDS (only partially ionizedionized))= weak electrolytes= weak electrolytes

CHCH33COCO22HH acetic acidacetic acid

HH22COCO33 carbonic acidcarbonic acid

HH33POPO44 phosphoric acidphosphoric acidHFHF hydrofluoric acidhydrofluoric acid

CHCH33COCO22H H (aq) H+(aq) + CHCH33COCO22- (aq)

Weak AcidWeak Acid

K = [CH3CO2-][H3O+]

[CH3CO2H]= 1.8 x 10-5 Equilibrium constants =

____________constants

K > 1 (larger) – _______ AcidK < 1 (smaller) – ______ Acid

Bronsted-Lowry Acids and Bronsted-Lowry Acids and BasesBases

Acid – any substance that can _________ a proton to any other substance.Base – any substance that can _________ a proton from any other substance.

Acids – Monoprotic and Acids – Monoprotic and PolyproticPolyprotic

Amphiprotic – can behave as Amphiprotic – can behave as _____________________._____________________.

Polyprotic bases – can accept ____________proton.

Label the following as Acid or BaseLabel the following as Acid or Base

HCl CO3

2-

CN-

H2SO4

NH4+

NH3

CH3CH2COOH

H2PO4-

HCO3-

H2O

Water is amphiprotic – Water is amphiprotic – self-ionization of waterself-ionization of water

Write the equation:

Conjugate Acid-Base PairsConjugate Acid-Base Pairs

• A pair of compounds or ions that differ by the presence of one H+ ion is called a _________________________.

• Every reaction between a Bronsted acid and Bronsted base involves ___________________and has two _______________________.

Conjugate Acid-BaseConjugate Acid-Base

Identify the conjugate acid-base pairs:

1) HCO3- (aq) + H2O (l) H3O+ (aq) + CO3

2- (aq)

2) HNO3 (aq) + NH3 (aq) NH4+ (aq) + NO3

– (aq)

KwKw

Kw =

Kw and pHKw and pHNeutral: [H3O+] = [OH-]

Kw = [H3O+][OH-]

[H3O+] = [OH-] = √1.0 x 10-14

[H3O+] = [OH-] = 1 x 10-7

pH = __________

Neutral = pH = ________

Kw and pHKw and pH

• Adding acid to pure H2O:

– Increases the [H3O+] ; some reacts with the [OH-]

– [OH-] is ______________

– Equilibrium is reached when [H3O+][OH-] = 1 x 10-14 (25oC)

– [H3O+] ____ 1 x 10-7 M; [OH-] _____ 1 x 10-7 M

• Adding base to pure H2O:

– Increases the [OH-] ; some reacts with the [H3O+]

– [H3O+] is _______________

– Equilibrium is reached when [H3O+][OH-] = 1 x 10-14 (25oC)

– [H3O+] ___1 x 10-7 M; [OH-] ____ 1 x 10-7 M

pHpH

Look at ionization constants to determine strength of acids and bases.

A solution of HCl has [HCl] = 4.0 x 10A solution of HCl has [HCl] = 4.0 x 10-3-3 M. M. What are the concentrations of HWhat are the concentrations of H33OO++ and OH and OH--

in this solution at 25in this solution at 25ooC?C?

What is the pH of a 0.0075 M solution of What is the pH of a 0.0075 M solution of HCl? What is the hydroxide ion HCl? What is the hydroxide ion concentration of the solution?concentration of the solution?

The pH of a solution of Ba(OH)The pH of a solution of Ba(OH)22 is 9.89 at 25 is 9.89 at 25ooC. C. What is the hydroxide ion concentration in the What is the hydroxide ion concentration in the

solution? How many grams of Ba(OH)solution? How many grams of Ba(OH)22 must have must have been dissolved?been dissolved?

Other pX ScalesOther pX Scales

In generalIn general pX = -log XpX = -log Xand so and so pOH = - log [OH-]pOH = - log [OH-]

Kw = [HKw = [H3OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC

Take the log of both sidesTake the log of both sides

-log (10-log (10-14-14) = - log [H) = - log [H3OO++] + (-log [OH] + (-log [OH--])])

pKw = _____ = ____ + _____pKw = _____ = ____ + _____

Ka Ka and and KbKb

Strong acids have _________ ionization constants (Ka) –Indicating higher ability of donating protons –And a ________ pH.

