acids and bases - yonsei university€¦ · b. ph of weak acid solutions c. ph of a mixture of...

Acids and Bases

Chapter 17

Copyright © Houghton Mifflin Company. All rights reserved. 17 | 2

What we learn from Chap 17

This chapter concerns the nature and uses of acids and bases. It is the second in our three-chapter discussion about equilibrium, and reinforces the idea of looking at acids and bases as an extension of the core, consistent understanding and application of equilibrium. The chapter opens with the phrase, “Home is where the heart is. Home is also where the phosphoric acid...is.” That is, our case study is on the practical science of acid-base chemistry. We make the case that acids and bases are used in many parts of the home, even in the construction of the home itself, a fact that we revisit several times in the chapter.


CHAPTER OUTLINEI. What Are Acids and Bases? II. Acid Strength

A. Strong and Weak Acids B. Why Do Acids Have Different Strengths?

III. The pH Scale A. Water and The pH Scale

IV. Determining the pH of Acidic Solutions A. Le Chatelier’s Principle and the Supply of Hydroxide Ion in Acid Solutions B. pH of Weak Acid Solutions C. pH of a Mixture of Monoprotic Acids

V. Determining the pH of Basic Solutions VI. Polyprotic Acids

A. Production and Uses of Phosphoric and Sulfuric Acids B. The pH of Polyprotic Acids

VII. Assessing the Acid-Base Behavior of Salts in Aqueous Solutions A. The Relationship of Ka to Kb B. Acid-Base Properties of Amino Acids

VIII. Anhydrides in Aqueous Solutions


Common Chemicals (Acid & Bases)


The Use of Acids & Bases


17.1 What are Acids & Bases

• Arrhenius Acids and Bases • Brønsted-Lowry Acid and Bases• Lewis Acid and Bases


Arrhenius Acids and Bases

– An acid produces hydrogen ions in solution.– HCl(aq) → H+(aq) + Cl−(aq)

– A base produces hydroxide ions in solution.– NaOH(aq) → Na+(aq) + OH−(aq)


A shell of 21 water molecules


Brønsted-Lowry Acid and Bases

• An acid is a proton donor.• A base is an proton acceptor

– HCl—Cl− : H3O+—H2O : a conjugate acid-base pair.

hydronium ion


Brønsted-Lowry Reaction


Lewis Acids and Bases

• An acid accepts a previously non-bonded pair of electrons (a lone pair) to form a coordinate covalent bond.

• A base donates a lone pair of electronsto form a coordinate covalent bond.

BH3(g) + :NH3(g) → BH3−NH3a Lewis acid a Lewis Base


Acid-Base Properties


17.2 Acid Strength

• Ka is the acid dissociation constant• Strong acids

– Dissociate essentially completely– Equilibrium is product favored– Have a very large Ka

• Weak acids– Partially dissociate– Have a small Ka– Equilibrium is reactant favored


Acid Strength


Acid Dissociation Constant

CH3COOH(l) H+(aq) + CH3COO-(aq)

[ ]

+ =-

3

3a

H CH COOK

CH COOH


H2O H+ NO3-


Acid Strength


Structure and Acid Strength

• Binary Acids : Polarity of the H-X bond (size)– The larger X (lower electron density) is, the

stronger the acid– HF < HCl < HBr < HI


Polarity (Cont.)HI HBr HCl HF

160.9 > 141.4 > 127.4 > 91.7 pm

297 < 368 < 431 < 569 kJ/mol

Bond length

Bond energy

109 > 108 > 1.3x106 >> 6.6x10-4Acid strength

HF + H2O → [F-·····H3O+] F- + H3O+

ion pairH-bonding

free ions←→



• Acid strength increases with electronegativity– H2O < HF



• H-O-Y- oxoacids– Acid strength increases with electronegativity of Y– HIO < HBrO < HClO

• Acid strength increases with number of oxygens (higher oxidation state)– HBrO<HBrO2<HBrO3<HBrO4


oxidation state (Cont.)

