Chapter 16: Acids and Bases, A Molecular Look Chemistry: The Molecular Nature of Matter, 6E...

Chapter 16: Acids and Bases, A Molecular Look

Chemistry: The Molecular Nature of Matter, 6E

Jespersen/Brady/Hyslop

Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E

Arrhenius Acids and Bases Acid produces H3O+ in water

Base gives OH–

Acid-base neutralization Acid and base combine to produce water

and a salt.e.g. HCl(aq) + NaOH(aq) H2O + NaCl(aq)

H3O+(aq) + Cl–(aq) + Na+(aq) + OH–(aq)

2H2O + Cl–(aq) + Na+

(aq) Many reactions resemble this without

forming H3O+ or OH– in solution2


Gas Phase Acid-Base Not covered by Arrhenius definition

e.g. NH3(g) + HCl(g) NH4Cl(s)

3


Brønsted-Lowry Definition Acid = proton donor Base = proton acceptor Allows for gas phase acid-base reactions

e.g. HCl + H2O H3O+ + Cl–

HCl = acid Donates H+

Water = base Accepts H+

4


Conjugate Acid-Base Pair Species that differ by H+ e.g. HCl + H2O H3O+ + Cl–

HCl = acid Water = base H3O+

Conjugate acid of H2O

Cl– Conjugate base of HCl

5


Formic Acid is Bronsted Acid

Formic acid (HCHO2) is a weak acid Must consider equilibrium

HCHO2(aq) + H2O CHO2–(aq) + H3O+

(aq) Focus on forward reaction

6


Formate Ion is Bronsted Base Now consider reverse reaction Hydronium ion transfers H+ to CHO2

–

7

H3O+ + CHO2

HCHO2 + H2O

conjugate pair

conjugate pair

acid base acid base


Learning Check

conjugate base

conjugate acid

HCl

NH3

HC2H3O2

CN–

HF

8

Identify the conjugate partner for each

Cl–

NH4+

C2H3O2–

HCN

F–


Learning Check Write a reaction that shows that HCO3

– is a Brønsted acid when reacted with OH–

HCO3–(aq) + OH–(aq)

Write a reaction that shows that HCO3– is

a Brønsted base when reacted with H3O+

(aq)

HCO3–(aq) + H3O+(aq)

9

H2CO3(aq) + H2O

H2O + CO32–(aq)


Your Turn!In the following reaction, identify the acid/base conjugate pairs. (CH3)2NH + H2SO4 → (CH3)2NH+ + HSO4

–

A. (CH3)2NH / H2SO4 (CH3)2NH+ / HSO4–

B. (CH3)2NH / (CH3)2NH+ H2SO4 / HSO4–

C. H2SO4 / HSO4– (CH3)2NH+ / (CH3)2NH

D. H2SO4 / (CH3)2NH (CH3)2NH+ / HSO4–

10


Amphoteric Substances Can act as either acid or base

Can be either molecules or ions

e.g. Hydrogen carbonate ion: AcidHCO3

–(aq) + OH–(aq) CO32–(aq) + H2O

BaseHCO3

–(aq) + H3O+(aq) H2CO3(aq) + H2O

[Amphiprotic substances can donate or accept a proton. This is a subtle but important difference from the word amphoteric]

11


Your Turn!Which of the following can act as an amphoteric substance?A. CH3COOH

B. HClC. NO2

–

D. HPO42–

12


Strengths of Acids and BasesStrength of Acid

Measure of its ability to transfer H+

Strong acids React completely with water e.g. HCl and HNO3

Weak acids Less than completely ionized e.g. CH3COOH and

CHOOH

Strength of Base classified in similar fashion: Strong bases

React completely with water e.g. Oxide ion (O2–) and OH–

Weak bases Undergo incomplete reactionse.g. NH3 and NRH2 (NH2CH3, methylamine)

13


Reactions of Strong Acids and Bases

In water Strongest acid = hydronium ion, H3O+

If more powerful H+ donor added to H2O

Reacts with H2O to produce H3O+

Similarly, Strongest base is hydroxide ion (OH–)