Strong bases have _________ ionization constants (Kb) – Indicating higher ability of accepting protons – And a _________ pH.

Ka and KbKa and Kb

Kb =

Ka =

HA(aq) + H2O(l) H3O+(aq) + A-

(aq)

B(aq) + H2O(l) BH+(aq) + OH-

(aq)

K and ReactionsK and Reactions

Reactions always go from the Reactions always go from the stronger A-B pair (_______ K) stronger A-B pair (_______ K) to the weaker A-B pair (_____ to the weaker A-B pair (_____

K).K).

ACIDSACIDS CONJUGATE BASESCONJUGATE BASESACIDSACIDS CONJUGATE BASESCONJUGATE BASES

STRONGSTRONGSTRONGSTRONG

weakweakweakweak

weakweakweakweak

STRONGSTRONGSTRONGSTRONG

K and ReactionsK and ReactionsA strong acid is __________ %dissociated.A strong acid is __________ %dissociated.

Therefore, a Therefore, a STRONG ACIDSTRONG ACID —a good H—a good H++ donor— donor—must have a must have a WEAK CONJUGATE BASEWEAK CONJUGATE BASE—a poor —a poor HH++ acceptor: acceptor:

HNOHNO33(aq) + H(aq) + H22O(liq) O(liq) H H33OO++(aq) + NO(aq) + NO33--(aq)(aq)

STRONG ASTRONG A basebase acid acid weak Bweak B

•Every A-B reaction has two acids and two bases.•Equilibrium always lies toward the _________ pair.•Here K is very __________.

•Every A-B reaction has two acids and two bases.•Equilibrium always lies toward the _________ pair.•Here K is very __________.

K and ReactionsK and Reactions

We know from experiment that HNO3 is a strong acid.

1. It is a stronger acid than H3O+

2. H2O is a stronger base than NO3-

3. K for this reaction is __________.

WEAK BASE

ACID

STRONG ACID

BASE

H3O+ + NO3-HNO3 + H2O

K and ReactionsK and ReactionsAcetic acid is only 0.42% ionized when [HOAc] = 1.0 M. It Acetic acid is only 0.42% ionized when [HOAc] = 1.0 M. It is a is a _____________________._____________________.

HOAc + HHOAc + H22O O HH33OO++ + OAc + OAc--

Because [HBecause [H33OO++] is small, this must mean] is small, this must mean

1. H1. H33OO++ is a stronger acid than HOAc is a stronger acid than HOAc

2. OAc2. OAc-- is a stronger base than H is a stronger base than H22OO

3. K for this reaction is ___________.3. K for this reaction is ___________.

Ka and StructureKa and Structure• Inductive effect:

adjacent atoms ____________ electrons.

• O is very electronegative, it will attract e-

• Once a proton is donated, a negative charge is left behind.

• The charge needs to be stabilized.

Ka and pKaKa and pKa

• The _____________________ structures that can stabilize the charge determines the acidity of the substance.

• Because: the negative charge left behind can be distributed over more atoms (providing stability).

• Exercise: Draw the resonance structures for NO3- and for

NO2-.

pKa = -logKa pKa = -logKa

Ka and StructureKa and Structure

Exercise: Draw the resonance structures for NO3

- and for NO2-.

Ka and StructureKa and Structure

• The number of resonance structures that can stabilize the charge determines the acidity of the substance.

• Exercise: Draw the resonance structures for acetate ion.

Ka and StructureKa and Structure

• Acidity is favored by:– Electronegative atoms attached.– The possibility of resonance structures for the

anion.

Ka and Ka and StructureStructure

• Methyl (CH3-) group increases the

electronic density on the O-, making the negative charge more unstable and then acetic acid is less acidic than formic acid.

• Propanoate ion is the stronger base (it is eager to accept a proton) because the negative charge is not favored.

Kb and StructureKb and Structure

• Which is the stronger base?

Kb and StructureKb and Structure

Substituent EffectsSubstituent Effects• The electron cloud in a σ-bond between two unlike atoms is not

uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge(δ-) and the other atom has a slight positive charge(δ+).

