polyprotic acids

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)