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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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Gas Phase Acid-Base Not covered by Arrhenius definition
e.g. NH3(g) + HCl(g) NH4Cl(s)
3
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Learning Check
conjugate base
conjugate acid
HCl
NH3
HC2H3O2
CN–
HF
8
Identify the conjugate partner for each
Cl–
NH4+
C2H3O2–
HCN
F–
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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)
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Your Turn!Which of the following can act as an amphoteric substance?A. CH3COOH
B. HClC. NO2
–
D. HPO42–
12
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Your Turn!In the reaction: HCl + H2O → H3O+ + Cl–
which species is the weakest base ?A. HClB. H2O
C. H3O+
D. Cl–
16
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Learning Check
Which is stronger?
H2S or H2O
CH4 or NH3
HF or HI
H2S
NH3
HI
21
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Lewis Acid-Base Reaction
Electrons in coordinate covalent bond come from O in hydroxide ion
33
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
SO2 as Lewis Acid
35
O2–
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
H+ Transfer from Lewis Perspectivee.g. H2O—H+ + NH3 H2O + H+—NH3
37
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Your Turn!Which of the following species can act as a Lewis base ?A. Cl–
B. Fe2+
C. NO2–
D. O2–
39
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Hydrated Metal Ions = Weak Brønsted Acids
45
M(H2O)n+(aq) + H2O M(OH)n+(aq) + H3O+(aq)
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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)
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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%
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Your Turn!Which of the following is an example of an alkoxide ion ?A. CH3OCH2
+
B. CH3CH2CHO–
C. CHCOO–
D. OH–
57
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
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
Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E
Sol-Gel Technologies and Products
61