1. Chapter 16: Acids and Bases, A Molecular Look
Chemistry: The Molecular Nature of Matter, 6E
Jespersen/Brady/Hyslop

2. 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 solution

3. Gas Phase Acid-Base Not covered by Arrhenius definition
e.g. NH3(g) + HCl(g)  NH4Cl(s)

4. 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+

5. 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

6. 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

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

8. Learning Check Identify the conjugate partner for each conjugate base
conjugate acid
HCl
NH3
HC2H3O2
CN– HF
Cl–
NH4+
C2H3O2–
HCN F–

9. 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)
H2O + CO32–(aq)
H2CO3(aq) + H2O

10. 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–

11. Amphoteric Substances
Can act as either acid or base
Can be either molecules or ions
e.g. Hydrogen carbonate ion:
Acid
HCO3–(aq) + OH–(aq)  CO32–(aq) + H2O
Base
HCO3–(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]

12. Your Turn!
Which of the following can act as an amphoteric substance? A. CH3COOH B. HCl C. NO2– D. HPO42–

13. Strengths of Acids and Bases
Strength 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 reactions
e.g. NH3 and NRH2 (NH2CH3, methylamine)

14. 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–

15. 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

16. Your Turn!
In the reaction: HCl + H2O → H3O+ + Cl– which species is the weakest base ? A. HCl B. H2O C. H3O+ D. Cl–

17. 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)

18. Learning Check
Identify the preferred direction of the following reactions:
H3O+(aq) + CO32–(aq) HCO3–(aq) + H2O
Cl–(aq) + HCN(aq) HCl(aq) + CN–(aq)
Dialog: How does an acid's strength affect the strength of its conjugate base?

19. Trends in Binary Acid Strength
Binary Acids = HnX
X = Cl, Br, P, As, S, Se, etc.
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

20. Trends in Binary Acid Strength
Binary 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

21. Learning Check Which is stronger? H2S or H2O CH4 or NH3 HF or HI H2S
How can I compare acid and base strengths?
How can I use the periodic table to compare the strengths of binary acids?

22. Trends in Oxoacid Strength
Oxoacids (HnX Om)
Acids of H, O, and one other element
HClO, HIO4, H2SO3, H2SO4, etc.
Acids with same number of oxygen atoms and differing X
Acid strength increases from
bottom to top within group
HIO4 < HBrO4 < HClO4
Acid strength increases from left to right within period as the electronegativity of the central atom increases H3PO4 < H2SO4 < HClO4
As electronegativity of element X , electron density is drawn away from O, which draws electron density away from the O—H bond. This makes the bond more polar and makes the molecule a better proton donor.

23. Trends in Oxoacid Strength
Oxoacids (HnXOm)
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

24. Learning Check Which is the stronger acid in each pair? H2SO4 or H3PO4
HNO3 or H3PO3
H2SO4 or H2SO3
HNO3 or HNO2
H2SO4
HNO3

25. 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

26. 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:

27. 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

28. 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

29. 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 acids
e.g. CH3CO2H < CH2ClCO2H < CHCl2CO2H < CCl3CO2H

30. Your Turn! Which of the following is the strongest organic acid? A B C

31. 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

32. Lewis Neutralization
Formation of coordinate covalent bond between electron pair donor and electron pair acceptor
NH3BF3 = addition compound
Made by joining two smaller molecules
Addition Compound

33. Lewis Acid-Base Reaction
Electrons in coordinate covalent bond come from O in hydroxide ion

34. Lewis Acids: Molecules or ions with incomplete valence shells
e.g. BF3 or H+
Molecules or ions with complete valence shells, but with multiple bonds that can be shifted to make room for more electrons
e.g. CO2
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 below
e.g. SO2

35. SO2 as Lewis Acid
O2–

36. 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

37. H+ Transfer from Lewis Perspective
e.g. H2O—H+ + NH3  H2O + H+—NH3

38. Learning Check Identify the Lewis acid and base in the following:
NH3 + H+ NH4+
Base Acid
F– BF3 BF4–
SeO3 + O2– SeO42–
Acid Base

39. Your Turn!
Which of the following species can act as a Lewis base ? A. Cl– B. Fe2+ C. NO2– D. O2–

40. 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)

41. 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)

42. 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 crystal
Fe2O3(s) + 6H+(aq)  2Fe3+(aq) + 3H2O

43. Your Turn! What is the acid formed by P2O3 when it reacts with water ?
A. H2PO4
B. H2PO2
C. H3PO4
D. H3PO3
P2O H2O → 2H3PO3

44. 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)

45. Hydrated Metal Ions = Weak Brønsted Acids
M(H2O)n+(aq) + H2O M(OH)n+(aq) + H3O+(aq)

46. Your Turn! The following reactions: Al(OH)3 + 3H+ → Al3+ + H2O
Al(OH)3 + OH– → Al(OH)4–
illustrate the concept of
A. neutralization
B. amphoteric property of Al(OH)3
C. oxidation of Al
D. reduction of OH–

47. 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

48. Acidity of Hydrated Metal Ions
Degree to which M(H2O)mn+ produces acidic solutions depends on
Charge on cation
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

49. Acidity of Hydrated Metal Ions
2. 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)

50. 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+

51. 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)
Be2+
Moderately weak
Other Alkaline earth metals (Ba2+, Ca2+ Sr2+, Mg2+)
Very Weak
Transition metal ions, Al3(often +3, +4 charges)
Quite acidic

52. 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)
acidic
basic

53. 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

54. Traditional Ceramic Synthesis
Pulverize components of ceramic into fine powders
Mix with
Water and pour into mold or
Binder and press into desired shape
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

55. 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

56. 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
100%

57. Your Turn!
Which of the following is an example of an alkoxide ion ? A. CH3OCH2+ B. CH3CH2CHO– C. CHCOO– D. OH–

58. Sol-Gel Process
Gradually add water to alcohol solution of alkoxide salts
Alkoxide ions gradually replaced by OH– ions
Hydrolysis reaction
Zr(C2H5O)4 + H2O  Zr(C2H5O)3OH + C2H5OH
When two Zr(C2H5O)3OH encounter each other, they undergo an acid-base reaction and lose H2O

59. Sol-Gel Process As more H2O is added: Sol-gel used in various ways
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
Dip coated on surface yields thin film ceramics
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 structure

60. Sol-Gel Process Sol-gel used in various ways
Remove solvent from wet gel at temperature above critical temperature of solvent yields very porous and extremely low density solid = aerogel
Adjust viscosity of gel suspension and spinning yields ceramic fibers
Precipitation of sol-gels yields ultrafine and uniform ceramic powders
TiN coating
Ceramic heat tiles

61. Sol-Gel Technologies and Products