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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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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

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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

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BL acid/base strengths

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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

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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

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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

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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 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

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Acid/base chemistry of complexesAqueous 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

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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

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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

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AnhydridesEx: H2O + SO3 H2SO4

anhydride acid form

Acidic

SO3 / H2SO4

“P2O5” / H3PO4

CO2/H2CO3

Basic

Na2O / NaOH

Amphoteric

Al2O3 / Al(OH)3

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Trends in acidity

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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

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H2SO4 SO42 (Td)

Sulfuric acidSulfate

H2SO3 SO32 (C3v)

Sulfurous acidSulfite

You should know these!

Common acids

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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!

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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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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

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polyoxocationslinear 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

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Polyoxoanions

VO43(aq) V2O5(s)

orthovanadate (Td)

2 VO43(aq) + H2O V2O7

4 (aq) + 2OH (aq)

V3O93 V3O10

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V4O124

H3O+

oxo bridge

H3O+

H3O+

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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+

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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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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 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

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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

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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)

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

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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

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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)

C6H6 C6H7+ SbF6

HSO3F / SbF5