Acids and Bases

1
Chapter 7

• Acids and Bases

2
History

• Svante Arrhenius in 1884
• acids contain hydrogen ions and bases contain
  hydroxide ions
• the solvent problem
• difference in properties of a compound in two
  different solvents
• The salt problem
• salts should be neutral

3
Ionization and Dissociation

• Dissociation
• the separation of ions
• Ionization
• the breaking of covalent bonds resulting in ion
  formation

4
Brønsted-Lowry Theory

• Acids
• proton donor
• Bases
• proton acceptor
• Water
• undergoes autoionization

5
Autoionization

• H2O(l) H2O(l) ? H3O(aq) OH-(aq)
• Kw H3OOH- 1.0 x 10-14 at 25C
• 1.2 x 10-15 at 0C and 4.8 x 10-13 at 100C
• Conjugate acid-base pairs
• one molecule donates a proton (acid), another
  accepts one (base)
• when the reaction is reversed, the hydronium ion
  donates a proton (acid) and the hydroxide ion
  accepts a proton (base)
• Amphiproticity
• acting as either an acid or a base

6
Hydronium Ion

• H3O
• tetrahedral molecular geometry
• naturally exists as a trihydrate in solution
• H9O4

7
Acid-Base Behavior

• Water functioning as a base
• HF(aq) H2O(l) ? H3O(aq) F-(aq)
• HF is the acid, H2O is the base
• H3O (conjugate acid), F- (conjugate base)
• Water functioning as an acid
• NH3(aq) H2O(l) ? NH4(aq) OH-(aq)
• H2O is the acid, NH3 is the base
• NH4 (conjugate acid), OH- (conjugate base)

8
Strong Acids and Bases

• H3OCl-(aq) NaOH-(aq) ? 2H2O(l) NaCl-(aq)
• H3O(aq) OH-(aq) ? 2H2O(l)
• Strong bases and strong acids dissociate
  completely in water
• O2-(aq) H2O(l) ? 2OH-(aq)
• HClO4(aq) H2O(l) ? H3O(aq) ClO4-(aq)

9
Acid-Base Chemistry in Other Solvents

• Ammonia (NH3) also undergoes autoionization
• NH3(l) NH3(l) ? NH4(NH3) NH2-(NH3)
• Acid-base reactions in ammonia
• NH4Cl-(NH3) NaNH2-(NH3) ?
• NaCl-(s) 2NH3(l)

10
Acid-Base Equilibrium Constants

• Acid ionization constant, Ka
• measure of how easily a proton can be removed
• HA(aq) H2O(l) ? H3O(aq) A-(aq)
• Ka expression written as
• pKa
• pKa -(log10Ka)
• the more negative pKa, the stronger the acid

11
Acid-Base Equilibrium Constants

• Base ionization constant, Kb
• measure of how easily a proton is accepted
• A-(aq) H2O(l) ? HA(aq) OH-(aq)
• Kb expression written as
• pKb
• pKb -(log10Kb)
• the more positive pKb, the weaker the base

12
Acid-Base Equilibrium Constants

• Ion product constant for water, Kw
• Kw KaKb H3OOH- 1 x 10-14
• pKw pKa pKb 14
• the stronger the base, the weaker the conjugate
  acid and vice versa

13
Brønsted-Lowry Acids

• If Ka gt 1 (negative pKa values),
• regarded as a strong acid
• undergo 100 ionization
• If Ka lt 1 (positive pKa values),
• regarded as a weak acid
• establish an equilibrium (less than 100
  ionization)

14
Binary Acids

• Most common are the hydrohalic acids
• HF is the only weak acid of the four
• HI is the strongest acid
• HX(aq) H2O(l) ? H3O(aq) X-(aq)

Acid pKa Bond Energy (kJ/mol)
HF(aq) 3 565
HCl(aq) -7 428
HBr(aq) -9 362
HI(aq) -10 295
H2O (l) 7 459
15
Oxyacids

• Ternary acids containing oxygen
• the ionizable hydrogen is bond to oxygen
• HNO3 (HONO2)
• the more oxygen atoms, the stronger the acid
• HNO3 (pKa -1.4) vs. HNO2 (pKa 3.3)
• (OH)nXOm
• (m0, pKa8)(m1, pKa2)(m2, pKa-1)(m3,
  pKa-8)

16
Polyprotic Acids

• Acids containing more than one ionizable hydrogen
• H2SO4(aq) H2O(l) ? H3O(aq) HSO4-(aq)
• (pKa -2)
• HSO4-(aq) H2O(l) ? H3O(aq) SO42-(aq)
• (pKa 1.9)

17
Brønsted-Lowry Bases

• OH- is the most important base
• NH3 is the next most important
• NH3(aq) H2O(l) ? NH4(aq) OH-(aq)
• Other equilibria
• PO43-(aq) H2O(l) ? HPO42-(aq) OH-(aq) (pKb
  1.35)
• S2-(aq) H2O(l) ? HS-(aq) OH-(aq) (2.04)
• F-(aq) H2O(l) ? HF(aq) OH-(aq) (10.55)

