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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 (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 Since pKa values are generally well-separated, only 1 or 2 species will be present at significant concentration at any pH H3PO4 + H2O H2PO H3O pKa1 = 2.1 H2PO H2O HPO H3O pKa1 = 7.4 HPO H2O PO 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
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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) slow very strong base OH O2
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Acid/base chemistry of complexes
Aqueous chemistry: Fe(NO3)3 [Fe(OH2)6]3+(aq) 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 acidity For aqueous ions: pKa vs z2 / (r++ d)
. Higher charge is more acidic pKa of [Fe(OH2)]3+ ~ 3 pKa of [Fe(OH2)]62+ ~ 9 . 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
“P2O5” / H3PO4 CO2/H2CO3 Basic Na2O / NaOH Amphoteric Al2O3 / Al(OH)3
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Trends in acidity
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Common acids You should know these! HNO3 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 You should know these! H2SO4 SO42 (Td)
Sulfuric acid Sulfate H2SO3 SO32 (C3v) Sulfurous acid Sulfite You should know these!
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Common acids You should know these! 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
Write formula as MOp(OH)q pKa 8 – 5p 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 (exptl value is 7.4) pKa3 (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
HlO H2O 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 ~ dimer H3O+ OH
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polyoxocations linear trimer is [Al3(OH2)14(OH)2]7+
charge/volume ratios Al(OH2) > dimer > trimer > Al(OH)3 3+ / Oh / 2 Oh 7+ / 3 Oh neutral Keggin ion [AlO4(Al(OH)2)12]7+ pH ≈ 4
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Polyoxoanions H3O+ VO43(aq) V2O5(s)
orthovanadate (Td) 2 VO43(aq) H2O V2O74 (aq) + 2OH (aq) V3O93 V3O105 V4O124 H3O+ H3O+ oxo bridge
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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 :CO [W(CO)6] BCl3 + :OEt2 BCl3:OEt2 D3h Fe3+(g) :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: BCl :NR3 Cl3B:NR3 Kf: NH3 < MeNH2 < Me2NH < Me3N inductive effect BMe :NR3 Me3B:NR3 Kf: NH3 < MeNH2 < Me2NH > Me3N inductive + steric Hrxn 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 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 H2O 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 nitrogen; it’s trigonal bipyramidal for M = P, As SbF5: Sb has Oh coordination (oligomerizes to Sb4F20 or Sb6F30) LB LA transient K2MnF6 (s) SbF5 (l) “MnF4” KSbF6 (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 ~ pKa ~ 5 pKa ~ 26 (superacid) C6H C6H7+ SbF6 HSO3F / SbF5
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