CH4. Acids and Bases
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Bronsted-Lowry
Bronsted-Lowry definitions:
Acid = proton donor; Base = proton acceptor
+ - HF (aq) + H2O H3O (aq) + F (aq) BL acid BL base
Fluoride ion is the conjugate base of HF
Hydronium ion is the conjugate acid of H2O
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1 Amphiprotic species
Amphiprotic – species that can act as BL acid or base
+ NH3 (aq) + H2O NH4 (aqu) + OH (aqu) BL base BL acid hydroxide
+ Kb = base dissociation constant = [NH4 ] [OH ] / [NH3]
H2O is amphiprotic - it‟s a base with HF, but an acid with NH3
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BL acid/base strength
Ka, the acidity constant, measures acid strength as:
+ - Ka = [H3O ] [A ] / [HA]
pKa = - log Ka
For strong acids - When pH = pKa, then [HA] = [A ]
pKa < 0
pKa(HCl) ≈ -7
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2 BL acid/base strengths
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Kw
Kw = water autodissociation (autoionization) constant
+ - 2 H2O H3O (aqu) + OH (aqu)
+ - -14 Kw = [H3O ] [OH ] = 1 x 10 (at 25°C)
Using the above, you should prove that for any conjugate acid-base pair:
pKa + pKb = pKw = 14
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3 Polyprotic acids
Since pKa values are generally well- separated, only 1 or 2 species will be present at significant concentration at any pH
- + H3PO4 + H2O H2PO4 + H3O pKa1 = 2.1
- 2- + H2PO4 + H2O HPO4 + H3O pKa1 = 7.4
2- 3- + HPO4 + H2O PO4 + H3O pKa1 = 12.7
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Solvent leveling
+ The strongest acid possible in aqueous solution is H3O + - Ex: HCl + H2O H3O (aq) + Cl (aq) there is no appreciable equilibrium, this reaction goes quantitatively; the acid form of HCl does not exist in aqueous solution
+ - Ex: KNH2 + H2O K (aq) + OH (aq) + NH3 (aq) this is solvent leveling, the stable acid and base species are the BL acid-base pair of the solvent
- NH2 = imide anion - NR2 , some substituted imide ions are less basic and can exist in aq soln 8
4 Solvent leveling
Only species with 0 < pKa < 14 can exist in aqueous solutions. + The acid/base range for water stability pKw, i.e. 14 orders of mag in [H ]. Other solvents have different windows and different leveling effects.
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Solvent leveling
+ 20 2EtOH EtOH2 (solv) + EtO (solv) K ~ 10
chemistry in the range of -3 < pKa < 17
+ NH3 NH4 (solv) + NH2 (solv) ammonium imide
chemistry in the range of 10 < pKa < 38
O2 OH NH3(l) + Na (m) Na (solv) + NH2 (solv) + ½ H2 (g) slow very strong base
Na+ (solv) + e (solv) 10
5 Acid/base chemistry of complexes
Aqueous chemistry:
H2O 3+ Fe(NO3)3 [Fe(OH2)6] (aq) + 3 NO3 (aq)
3+ 5+ + 2 [Fe(OH2)6] (aq) [Fe2(OH2)10OH] (aq) + H3O (aq) dimer Hexaaquairon(III), pKa ~ 3
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Aqua, hydroxo, oxoacids
n+ 2+ aqua acid M(OH2)x ex: [Cu(OH2)6] hexaaquacopper(II) cation
hydroxoacid M(OH)x ex: B(OH)3 , Si(OH)4 pKa ~ 10
oxoacid MOp(OH)q p and q designate oxo and hydroxo ligands
+ ex: H2CO3 (aq) + H2O HCO3 (aq) + H3O (aq) carbonic acid bicarbonate
pKa ~ 3.6 CO2 (g) + H2O
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6 Trends in acidity
For aqueous ions: 2 pKa vs z / (r++ d) 1. Higher charge is more acidic
3+ pKa of [Fe(OH2)] ~ 3 2+ pKa of [Fe(OH2)]6 ~ 9 2. Smaller radius is more acidic Mn2+ Cu2+ early TM late TM lower Z* higher Z* => larger radius => smaller radius less acidic more acidic
+ + Na (aqu) = [Na(OH2)6] has pKa > 14 so it‟s a spectator ion in aqu soln 13
Anhydrides
Ex: H2O + SO3 H2SO4 anhydride acid form Acidic
SO3 / H2SO4
“P2O5” / H3PO4
CO2/H2CO3
Basic
Na2O / NaOH
Amphoteric
Al2O3 / Al(OH)3 14
7 Trends in acidity
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Common acids
HNO3 NO3 (D3h) Nitric acid Nitrate
HNO NO (C ) 2 2 2v You should know these! Nitrous acid Nitrite
3 H3PO4 PO4 (Td) Phosphoric acid Phosphate
2 H3PO3 HPO3 (C3v) Phosphorous acid Phosphite
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8 Common acids
2 H2SO4 SO4 (Td) Sulfuric acid Sulfate You should know these!
2 H2SO3 SO3 (C3v) Sulfurous acid Sulfite
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Common acids
HClO4 ClO4 (Td) Perchloric acid Perchlorate
HClO3 ClO3 (C3v) Chloric acid Chlorate You should know these!
