Chapter 14Aqueous Equilibria: Acids and Bases
Polyprotic AcidsPolyprotic Acids Acids that contains more than one dissociable proton Dissociate in a stepwise manner
◦ Each dissociation step has its own Ka Stepwise dissociation constants decreases in the order
Ka1 > Ka2 > Ka3
◦ More difficult to remove a positively charge proton from negative ion
Polyprotic AcidsPolyprotic Acids Diprotic acid solutions contain a mixture of acids:
H2A, HA, H2O
◦ Strongest acid – HA Principle reaction – dissociation of H2A
All of H3O+ come from the first ionization
H2SO4(aq) + H2O(l) H3O+(aq) + HSO4-(aq)
HSO4-(aq) + H2O(l) H3O+(aq) + SO4
2-
Polyprotic acidsPolyprotic acids H2CO3(aq) + H2O(l) H3O+( aq) + HCO3
-(aq)
Ka1 = 4.3 x 10-7
HCO3-(aq) + H2O(l) H3O+( aq) + CO3
2-(aq)
Ka2 = 4.8 x 10-11
Polyprotic AcidsPolyprotic Acids
Equilibria in Solutions of Weak Equilibria in Solutions of Weak BasesBases
NH41+(aq) + OH1-(aq)NH3(aq) + H2O(l)
BaseAcidAcidBase
[NH41+][OH1-]
[NH3]Kb =
BH1+(aq) + OH1-(aq)B(aq) + H2O(l)
[BH1+][OH1-]
[B]Kb =
Base-Dissociation Constant:
Equilibria in Solutions of Weak Equilibria in Solutions of Weak BasesBases
Equilibria in Solutions of Weak Equilibria in Solutions of Weak BasesBases
Calculate the [-OH] and pH of a 0.40 M NH3 solution. At 25 °C, Kb = 1.8 x 10-5.
ExampleExample Morphine (C17H19NO3), a narcotic used in painkillers,
is a weak organic base. If the pH of a 7.0 x 10-4 M solution of morphine is 9.5, what is the value of Kb?
Relation Between Relation Between KKaa and and KKbb
H3O1+(aq) + OH1-(aq)2H2O(l)
H3O1+(aq) + NH3(aq)NH41+(aq) + H2O(l)
NH41+(aq) + OH1-(aq)NH3(aq) + H2O(l)
[NH41+][OH1-]
[NH3]
[H3O1+][NH3]
[NH41+]
x
Kw
Ka
Kb
= [H3O1+][OH1-] = Kw
= (5.6 x 10-10)(1.8 x 10-5) = 1.0 x 10-14
Ka x Kb =
Relation Between Relation Between KKaa and and KKbb
pKa + pKb = pKw = 14.00
Ka x Kb = Kw
Kb = Ka
KwKa = Kb
Kw
conjugate acid-base pair
ExampleExample
Find the pH of a 0.100 M NaCHO2 solution. The salt completely dissociate into Na+(aq) and CHO2
-(aq) and Na+ ion has no acid or base properties. Ka (HCHO2)= 1.8 x 10-4
ExampleExample What is the pH of 0.10M sodium nicotinate at 25oC?
The Ka for nicotinic acid is 1.4 x 10-5.
Acid-Base Properties of SaltsAcid-Base Properties of Salts pH of a salt solution is determined by the acid-base
properties of the consistuent cations and anions◦ In an acid-base reaction, the influence of the
stronger partner is predominant◦ Strong acid + Strong Base Neutral solution◦ Strong acid + Weak Base Basis solution◦ Weak acid + Strong Base Acidic solution
Acid-Base Properties of SaltsAcid-Base Properties of Salts Acidic cation + neutral anion Acidic salt
NH4+ + Cl- NH4Cl
Neutral cation + neutral anion neutral salt
Na+ Cl- NaCl
Neutral cation + basic anion basic salt
Na+ CN- NaCN
Acid-Base Properties of Acid-Base Properties of SaltsSalts Acidic cation + basic anion (50 :50 mixture) must
compare Ka and Kb
• Ka > Kb: The solution will contain an excess of H3O1+ ions (pH < 7).
• Ka < Kb: The solution will contain an excess of OH1- ions (pH > 7).
• Ka = Kb: The solution will contain approximately equal concentrations of H3O1+ and OH1- ions (pH ≈ 7).
Salts
Cation Anion
Acidic neutral Basic neutral
Cations f rom weak acids produce H3O+ or H+
E. g NH4+
Cations f rom strong bases do not react with H2OE . g Group 1A, 2A
Anions f rom weakacids produc -OHEg CN-, F-
Anions f rom strong acidsdo not react with H2OE .g Cl-, Br-, NO3
-
Acid-Base Properties of SaltsAcid-Base Properties of SaltsSalts That Yield Acidic Solutions
Hydrated cations of small, highly charged metal ions, such as Al3+.
Acid-Base Properties of SaltsAcid-Base Properties of Salts
ExamplesExamples Classify each of the following salt solution as acidic
basic or neutral. Write a hydrolysis equation for each ion. ◦ KBr
◦ NaNO2
◦ NH4Br
ExampleExample Calculate the pH of a 0.10M solution of sodium
fluoride (NaF) at 25oC.
Ka = 7.1 x 10-4
ExamplesExamples Calculate Ka for the cation, and Kb for the anion in an
aqueous NH4CN solution. Is the solution acidic, basic or neutral? Write the hydrolysis reaction of the salt (Kb for NH3 = 1.8 x 10-5, Ka for HCN = 4.9 x 10-10)
ExampleExample Predict whether 0.35M NH4Br solution is acidic,
basic or neutral. Calculate its pH. Kb = 1.8 x10-5
Lewis Acids and BasesLewis Acids and Bases
Lewis Base: An electron-pair donor. • All Lewis bases are Bronsted-Lowry bases
Lewis Acid: An electron-pair acceptor.• Include cations and neutral molecule having vacant valence orbitals that can accept a share in a pair of electrons from a Lewis Base
Lewis Acids and BasesLewis Acids and Bases
Lewis Acids and BasesLewis Acids and Bases
Lewis Acids and BasesLewis Acids and Bases
ExamplesExamplesFor each of the following
reactions, identify the Lewis acid and the Lewis base◦CO2(g) + -OH(aq) HCO3
-(aq)
◦AlCl3(aq) + Cl-(aq) AlCl4-(aq)
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