• If the electronegative atom is then joined to a chain of atoms, usually carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the -I effect.

• Some groups, such as the alkyl group are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing. This is electron releasing character is indicated by the +I effect.

The inductive effectThe inductive effect

Substance Definition Examples

Arrhenius acid Provides _____in water

Strong acids: HCl, HNO3

Weak acids: HF, HCN

Cations of weak bases: NH4+

Anions of some polyprotic acids: HSO4 -

Arrhenius base Provides _____in water Strong bases: NaOH, Ba(OH)2

Bronsted-Lowry acid Proton ________

Strong acids: HCl, HNO3

Weak acids: HF, HCN

Cations of weak bases: NH4+

Anions of some polyprotic acids: HSO4 -

Bronsted-Lowry base Proton _________

Strong bases: NaOH, Ba(OH)2

Weak bases: NH3

Anions of weak acids: OH -, CN -

Anions of most polyprotic acids: CO32-, H2PO4

-

Lewis acid Electron Pair _______

Molecules with an incomplete octet: BF3

Small highly charged cations: Cu2+, Fe+3

Molecules with polar bonds: CO2

Lewis base Electron Pair ________Molecules with a lone pair of electrons: H2O, NH3, CO

Anions: Cl -, OH -, CN -

Summary:Summary:

Identify the acidic protons and the proton accepting atoms in the

following:

Answer using table 17.3:

• Which is the stronger acid?– Benzoic acid or acetic acid

Which has the stronger conjugate base?– Acetic acid or benzoic acid

• Which is the stronger base?– Ammonia or acetate ion

Which has the stronger conjugate acid?– Ammonia or acetate ion

Aqueous Solutions of SaltsAqueous Solutions of Salts

• Some ions are Bronsted bases:

• Some ions are Bronsted acids:

• Anions that are conjugate bases of strong acids (Cl-, NO3-)

are such weak bases, they ________________ on pH.• Alkali metal and alkaline earth cations have __________pH.• Anions of polyprotic acids can be either _______________.• Transition metal ions M2+ and M3+ can act as ___________.• ___________anions are conjugate bases of weak acids

(CO32-, F-, CN-) or ions [Al(H2O)6]3+.

CO32- (aq) + H2O(l) HCO3

- (aq) + OH- (aq) Kb = 2.1 x 10-4

[Al(H2O)6]3+ (aq) + H2O(l) [Al(H2O)5 (OH)2+ (aq) + H3O+ (aq) Ka = 7.9 x 10-6

Table 17.4 Acid and Base Table 17.4 Acid and Base Properties of Ions in Aqueous Properties of Ions in Aqueous

SolutionSolution

For each of the following salts in water, predict whether the pH will be acidic, basic,

or neutral.

NaNO3

FeCl2

NH4NO3

NH4F

Ka and Kb RelationshipKa and Kb Relationship

• Ka x Kb = Kw

Reaction between an Acid and a Reaction between an Acid and a BaseBase

• Write the net ionic equation for the possible reaction between acetic acid and sodium hydrogen sulfate, NaHSO4. Does the equilibrium lie to the left or right?

CH3CO2H (aq) + HSO4- (aq) CH3CO2- (aq) + H2SO4 (aq)

Types of acid-base ReactionsTypes of acid-base Reactions

• Strong acid + Strong base (neutralizations)

• Weak acid + Strong base• Strong acid + Weak base• Weak acid + Weak base

Strong acid + Strong baseStrong acid + Strong base

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

H3O+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) Na+(aq) + Cl-(aq) + 2 H2O (l)

H3O+(aq) + OH-(aq) 2 H2O (l)K = 1/Kw = 1 x 1014

• Really large! Reaction goes to completion• Na+ and Cl- have no effect on pH• Final solution should be neutral:

Mixing equal molar amounts of a strong acid with a strong base produces a __________ solution (pH = _____ at 25oC).

Weak acid + Strong baseWeak acid + Strong base

HCO2H(aq) + NaOH(aq) HCO2Na(aq) + H2O(l)HCO2H(aq) + OH-(aq) H2O (l) + HCO2

- (aq)

• OH- is stronger base than HCO2-

• Reaction should proceed to the right forming HCO2Na; Na+ is neutral ion and HCO2

- is a basic ion (conjugate base of a weak acid).