18
Multiple Ionizations in Bases

• S2-(aq) H2O(l) ? HS-(aq) OH-(aq) (2.04)
• HS-(aq) H2O(l) ? H2S(aq) OH-(aq) (6.96)
• and
• PO43-(aq) H2O(l) ? HPO42-(aq) OH-(aq) (1.35)
• HPO42-(aq) H2O(l) ? H2PO4-(aq) OH-(aq) (6.79)
• H2PO4-(aq) H2O(l) ? H3PO4(aq) OH-(aq) (11.88)

19
The Acidity of Metal Ions

• Dissolving metal ions in water give hydrated ions
• weakly held water molecules (low charge density)
  are neutral
• strongly held water molecules (high charge
  density) tend to be acidic
• Fe(OH2)63(aq) H2O(l) ? H3O(aq)
  Fe(OH2)5(OH)2(aq)

20
The Acidity of Metal Ions

• Ions with 3 or higher are always acidic
• reaction with water is called hydrolysis

Acidity of Some Metal Ions Acidity of Some Metal Ions Acidity of Some Metal Ions Acidity of Some Metal Ions
Li Be2
slightly acidic weakly acidic
Na Mg2 Al3
neutral weakly acidic acidic
K Ca2 Sc3 Ti4
neutral slightly acidic acidic very acidic
21
pKa Values for Metal Ions

• As many as there are positive charges
• Fe(OH2)63(aq) H2O(l) ? H3O(aq)
  Fe(OH2)5(OH)2(aq) (pKa1)
• Fe(OH2)5(OH)2(aq) H2O(l) ? H3O(aq)
  Fe(OH2)4(OH)2(aq) (pKa2)
• Fe(OH2)4(OH)2(aq) H2O(l) ? H3O(aq)
  Fe(OH)3(aq) 3H2O(l) (pKa3)

22
Basicity of Nonmetal Anions

• Dissolving anions in water give neutral to basic
  species
• the lower the charge density, the more basic the
  solution

Basicity of Some Nonmetal Ions Basicity of Some Nonmetal Ions Basicity of Some Nonmetal Ions
N3- O2- F-
very basic very basic weakly basic
P3- S2- Cl-
very basic basic neutral
As3- Se2- Br-
very basic basic neutral
Te2- I-
weakly basic neutral
23
Basicity of Nonmetal Anions

• Weakly basic
• F-(aq) H2O(l) ? HF(aq) OH-(aq)
• Basic
• S2-(aq) H2O(l) ? HS-(aq) OH-(aq)
• Very Basic
• O2-(aq) H2O(l) ? 2OH-(aq)

24
Basicity of Oxyanions

• Depends upon the number of oxygen atoms and ion
  charge
• XOn-
• as n decreases, the basicity increases
• the greater oxygen content weakens the O-H bond,
  making hydrolysis more difficult
• XO4n-
• as the charge increases, the basicity increases

25
Basicity of Oxyanions
Basicity of Some XOn- Oxyanions Basicity of Some XOn- Oxyanions Basicity of Some XOn- Oxyanions
Classification Type Examples
Neutral XO4- ClO4-, MnO4-
Neutral XO3- NO3-,ClO3-
Weakly basic XO2- NO2-,ClO2-
Moderately basic XO- ClO-
Basicity of Some XO4n- Oxyanions Basicity of Some XO4n- Oxyanions Basicity of Some XO4n- Oxyanions
Classification Type Examples
Neutral XO4- ClO4-,MnO4-
Weakly basic XO42- SO42-,CrO42-
Moderately basic XO43- PO43-,VO43-
Strongly basic XO44- SiO42-
26
Oxides

• Show many different properties
• most metal oxides
• basic
• most nonmetal oxides
• acidic
• weak metal oxides
• amphoteric
• a few nonmetal and metal oxides
• neutral

27
Acid-Base Reactions of Oxides

• Reaction of nonmetal acidic oxides with basic
  metal oxides
• CaO(s) SO2(g) ? CaSO3(s)
• 2CaSO3(s) O2(g) ? 2CaSO4(s)
• CaCO3(s) heat ? CaO(s) CO2(g)

28
Acid-Base Reactions of Oxides

• Reaction of acidic, nonmetal oxides with bases
• CO2(g) 2NaOH(aq) ? Na2CO3(aq) H2O(l)
• Reaction of basic, metal oxides with acids
• MgO(s) 2HNO3(aq) ? Mg(NO3)2(aq) H2O(l)

29
Comparing Acidities

• Measure the free energy change of the reaction of
  different acids with the same base
• CaO(s) CO2(g) ? CaCO3(s)
• ?G -134 kJ/mol
• CaO(s) SO3(g) ? CaSO4(s)
• ?G -347 kJ/mol
• the larger the ?G, the stronger the acid