More powerful H+ acceptors React with H2O to produce OH–

14


Position of Acid-Base Equilibrium Acetic acid (HC2H3O2) is weak acid

Ionizes only slightly in water

HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2–(aq)

weaker acid weaker base stronger acid stronger base

Hydronium ion Better H+ donor than acetic acid Stronger acid

Acetate ion Better H+ acceptor than water Stronger base

Position of equilibrium favors weaker acid and base

15


Your Turn!In the reaction: HCl + H2O → H3O+ + Cl–

which species is the weakest base ?A. HClB. H2O

C. H3O+

D. Cl–

16


In General Stronger acids and bases tend to

react with each other to produce their weaker conjugates Stronger Brønsted acid has weaker

conjugate base Weaker Brønsted acid has stronger

conjugate base Can be applied to binary acids (acids

made from hydrogen and one other element)

17


Learning CheckIdentify the preferred direction of the following reactions:

H3O+(aq) + CO32–(aq) HCO3

–(aq) + H2O

Cl–(aq) + HCN(aq) HCl(aq) + CN–

(aq)

18


Trends in Binary Acid StrengthBinary Acids = HnX

X = Cl, Br, P, As, S, Se, etc.1. Acid strength increases from left to right

within same period (across row) Acid strength increases as

electronegativity of X increases e.g. HCl is stronger acid than H2S which

is stronger acid than PH3

or PH3 < H2S < HCl

19


Trends in Binary Acid StrengthBinary Acids = HnX

X = Cl, Br, P, As, S, Se, etc.2. Acid strength increase from top to

bottom within group Acid strength increases as size of X

and bond length increases e.g. HCl is weaker acid than HBr which

is weaker acid than HI or HCl < HBr < HI

20


Learning Check

Which is stronger?

H2S or H2O

CH4 or NH3

HF or HI

H2S

NH3

HI

21


Trends in Oxoacid StrengthOxoacids (HnX Om)

Acids of H, O, and one other element HClO, HIO4, H2SO3, H2SO4, etc.

1. Acids with same number of oxygen atoms and differing X

a. Acid strength increases from bottom to top within group

HIO4 < HBrO4 < HClO4

b. Acid strength increases from left to right within period as the electronegativity of the central atom increases H3PO4 < H2SO4 < HClO4

22


Oxoacids (HnXOm)2. For same X

Acid strength increases with number of oxygen atoms H2SO3 < H2SO4

More oxygens, remove more electron density from central atom, weakening O—H bond make H more acidic

Trends in Oxoacid Strength

23


Learning Check

Which is the stronger acid in each pair?

H2SO4 or H3PO4

HNO3 or H3PO3

H2SO4 or H2SO3

HNO3 or HNO2

24

H2SO4

HNO3

H2SO4

HNO3


Your Turn!Which corresponds to the correct order of acidity from weakest to strongest acid ?A. HBrO3, HBrO, HBrO2

B. HBrO, HBrO2, HBrO3

C. HBrO, HBrO3, HBrO2

D. HBrO3, HBrO2, HBrO

25


Alternate Definition of Acid Strength Acid strength can be analyzed in terms

of basicity of anion formed during ionization

Basicity Willingness of anion to accept H+ from

H3O+

Consider HClO3 and HClO4:

26

O

ClO OH

O

Cl

O

O OH

HClO3 HClO4


Comparing Basicity

Lone oxygens carry most of the negative charge ClO4

– has 4 O atoms, so each has –¼ charge

ClO3– has 3 O atoms, so each has –1/3 charge

ClO4– weaker base than ClO3

– Thus conjugate acid, HClO4, is stronger acid

HClO4 stronger acid as more fully ionized

27

O

ClO OH

O

Cl

O

O OH

HClO3 HClO4


Learning Check Arrange the following in order of

increasing acid strength:

HBr, AsH3, H2Se AsH3 < H2Se < HBr

H2SeO4, H2SO4, H2TeO4

H2TeO4 < H2SeO4 < H2SO4

HBrO3, HBrO, HBrO4, HBrO2

HBrO < HBrO2 < HBrO3 < HBrO4

28


Strength of Organic Acids Organic acid —COOH Presence of electronegative atoms

(halide, nitrogen or other oxygen) near —COOH group Withdraws electron density from O—H bond Makes organic acid, stronger acidse.g.

CH3CO2H < CH2ClCO2H < CHCl2CO2H < CCl3CO2H

29


Your Turn!Which of the following is the strongest organic acid?

