Prentice Hall ©2004 Chapter 14 Aqueous Equilibria: Acids & Bases by Dr Ayesha Mohy-ud-din.

Prentice Hall ©2004

Chapter 14Chapter 14

Aqueous Equilibria: Acids & Bases

byDr Ayesha Mohy-ud-din


Acid–Base ConceptsAcid–Base Concepts


pH – A Measure of AciditypH – A Measure of Acidity

• Problem 14.8 Calculate the pH of each of the following

Problem 14.9 Calculate the concentrations of H3O+ and OH– in each of the following solutions:

(a) Human blood (pH 7.40)(b) A cola beverage (pH 2.8)

Problem 14.10 Calculate the pH of(a)0.050 M HClO4 (b) 6.0 M HCl (c) 4.0 M KOH (d) 0.010 M Ba(OH)2

Problem 14.11 Calculate the pH of a solution prepared by dissolving 0.25 g of BaO in enough water to make 0.500 L of

solution


Acid Ionization ConstantsAcid Ionization Constants

• Acid Ionization Constant: the equilibrium constant for the ionization of an acid.

HA(aq) + H2O(l) æ H3O+(aq) + A–(aq)

[HA]

]][A[H3O

aK



7.1 x 10 –4

4.5 x 10 –4

3.0 x 10 –4

1.7 x 10 –4

8.0 x 10 –5

6.5 x 10 –5

1.8 x 10 –5

4.9 x 10 –10

1.3 x 10 –10

HF

HNO2

C9H8O4 (aspirin)

HCO2H (formic)

C6H8O6 (ascorbic)

C6H5CO2H (benzoic)

CH3CO2H (acetic)

HCN

C6H5OH (phenol)

F–

NO2 –

C9H7O4 –

HCO2 –

C6H7O6 –

C6H5CO2 –

CH3CO2 –

CN –

C6H5O –

ACID Ka CONJ. BASE Kb

1.4 x 10 –11

2.2 x 10 –11

3.3 x 10 –11

5.9 x 10 –11

1.3 x 10 –10

1.5 x 10 –10

5.6 x 10 –10

2.0 x 10 –5

7.7 x 10 –5


CalculatingEquilibriumConcentrationinSolutions ofWeak Acids


HA æ H+ + A

(M): 0.50 0.00 0.00 (M): –x +x +x

Equilib (M): 0.50 –x x x


• Initial Change Equilibrium Table: Determine the pH

of 0.50 M HA solution at 25°C. Ka = 7.1 x 10–4.

InitialChange

(aq) (aq)-(aq)



• pH of a Weak Acid (Cont’d):

1. Substitute new values into equilibrium expression.

2. If Ka is significantly (>1000 x) smaller than [HA] the expression

(0.50 – x) approximates to (0.50).

3. The equation can now be solved for x and pH.

4. If Ka is not significantly smaller than [HA] the quadratic

equation must be used to solve for x and pH.



• The Quadratic Equation:

• The expression must first be rearranged to:

• The values are substituted into the quadratic and

solved for a positive solution to x and pH.

aacbb

x2

42

02 cbxax



• Percent Dissociation: A measure of the strength of an acid.

• Stronger acids have higher percent dissociation.

• Percent dissociation of a weak acid decreases as

its concentration increases.

100%[HA]

][H3OonDissociati %


Base Ionization ConstantsBase Ionization Constants

• Base Ionization Constant: The equilibrium constant for the ionization of a base.

• The ionization of weak bases is treated in the same

way as the ionization of weak acids.

B(aq) + H2O(l) æ BH+(aq) + OH–(aq)

• Calculations follow the same procedure as used for

a weak acid but [OH–] is calculated, not [H+].


Base Ionization ConstantsBase Ionization Constants

5.6 x 10 –4

4.4 x 10 –4

4.1 x 10 –4

1.8 x 10 –5

1.7 x 10 –9

3.8 x 10 –10

1.5 x 10 –14

C2H5NH2 (ethylamine)

CH3NH2 (methylamine)

C8H10N4O2 (caffeine)

NH3 (ammonia)

C5H5N (pyridine)

C6H5NH2 (aniline)

NH2CONH2 (urea)

C2H5NH3+

CH3NH3+

C8H11N4O2+

NH4+

C5H6N+

C6H5NH3+

NH2CONH3+

BASE Kb CONJ. ACID Ka

1.8 x 10 –11

2.3 x 10 –11

2.4 x 10 –11

5.6 x 10 –10

5.9 x 10 –6

2.6 x 10 –5

0.67

Note that the positive charge sits on the nitrogen.


Diprotic & Polyprotic AcidsDiprotic & Polyprotic Acids

• Diprotic and polyprotic acids yield more than one

hydrogen ion per molecule.

• One proton is lost at a time. Conjugate base of first

step is acid of second step.

• Ionization constants decrease as protons are

removed.


Diprotic & Polyprotic AcidsDiprotic & Polyprotic Acids

Very Large1.3 x 10 –2

6.5 x 10 –2

6.1 x 10 –5

1.3 x 10 –2

6.3 x 10 –8

4.2 x 10 –7

4.8 x 10 –11

9.5 x 10 –8

1 x 10 –19

7.5 x 10 –3

6.2 x 10 –8

4.8 x 10 –13

H2SO4

HSO4–

C2H2O4

C2HO4–

H2SO3

HSO3–

H2CO3

HCO3–

H2SHS–

H3PO4

H2PO4–

HPO42–

ACID Ka CONJ. BASE Kb

HSO4 –

SO4 2–

C2HO4–

C2O42–

HSO3 –

SO3 2–

HCO3–

CO3 2–

HS–

S 2–

H2PO4–

HPO42–

PO43–

Very Small7.7 x 10 –13

1.5 x 10 –13

1.6 x 10 –10

7.7 x 10 –13

1.6 x 10 –7

2.4 x 10 –8

2.1 x 10 –4

1.1 x 10 –7

1 x 10 –5

1.3 x 10 –12

1.6 x 10 –7

2.1 x 10 –2


Molecular Structure and Acid StrengthMolecular Structure and Acid Strength

• The strength of an acid depends on its tendency to

ionize.

• For general acids of the type H–X:

1. The stronger the bond, the weaker the acid.

2. The more polar the bond, the stronger the acid.

• For the hydrohalic acids, bond strength plays the

key role giving: HF < HCl < HBr < HI



• The electrostatic potential maps show all the hydrohalic

acids are polar. The variation in polarity is less

significant than the bond strength which decreases

from 567 kJ/mol for HF to 299 kJ/mol for HI.



• For binary acids in the same group, H–A bond strength decreases with increasing size of A, so acidity increases.

• For binary acids in the same row, H–A polarity increases with increasing electronegativity of A, so acidity increases.



• For oxoacids bond polarity is more important. If we consider the main element (Y):

Y–O–H

• If Y is an electronegative element, or in a high

oxidation state, the Y–O bond will be more covalent

and the O–H bond more polar and the acid stronger.



• For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity.



• For oxoacids having the same central atom but different numbers of attached groups, acid strength increases with increasing central atom oxidation number.

• As shown on the next slide, the number of oxygen atoms increases the positive charge on the chlorine which weakens the O–H bond and increases its polarity.



• Oxoacids of Chlorine:

Prentice Hall ©2004 Chapter 14 Aqueous Equilibria: Acids & Bases by Dr Ayesha Mohy-ud-din.

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Transcript of Prentice Hall ©2004 Chapter 14 Aqueous Equilibria: Acids & Bases by Dr Ayesha Mohy-ud-din.