S OO

O

O

H H····

····

-

2+

··

···· ···· ··-

S OO

O

H H····

····

-

+··

···· ··

S OO

O

O

H H····

····

·· ···· ··

S OO

O

H H····

···· ··

·· ··

Ka ≈103 Ka =1.3x10-2


Strengths of Organic Acids

C OC

O

H H····

·· ··H

H

OCH H····

H

H

C

H

H

Ka = 1.8x10-5 Ka =1.3x10-16

acetic acid ethanol


Organic Acids (Cont.)Focus on the Anions Formed

OCH····

H

H

C

H

H

C

O

C

O

H-

··

·· ··

H

H

····

C

O

C

O

H

-···· ··

H

H

····

··

-


Organic Acids (Cont.)

C

H

H

C

O

C

O

H-

···· ··

H

H

····

CH

H

H

C

O

O

-··

·· ··

····

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

Ka = 1.8x10-5

Ka = 1.3x10-5


Organic Acids (Cont.)

C

O

C

O

H-

···· ··

H

H

····

Ka = 1.8x10-5

Ka = 1.4x10-3

C

O

C

O

H-

···· ··

H

Cl

····


Ion Product of Water

– In any aqueous solution, there is an equilibrium between H30+(H) ions and OH-ions.

H2O(l) + H2O(l) H3O+(aq) + OH–(aq)

KW = [H3O+]×[OH–] = 1.0 × 10-14 at 25°C :(ion product const. of water)

←→


17.3 pH Scale

[H+] = [OH-][H+] > [OH-][H+] < [OH-]

Solution isneutralacidicbasic

[H+] = 1×10-7

[H+] > 1×10-7

[H+] < 1×10-7

pH = 7pH < 7pH > 7

At 25oC

pH = -log [H+]

[H+] = 10-pH


pH


pH and pOH

Kw = [H+][OH-]pKw = pH + pOH

pH + pOH = 14.00pOH = -log[OH-][OH-] = 10-pOH


Sample Problem

The pH of orange juice is about 4.2. What is pOH, [H+] and the [OH-]?

pOH = 14.0 – pH = 14.0 – 4.2 = 9.8

[H+] = 10-pH = 10-4.2 = 6.3×10-5

[OH-] = 10-pOH = 10-9.8 = 1.6×10-10


17.4 pH of Strong Acid

• Calculate the pH of 0.15 M solution of HCl.• Strong acid are completely dissociated,

therefore, [H+] = [HCl]pH = -log [H+] = -log(0.15) = 0.82


pH of a Weak Acid

• Weak acids partially dissociate.• Let’s abbreviate CH3COOH as HAc• Two important equilibria are:HAc(aq) H+(aq) + Ac-(aq) Ka= 1.8×10-5

H2O(l) H+(aq) + OH-(aq) Kw= 1.0×10-14

Ka>>Kw hence the dissociation of HAc is the major contributor to the [H+] in solution.


[ ]

+ = = ×-

-51.8 10a

H AcK

HAc

Calculate the pH of a 0.15M solution of HAc.

HAc H+ + Ac-

0.15-x x x

pH of a Weak Acid

( )( )

( )

( )

5

25

-3

-3

x x1.8 10

0.15-xassume that x is small compared toconcentration of the acid.

x1.8 10

0.15x = 1.6 10

pH=-log 1.6 10 2.80

−

−

× =

× =

×

× =

M


pH of a Weak Acid

0

x 100% 5%M

× ≤

-31.6 10 M 100% = 1.1% 5%0.15 M×

× ≤

Applying the 5% rule:

If the 5% rule is not satisfied, the quadratic equation must be used.