30

OHC

O

CH

H

H

OHC

O

CF

H

H

OHC

O

CBr

H

H

OHC

O

CI

H

H

OHC

O

CCl

H

H

A B C

D E


Lewis Definition of Acid and Base Broadest definition of species that can

be classified as either acid or base Definitions based on electron pairs Lewis acid

Any ionic or molecular species that can accept pair of electrons

Formation of coordinate covalent bond Lewis base

Any ionic or molecular species that can donate pair of electrons

Formation of coordinate covalent bond31


Lewis Neutralization Formation of coordinate covalent bond

between electron pair donor and electron pair acceptor

NH3BF3 = addition compound Made by joining two smaller molecules 32

Addition Compound


Lewis Acid-Base Reaction

Electrons in coordinate covalent bond come from O in hydroxide ion

33


Lewis Acids:1. Molecules or ions with incomplete valence

shells e.g. BF3 or H+

2. Molecules or ions with complete valence shells, but with multiple bonds that can be shifted to make room for more electrons

e.g. CO2

3. Molecules or ions that have central atoms that can expand their octets

Capable of holding additional electrons Usually, atoms of elements in Period 3 and

belowe.g. SO2 34


SO2 as Lewis Acid

35

O2–


Lewis Bases: Molecules or ions that have unshared

electron pairs and that have complete shells e.g. O2– or NH3

Lewis Definition is Most General All Brønsted acids and bases are Lewis

acids and bases All Arrhenius acids and bases are Brønsted

acids and bases

36


H+ Transfer from Lewis Perspectivee.g. H2O—H+ + NH3 H2O + H+—NH3

37


Learning CheckIdentify the Lewis acid and base in the

following: NH3 + H+ NH4

+

Base Acid

F– + BF3 BF4

–

Base Acid

SeO3 + O2– SeO42–

Acid Base

38


Your Turn!Which of the following species can act as a Lewis base ?A. Cl–

B. Fe2+

C. NO2–

D. O2–

39


Acid-Base Properties of Elements and Their Oxides

Nonmetal oxides React with H2O to form acids Upper right hand corner of periodic

table Acidic Anhydrides Neutralize bases Aqueous solutions red to litmus SO3(g) + H2O H2SO4(aq)

N2O5(g) + H2O 2HNO3(aq)

CO2(g) + H2O H2CO3(aq) 40


Acid-Base Properties of Elements and Their Oxides

Metal oxides React with H2O to form hydroxide

(Base) Group 1A and 2A metals (left hand side

of periodic table) Basic Anydrides Neutralize acids Aqueous solutions blue to litmus Na2O(s) + H2O 2NaOH(aq)

CaO(s) + H2O Ca(OH)2(aq)41


Metal Oxides MxOy

Solids at room temperature Many insoluble in H2O Why?

Too tightly bound in crystal Can't remove H+ from H2O Do dissolve in solution of strong acid Now H+ free, can bind to O2– and remove

from crystalFe2O3(s) + 6H+(aq) 2Fe3+(aq) + 3H2O

42


Your Turn!What is the acid formed by P2O3 when it reacts with water ?A. H2PO4

B. H2PO2

C. H3PO4

D. H3PO3

P2O3 + 3H2O → 2H3PO3

43


Metal Ions in Solution (Once Anion is Removed)

Exist with sphere of water molecules with their negative poles directed toward Mn+

Mn+(aq) + mH2O M(H2O)mn+(aq)

Lewis Acid Lewis Base hydrated metal ion = addition

compound n = charge on metal ion

= 1, 2, or 3 depending on metal atom For now assume m = 1 (monohydrate)

44


Hydrated Metal Ions = Weak Brønsted Acids

45

M(H2O)n+(aq) + H2O M(OH)n+(aq) + H3O+(aq)


Your Turn!The following reactions:

Al(OH)3 + 3H+ → Al3+ + H2O

Al(OH)3 + OH– → Al(OH)4–

illustrate the concept ofA. neutralizationB. amphoteric property of Al(OH)3

C. oxidation of AlD. reduction of OH–

46


Hydrated Metal Ions Can Act as Weak Acids

Electron deficiency of metal cations causes them to induce electron density towards metal from water of hydration

Higher charge density = more acidic metal

Acidity increases left to right across period Acidity decreases top to bottom down group

volumeioniccharge ionic

density charge

47


Acidity of Hydrated Metal Ions

Degree to which M(H2O)mn+

produces acidic solutions depends on1. Charge on cation2. Cation's size

1. As charge increases on Mn+, acidity increases Increases metal ion’s ability to draw

electron density to itself and away from O—H bond

48


Acidity of Hydrated Metal Ions2. As size of cation decreases,

acidity increases Smaller, more concentrated charge Means greater pull of electron density

from O—H bond Net result

Very small, highly charged cations are very acidic

[Al(H2O)6]3+(aq) + H2O [Al(H2O)5(OH)]2+(aq)