• Solution is basic.Mixing equal amounts of a strong base with a weak acid produces a ______ whose anion is the conjugate base of the weak acid. The solution is __________, pH depending on the value of _____________________.

Strong acid + Weak baseStrong acid + Weak base

HCl + NH3

H3O+ (aq) + NH3 (aq) H2O (l) + NH4+ (aq)

• H3O+ is stronger acid than NH4+

• Equilibrium lies to the right• NH4Cl; NH4

+ is an acidic ion, Cl- will not affect pHMixing equal amounts of a strong acid and a weak base produces a ________ whose cation is the conjugate acid of the weak base. The solution is __________, with the pH depending on the value of ______________.

Weak acid + Weak baseWeak acid + Weak baseAcetic acid + ammonia:CH3CO2H (aq) + NH3 (aq) NH4

+ (aq) + CH3CO2- (aq)

• CH3CO2H is stronger acid than NH4+

• Equilibrium lies to the right (product favored).• Product is NH4CH3CO2

• Ka for NH4+= 5.6 x 10-10

• Kb for CH3CO2- = 5.6 x 10-10

• Ka = Kb ; solution will be neutralMixing equal molar amounts of a weak acid and a weak base produces a ________ whose cation is the conjugate acid of the weak base and whose anion is the conjugate base of the weak acid. The solution pH depends on the ________________________________.

Determine Ka for butanoic acid. A solution prepared from 0.055 mol of butanoic acid dissolved in

sufficient water to give 1.0 L of solution has a pH of 2.72 at 25oC.

CH3CH2CH2CO2H (aq) + H2O (l) H3O+ (aq) + CH3CH2CH2CO2

- (aq)

What are the equilibrium concentrations of HF, F- ion, and H3O+ ion in a 0.015 M solution of HF? What

is the pH of the solution?

HF (aq) + H2O (l) H3O+(aq) + F-(aq)

What is the pH of the solution that results from mixing 25 mL of 0.016 M NH3 and 25 mL of 0.016 M

HCl?

1) Write balanced equation2) Find amount of product (salt that determines the pH)3) For pH solve an equilibrium problem

Polyprotic Acids and BasesPolyprotic Acids and Bases

• The pH of many inorganic polyprotic acids depends primarily on the hydronium ion generated in the first ionization step.

• Same principle is applied for conjugate bases of polyprotic acids.

• Phosphoric acid:First ionization step: Ka1 =

Second ionization step: Ka2 =

Third ionization step: Ka3 =

Lewis Acids and BasesLewis Acids and Bases

• Gilbert N Lewis (1930s)• Lewis acid is a substance that can

a pair of electrons from another atom to form a new bond.

• Lewis base is a substance that can a pair of electrons to another atom to form a new bond.A + B: BAacid base acid-base adduct

coordinate covalent bond

Cationic Lewis AcidsCationic Lewis Acids

• Complex ions or Coordination complexes (due to coordinate covalent bond).

• [Fe(H2O)6]3+ ; [Co(H2O)6]2+; [Cu(H2O)6]2+; [Cu(NH3)4]2+

• OH- is a Lewis base• Amphoteric substance: metal hydroxide

that can behave as an acid or a base: Al(OH)3

Al(OH)3 (s) + OH- (aq) [Al(OH)4]- (aq)

Al(OH)3 (s) + 3 H3O+ (aq) Al3+ (aq) + 6 H2O (l)

Molecular Lewis AcidsMolecular Lewis Acids

• Explains why nonmetals behave as _______.

• C in CO2 is slightly positive, a negatively charged Lewis base OH- can attack this atom to give bicarbonate ion:

CO2 (g) + OH- (l) HCO3- (aq)

• Substances which lone pairs of electrons (central atom) can behave as Lewis _______. Substances which lack an electron pair behave as a Lewis _______.

RememberRemember

• Go over all the contents of your textbook.

• Practice with examples and with problems at the end of the chapter.

• Practice with OWL.• Work on your OWL assignment for

Chapter 17.