30
Comparing Basicities

• Measure the free energy change of the reaction of
  different bases with the same acid
• Na2O(s) H2O(l) ? 2NaOH(s)
• ?G -142 kJ/mol
• CaO(s) H2O(l) ? Ca(OH)2(s)
• ?G -59 kJ/mol
• the larger the ?G, the stronger the base

31
Acid/Base Chemistry in Geology

• Silicate rocks can be classified according to
  their oxide content
• combination of basic metal oxides and acidic
  silicon dioxide
• gt66 SiO2, acidic
• 52-66 SiO2, intermediate
• 45-52 SiO2, basic
• lt45 SiO2, ultrabasic

32
Pearson Hard-Soft Acid/Base (HSAB) Concept

• Lewis acids/bases can be classified as either
  hard or soft
• a reaction will proceed in which the softer acids
  would pair with the softer bases
• NaI AgNO3 ? AgI NaNO3 ???
• AgI NaNO3 ? NaI AgNO3 ???

33
Hard Acids

• class a metal ions
• consist of most of the metal ions
• low electronegativities
• high charge densities
• H B3 C4 Fe3

34
Soft Acids

• class b metal ions
• lower right portion of the metallic elements
• lower charge densities
• higher electronegativities
• Au Ag Cu

35
Borderline Acids

• intermediate metal ions
• intermediate charge densities
• intermediate electronegativities
• Cu, charge density 51 C/mm3, soft
• Cu2, charge density 116 C/mm3, borderline
• Fe3, charge density gt200 C/mm3, hard
• Fe2, charge density 100 C/mm3, borderline

36
HSAB Acid Classification
37
Hard Bases

• class a ligands
• high charge densities
• high electronegativities
• fluorine and oxygen-bonded species
• oxide
• hydroxide
• nitrate
• phosphate
• fluoride

38
Soft Bases

• class b ligands
• low charge densities
• low electronegativities
• carbon
• sulfur
• phosphorus
• iodine

39
Borderline Bases

• bromide
• azide
• ambidentate ligands
• thiocyanate
• -NCS, borderline
• -SCN, soft base

40
Applications of the HSAB Concept

• Prediction of chemical reactions

HgF2(g) BeI2(g) ? BeF2(g) HgI2(g)
soft-hard hard-soft hard-hard soft-soft
AgBr(s) I-(aq) ? AgI(s) Br-(aq)
soft-borderline soft soft-soft borderline
41
Applications of the HSAB Concept

• Qualitative analysis
• Group I metals
• cations which form insoluble chlorides
• Group II metals
• cations which form soluble chlorides and very
  insoluble sulfides
• Group III metals
• cations which form soluble chlorides and
  insoluble sulfides
• Group IV metals
• cations which form soluble chlorides and soluble
  sulfides
• Group V metals
• cations which form few, insoluble salts

42
Applications of the HSAB Concept

• Qualitative analysis

Group I Group II Group III Group IV Group V
AgCl HgS MnS CaCO3 Na
PbCl2 CdS FeS SrCO3 K
Hg2Cl2 CuS CoS BaCO3 NH4
SnS2 NiS Mg2
As2S3 ZnS
Sb2S3 Al(OH)3
Bi2S3 Cr(OH)3
43
Applications of the HSAB Concept

• Qualitative analysis
• Group I
• Group II

Ag(OH2)n(aq) Cl(H2O)m-(aq) ? AgCl(s) (nm)H2O(l)
soft-hard borderline-hard soft-bord. hard-hard
Cd(OH2)n2(aq) S(H2O)m2-(aq) ? CdS(s) (nm)H2O(l)
soft-hard soft-hard soft-soft hard-hard
44
HSAB in Geochemistry

• Classification of the chemical elements
• siderophiles
• metals found in elemental form on the surface of
  the Earth
• chalcophiles
• sulfides
• lithophiles
• metals and nonmetals which occur as oxides,
  silicates, sulfates, or carbonates
• atmophiles
• natural elements in the atmosphere

45
HSAB in Geochemistry

• Lithophiles
• hard acids
• prefer oxygen-bound hard bases
• Al2O3 (bauxite)
• CaCO3 (limestone, chalk, marble)

46
HSAB in Geochemistry

• Chalcophiles
• borderline/soft acids
• prefer soft bases such as sulfide
• ZnS (sphalerite, wurtzite)
• HgS (cinnabar)

47
Interpretation of the HSAB Concept

• Hard acid hard base combination
• pairing of a low electronegativity cation with a
  high electronegativity anion
• ionic behavior
• Soft acid soft base combination
• pairing of a higher electronegative cation with a
  lower electronegative anion
• covalent behavior

48
Biological Aspects

• Toxic elements
• elements which are toxic at very low
  concentrations
• beryllium
• arsenic
• selenium
• cadmium
• indium
• tellurium
• mercury
• thallium
• lead

49
Biological Aspects

• Cysteine, an amino acid
• zinc (borderline acid) normally binds to the
  thiol site (soft base)
• all the toxic metals (except beryllium) are
  softer acids than zinc
• the toxic nonmetals are soft bases which
  preferentially bind to iron(II) and zinc