+ H3O+

(aq) 49


Your Turn!In the following list of pairs of ions, which is the more acidic ? Fe2+ or Fe3+; Cu2+ or Cu+; Co2+ or Co3+

A. Fe3+, Cu+, Co2+

B. Fe2+, Cu2+, Co3+

C. Fe3+, Cu2+, Co3+

D. Fe2+, Cu2+, Co2+

50


Trends in Acidity of Mn+

Acidity increases up group (column) as cation size decreases

Acidity increases across period (row) as cation size decreases

Alkali Metal Ions (Li+, Na+, K+, Rb+, Cs+)

All weak (+1, large size)

51

Be2+ Moderately weakOther Alkaline earth metals (Ba2+, Ca2+ Sr2+, Mg2+)

Very Weak

Quite acidicTransition metal ions, Al3(often +3, +4 charges)


Learning Check Identify each of the following as acidic or

basic and give their reaction with water: P2O5

P2O5(s) + 3H2O 2H3PO4(aq)

2H3PO4(aq) 2H+(aq) + 2H2PO4–

(aq)

MnO2

MnO2(s) + 2H2O Mn2+(aq) + 4OH–(aq)

52

acidic

basic


Ceramic Materials Date back to prehistoric times

Pottery as far back as 13,000 years old Today found in brick, cement, and glass

Porcelain dinnerware, tiles, sinks, toilets, artistic pottery and figurines

Composed of Silicates — compounds containing anions composed of silicon and oxygen

Advanced ceramic materials Made in chemistry laboratories High-tech applications Found in cell phones and diesel engines

53


Traditional Ceramic Synthesis1. Pulverize components of ceramic into fine

powders2. Mix with

Water and pour into mold or Binder and press into desired shape

3. Heat in kiln, tC > 1000 ˚C Sintering – particles fuse together to form

ceramic Problems:

Hard to form uniform, very small particles Ceramics formed often have small cracks

which decreases their strength Composition not easily reproducible

54


Sol-Gel Process Synthesis of ceramics that avoid problems of

particle size and uniformity Based on acid–base reactions Starting materials are

Metal salts or Compounds where metal or metalloid (e.g., Si) is

bonded to some number of alkoxide groups

55


Sol-Gel Process Metal alkoxide salts generally soluble in

alcohols Alcohols are very weak acids

Essentially no tendency to lose H+ Alkoxide ions very strong bases

React with water to form alcohol and OH– C2H5O– + H2O C2H5OH + OH–

Basis of sol-gel process

56

100%


Your Turn!Which of the following is an example of an alkoxide ion ?A. CH3OCH2

+

B. CH3CH2CHO–

C. CHCOO–

D. OH–

57


Sol-Gel Process Gradually add water to alcohol solution

of alkoxide salts Alkoxide ions gradually replaced by OH–

ionsHydrolysis reactionZr(C2H5O)4 + H2O Zr(C2H5O)3OH +

C2H5OH When two Zr(C2H5O)3OH encounter each

other, they undergo an acid-base reaction and lose H2O

58


Sol-Gel Process As more H2O is added:

More alkoxide ions converted to alcohols Form more oxide linkages bridging Zr ions Result is very fine particles of metal oxides with

residual OH ions suspended in alcohol (gel-like) Sol-gel used in various ways

1.Dip coated on surface yields thin film ceramics2.Cast into mold produces semisolid gelatin-like

material = wet gel Dry wet gel by evaporation gives porous gel =

xerogel Heating xerogel yields dense ceramic or glass

with uniform structure59


Sol-Gel Process Sol-gel used in various

ways3. Remove solvent from wet gel

at temperature above critical temperature of solvent yields very porous and extremely low density solid = aerogel

4. Adjust viscosity of gel suspension and spinning yields ceramic fibers

5. Precipitation of sol-gels yields ultrafine and uniform ceramic powders

60

TiN coating

Ceramic heat tiles


Sol-Gel Technologies and Products

61

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Chapter 16: Acids and Bases, A Molecular Look Chemistry: The Molecular Nature of Matter, 6E...

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