17.5 pH of a Strong Base

• Calculate the pH of a 0.25 M KOH solution.• Strong bases are completely dissociated,

therefore, [OH-] = [KOH]

pOH = -log [OH-] =-log(0.25) = 0.60pH = 14.00 – 0.60 = 13.40


pH of a Weak Base

• Calculate the pH of a 0.25 M NH2OH solution.• The two important equilibria are:NH2OH(aq) + H2O(l) NH3OH+(aq) + OH-(aq) Kb= 1.1×10-8

H2O(l) H+(aq) + OH-(aq) Kw= 1.0×10-14

Kb>>Kw hence the hydrolysis of NH2OH is the major contributor to the [OH-].


pH of a Weak Base+ -

3b

2

[NH OH ][OH ]K = [NH OH]

NH2OH(aq) + H2O(l) NH3OH+(aq) + OH-(aq)0.25 M-x x x

( ) ( )

( ) ( )

( )

-8

-8

5

5

x x1.1 10

0.25 xassume that x is small comparedto the concentration of the base

x x1.1 10

0.25x 5.2 10

pOH log 5.2 10 4.28

pH 14.00 4.28 9.72

−

−

× =−

× =

= ×

= − × =

= − =


pH of a Weak Base

-55.2 10 M 100% = 0.021% 5%0.25 M×

× ≤

Checking with the 5% rule:


Strengths of Amines as Bases

CH

H

H

C

H

H

CH

H

H

C

H

H

CH

H

H

C

H

H

pKb = 4.74 pKb = 3.38 pKb = 3.37

methylamine ethylamine propylamine

NH2 NH2 NH2


17.6 Polyprotic Acids

• Polyprotic acids have more than one ionizable hydrogen.

H3PO4 H2PO4− + H+ Ka1= 7.4×10-3

H2PO4− HPO4

2- + H+ Ka2= 6.2×10-8

HPO42- PO4

3- + H+ Ka3= 4.8×10-13


Polyprotic Acids


17.7 Acid-Base Behavior of Aqueous Salt Solutions

• Salt : cation (H+) + anion (OH-)• Aqueous salt solutions can be acidic,

basic or neutral.• The nature of the cation and the anion

determine the pH of the solution


• Cations• Spectator(neutral) : derived from strong bases. Li+, Na+,

K+ ; Ca2+, Sr2+, Ba2+

• Acidic: all other cations, including those of the transition metals.

• Anions • Spectator(neutral) : derived from strong acids

Cl-, Br-, I-, NO3-

• Basic: anions derived from weak acids, such as F-, NO2

-


Acid-Base Behavior of Aqueous Salt Solutions

If cation is acidic, anion basic, compare Ka , Kb values.NH4F: Ka = 5.6×10-10, Kb = 1.4×10-11; acidic


The Relationship of Ka to Kb

HA(aq) H+(aq) + A-(aq) Ka

A-(aq) + H2O(l) HA(aq) + OH-(aq) Kb

H2O(l) H+(aq) + OH-(aq) Kw

Kw = Ka × Kb

= = w wa b

b a

K KK KK K



• Neutral solutionsSalts containing an alkali or alkaline earth metal and the

conjugate base of a strong acid.NaCl(s) + H2O(l)→ Na+(aq) + Cl-(aq)

• Basic solutionsSalts made from strong base and a weak acid.

CH3COONa(s) →Na+(aq) + CH3COO-(aq)CH3COO-(aq) + H2O(l) CH3COOH(aq) + OH-(aq)



• Acidic SolutionsSalts made from a strong acid and a weak base.

NH4Cl(s) →NH4+(aq) + Cl-(aq)

NH4+ (aq) NH3(aq) + H+(aq)


Anhydrides in Aqueous Solution

• Basic Anhydride– A binary compound formed between

metals with very low electronegativity and oxygen.

Na2O(s) + H2O(l) → 2NaOH(aq)• Acid Anhydride

– A binary compound formed between nonmetals and oxygen.

CO2(g) + H2O(l) → H2CO3(aq)

acids and bases - yonsei university€¦ · b. ph of weak acid solutions c. ph of a mixture of...

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