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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 conjugate

acid-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 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

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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 leveling

2EtOH 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 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

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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 ~ 3.6CO2 (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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Anhydrides

Ex: 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!

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Common acids

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H2SO4 SO42

(Td)

Sulfuric acid Sulfate

H2SO3 SO32

(C3v)

Sulfurous acid Sulfite

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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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

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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 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

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 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)

C6H6 C6H7+ SbF6

HSO3F / SbF5