Date post: | 13-Dec-2015 |
Category: |
Documents |
Upload: | tracy-daniel |
View: | 233 times |
Download: | 2 times |
1
CH4. Acids and Bases
2
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
3
Amphiprotic species
Amphiprotic – species that can act as BL acid or base
NH3 (aq) + H2O NH4+ (aqu) + OH (aqu)
BL acid hydroxide
Kb = base dissociation constant = [NH4+] [OH] / [NH3]
H2O is amphiprotic - it’s a base with HF, but an acid with NH3
BL base
4
BL acid/base strength
Ka, the acidity constant, measures acid strength as:
Ka = [H3O+] [A-] / [HA]
pKa = - log Ka
When pH = pKa, then [HA] = [A-]
For strong acids
pKa < 0
pKa(HCl) ≈ -7
5
BL acid/base strengths
6
Kw
Kw = water autodissociation (autoionization) constant
2 H2O H3O+ (aqu) + OH- (aqu)
Kw = [H3O+] [OH-] = 1 x 10-14 (at 25°C)
Using the above, you should prove that for any conjugateacid-base pair:
pKa + pKb = pKw = 14
7
Polyprotic acids
H3PO4 + H2O H2PO4- + H3O+ pKa1 = 2.1
H2PO4- + H2O HPO4
2- + H3O+ pKa1 = 7.4
HPO42- + H2O PO4
3- + H3O+ pKa1 = 12.7
Since pKa values are generally well-separated, only 1 or 2 species will be present at significant concentration at any pH
8
Solvent levelingThe 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
9
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.
10
Solvent leveling2EtOH EtOH2
+(solv) + EtO (solv) K ~ 1020
chemistry in the range of -3 < pKa < 17
NH3 NH4+(solv) + NH2
(solv)
ammonium imide
chemistry in the range of 10 < pKa < 38
Na (m) Na+ (solv) + NH2(solv) + ½ H2 (g)
Na+ (solv) + e (solv)
NH3(l)
slowvery strong base
OH
O2
11
Acid/base chemistry of complexes
Aqueous chemistry:
Fe(NO3)3 [Fe(OH2)6]3+(aq) + 3 NO3(aq)
2 [Fe(OH2)6]3+ (aq) [Fe2(OH2)10OH]5+ (aq) + H3O+(aq)
Hexaaquairon(III), pKa ~ 3
H2O
dimer
12
Aqua, hydroxo, oxoacids
aqua acid M(OH2)xn+ ex: [Cu(OH2)6]2+ 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
13
Trends in acidityFor aqueous ions:
1. Higher charge is more acidic
pKa of [Fe(OH2)]3+ ~ 3
pKa of [Fe(OH2)]62+ ~ 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
pKa vs z2 / (r++ d)
Na+ (aqu) = [Na(OH2)6]+ has pKa > 14 so it’s a spectator ion in aqu soln
14
AnhydridesEx: H2O + SO3 H2SO4
anhydride acid form
Acidic
SO3 / H2SO4
“P2O5” / H3PO4
CO2/H2CO3
Basic
Na2O / NaOH
Amphoteric
Al2O3 / Al(OH)3
15
Trends in acidity
16
Common acidsHNO3 NO3
(D3h)
Nitric acid Nitrate
HNO2 NO2 (C2v)
Nitrous acid Nitrite
H3PO4 PO43 (Td)
Phosphoric acid Phosphate
H3PO3 HPO32 (C3v)
Phosphorous acid Phosphite
You should know these!
Common acids
17
H2SO4 SO42 (Td)
Sulfuric acidSulfate
H2SO3 SO32 (C3v)
Sulfurous acidSulfite
You should know these!
Common acids
18
HClO4 ClO4 (Td)
Perchloric acid Perchlorate
HClO3 ClO3 (C3v)
Chloric acid Chlorate
HClO2 ClO2 (C2v)
Chlorous acid Chlorite
HOCl OCl
Hypochlorous acid Hypochlorite
You should know these!
19
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)
20
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
21
Amphoteric oxides
[Al(OH2)6]3+ Al2O3 / Al(OH)3 [Al(OH)4]
Oh Td
2 [Al(OH2)6]3+(aq) [Al2(OH2)10(OH)]5+(aq) + H3O+(aq)
pKa ~ 2 dimer
H3O+ OH
22
polyoxocations
linear trimer is [Al3(OH2)14(OH)2]7+
charge/volume ratios
Al(OH2)63+ > dimer > trimer --- > Al(OH)3
3+ / Oh 5+ / 2 Oh 7+ / 3 Oh neutral
Keggin ion
[AlO4(Al(OH)2)12]7+
pH ≈ 4
23
Polyoxoanions
VO43(aq) V2O5(s)
orthovanadate (Td)
2 VO43(aq) + H2O V2O7
4 (aq) + 2OH (aq)
V3O93 V3O10
5
V4O124
H3O+
oxo bridge
H3O+
H3O+
24
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
Fe3+(g) + 6 :OH2 → [Fe(OH2)6]3+
25
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
26
log K and ligand type
27
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)
28
Donor/Acceptor numbersCommonly 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
29
Descriptive chemistry - Group 13Expect 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
Friedel-Crafts
RC(O)-X: + “AlCl3” RC(O) + AlCl3X
C6H6 C6H5C(O)R
30
Descriptive chemistry - Group 14
CX4 is not a Lewis Acid
Acidity SiF4 > SiCl4 > SiBr4 > SiI4 (inductive effect)
ex: 2KF(s) + SiF4(g) K2SiF6(s)
LB LA SiF62 Oh
SnF4 and PbF4 have Oh not Td coordination (heavier congener, higher CN)
each M has 2 unique axial F and 4 shared F
31
Descriptive chemistry - Group 15
MF5 does not exist for nitrogen; it’s 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
32
Descriptive chemistry - Group 16
Inductive effect stabilizes conjugate base (anionic form)
sulfuric acid fluorosulfonic HSO3F / SbF5
pKa ~ 2 pKa ~ 5 pKa ~ 26 (superacid)
C6H6 C6H7+ SbF6
HSO3F / SbF5