HClO2 ClO2 (C2v) Chlorous acid Chlorite
HOCl OCl Hypochlorous acid Hypochlorite
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9 Pauling‟s rules for pKa„s of oxoacids
1. Write formula as MOp(OH)q
2. pKa 8 – 5p
3. Each succeeding deprotonation increases the pKa by 5
Ex: rewrite HNO3 as NO2(OH)
p = 2; pKa 8 – 5(2) 2 (exptl value is 1.4)
Ex: rewrite H3PO4 as PO(OH)3
p = 1; pKa1 8 – 5(1) 3 (exptl value is 2.1)
pKa2 8 (exptl value is 7.4)
pKa3 13 (exptl value is 12.7)
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pKa values
p Pauling pKa calcn exptl Cl(OH) 0 8 7.5 ClO(OH) 1 3 2.0
ClO2(OH) 2 2 1.2
ClO3(OH) 3 7 ≈ 10
HlO4 + 2H2O H5IO6
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10 Amphoteric oxides
3+ [Al(OH2)6] Al2O3 / Al(OH)3 [Al(OH)4] + Oh H3O OH Td
3+ 5+ + 2 [Al(OH2)6] (aq) [Al2(OH2)10(OH)] (aq) + H3O (aq)
pKa ~ 2 dimer
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polyoxocations
7+ linear trimer is [Al3(OH2)14(OH)2]
Keggin ion
7+ [AlO4(Al(OH)2)12] pH ≈ 4 charge/volume ratios
3+ Al(OH2)6 > dimer > trimer --- > Al(OH)3 3+ / Oh 5+ / 2 Oh 7+ / 3 Oh neutral
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11 Polyoxoanions
+ H3O 3 VO4 (aq) V2O5(s) orthovanadate (Td)
3 4 2 VO4 (aq) + H2O V2O7 (aq) + 2OH (aq)
+ H3O
3 5 V3O9 V3O10
+ H3O oxo bridge 4 23 V4O12
Lewis acids and bases
A + B: A:B LA LB complex
LA = electron pair acceptor; LB = electron pair donor Lewis definition is more general than BL definition, does not require aqueous or protic solvent Ex: W + 6 :CO [W(CO)6]
BCl3 + :OEt2 BCl3:OEt2
D3h
3+ 3+ Fe (g) + 6 :OH2 → [Fe(OH2)6] 24
12 LA/LB strengths
LA strength is based on reaction Kf LA/LB strengths depend on specific acid base combination
Ex: BCl3 + :NR3 Cl3B:NR3
Kf: NH3 < MeNH2 < Me2NH < Me3N inductive effect
BMe3 + :NR3 Me3B:NR3
Kf: NH3 < MeNH2 < Me2NH > Me3N inductive + steric
Hrxn 58 74 81 74 kJ/mol
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log K and ligand type
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13 Drago-Wayland equation
A (g) + :B (g) A:B (g) Gas phase reactions (omits solvation effects)
-Hrxn = EA EB + CA CB look up E, C values for reactants (Table 4.4)
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Donor/Acceptor numbers
Commonly used to choose appropriate solvents (Table 4.5)
Donor Number (DN) is derived from Hrxn (SbCl5 + :B Cl5Sb:B) higher DN corresponds to stronger LB
Acceptor Number (AN) is derived from stability of Et3P=O:A complex higher AN corresponds to stronger LA
Ex: THF (tetrahydrofuran) C4H8O DN AN ε dielectric constant THF 20 8 7
H2O 18 55 82
+ Some Li salts and BF3 have similar solubilities in THF, H2O
NH3 is much more soluble in H2O
Most salts are much more soluble in H2O 28
14 Descriptive chemistry - Group 13
Expect inductive effect BF3 > BCl3 > BBr3 but the opposite is true
ex: BF3 is stable in H2O, R2O (ethers)
BCl3 rapidly hydrolyzes due to nucleophilic attack of :OH2
the lower acidity of BF3 is due to unusually favorable B–X bonding in the planar conformation due to interaction
“AlCl3“ is a dimer (Al2Cl6) General trend larger central atom, tends to have higher CN
Al2Me6 is isostructural with Al2Cl6
C6H6 C6H5C(O)R Friedel-Crafts
RC(O)-X: + “AlCl3” RC(O) + AlCl3X
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Descriptive chemistry - Group 14
CX4 is not a Lewis Acid
Acidity SiF4 > SiCl4 > SiBr4 > SiI4 (inductive effect)
ex: 2KF(s) + SiF4(g) K2SiF6(s) 2 LB LA SiF6 Oh
SnF4 and PbF4 have Oh not Td coordination (heavier congener, higher CN) each M has 2 unique axial F and 4 shared F
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15 Descriptive chemistry - Group 15
MF5 does not exist for M=N; trigonal bipyramidal for M = P, As
SbF5: Sb has Oh coordination (oligomerizes to Sb4F20 or Sb6F30)
LB LA transient
K2MnF6 (s) + 2 SbF5 (l) “MnF4” + 2KSbF6 (s) F transfer
KF, H2O2 aqu HF
KMnO4 Sb2O3 MnF3 + ½ F2 (g)
Dove (1980‟s), chemical synthesis of F2 gas
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Descriptive chemistry - Group 16
Inductive effect stabilizes conjugate base (anionic form)
sulfuric acid fluorosulfonic HSO3F / SbF5
pKa ~ 2 pKa ~ 5 pKa ~ 26 (superacid)
HSO F / SbF 3 5 + C6H6 C6H7 SbF6
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