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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5852_Библиотеки_им_академика_М_И_Перельмана.pdf
1.17 INTRODUCTION
or generalizations were based mainly on observable properties such as taste and their effects on other
acids were dened as substances that have sour taste, turn
blue litmus red, and have ability to dissolve substances insoluble in water and react with certain metals to
produce hydrogen gas. b the substances that have bitter taste, turn red litmus blue and
neutralize acids
1.18 ARRHENIUS CONCEPT OF ACIDS AND BASES
Acid: an acid is a substance that furnishes hydrogen ions in its
aqueous solution33
in their aqueous solutions.
Chapter Outline (Part C)
1.17 Introduction, 1.18 Arrhenius Concept of Acids and Bases, 1.18.1 Dissociation Constants of Acids and Their Strength,
1.18.2 Limitations of Arrhenius Concept, 1.19 Bronsted–Lowry Concept of Acids and Bases, 1.19.1 Conjugate Acid–Base Pairs,
1.19.2 Relative Strength of Acids and Bases, 1.19.3 Advantages of Bronsted–Lowry Concept Over Arrhenius Concept,
1.19.4 Limitations of Bronsted–Lowry Concept, 1.20 Lewis–Concept of Acids and Bases, 1.20.1 Limitations of Lewis Concept,
1.20.2 Effect of Substituents on the Strength of Lewis Acids and Bases
The greatest force is derived from the
power of thought.
–Swami Vivekananda
STRUCTURE AND
PROPERTIES
1
PART - C
Acids and Bases
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
The pH Scale
Increasing
acidity
Neutral
Increasing
alkalinity
Battery acid
Lemon juice
Vinegar
Normal range precipitation
Normal range
of stream water
Milk
Baking soda,
sea water
Milk of magnesia
Ammonia
Lye
Acid rain
72
HCl
Hydrochloric acid
Water
Water
Water
Nitric acid
HNO
3
Acetic acid
CH3COOH
H
+
+ Cl
–
H+ + CH3COO
–
H+ + NO
–
3
H
+
(aq)
+ A
–
(aq)
Acid
Water
HA
NaOH
Sodium hydroxide
Water
Na+ + OH
–
NH
+
4
+ OH
–
Ammonium hydroxide
Water
NH4OH
Base
Water
B
+
(aq)
+ OH
–
(aq)
BOH
HA H+(aq) + A−(aq)
2434
or more hydrogen ions. Such acids are called polyprotic acids.
Base:
4
+ in water while bases are
−
in water.
1.18.1 Dissociation Constants of Acids and Their Strength
+
+
dilution.
24
weak
acids.
+− in water.
Structure and Properties
Acid Base Acid Base
HCl + H2O
H
3
O+ + Cl
–
Acid Base Acid Base
Acid BaseAcid Base
HCl + NH
3
HNO3 + F
–
HF + NO
–
3
NH
+
4
+ Cl
Strong acids: a3
24
Weak acids: a3
Strong bases: b
Weak bases: 4
2
1.18.2 Limitations of Arrhenius Concept
323
223
not contain any hydrogen but even then these act as acids in aqueous medium.
1.19 BRONSTED–LOWRY CONCEPT OF ACIDS AND BASES
+) to another substance
acid, +) from another substance.
2
73
2
2
1.
2.
NH4+ + S
2–
HS– + NH
3
Acid Base Acid Base
–
HSO
4
Acid Base
+ CO
2–
3
HSO
–
+ SO
3
Acid Base
24–
74
Acid Base Acid Base
Acid Base Acid Base
CH
3
COOH + H2O
HCl + H
2
O
H
3
O+ + Cl
–
H3O+ + CH3COO
–
NH
4
+
+ Cl
Acid-1 Base-2 Acid-2 Base-2
HCl + NH
3
HF + NO
–
3
Acid-1 Base-2 Acid-2 Base-2
HNO
3
+ F
–
H
H
H
H
H
H
H
H
H
H
H
H
O O
O
O O
O
+
–
3.
not accept proton.
4.
Amphoteric compounds5. Some molecules and ions may exhibit either acidic or basic behaviour under
amphiprotic or amphoteric
3
amphiprotic.
H2O + NH
Acid-1 Base-2
HCl + H2O
Acid-1 Base-2
3
+
+ OH
4
–
NH
Acid-2 Base-1
–
+
H3O
+ Cl
Acid-2 Base-1
Structure and Properties
HCl
H
2SO4
CH3COOH
H
+
+ Cl
–
H+ + HSO
–
4
H+ + CH3COO
–
+
H2O + H2O H3O
Base-1 Acid-2 Acid-1 Base-2
+ OH
–
75
3
¯
3
3
–
NH3 NH
2
+ H
+
NH3 + H
+
NH
+
4
all bronsted acids are arrhenius acids but all bronsted bases are not arrhenius bases. 6.
7.
1.19.1 Conjugate Acid–Base Pairs
conjugate base of the acid
conjugate acid of base.
Base Proton Conjugate acid
NH
HS
HCO
+
+
4
−
−
3
H
O + H+ H3O
2
NH
3
S2− + H
2–
CO
+ H
3
+ H
+
+
+
+
HCl + H2O H3O
+ Cl
–
3+3+ is an
− is a base. Such pairs of substance which are formed from one another by gain or loses of a
conjugate acid–base pairs.
Acid-1 + Base-2
Acid-2 + Base-1
76
Strong acid
H
3
O + Cl
–
Weak conjugate base
HCl + H
2
O
CH3COOH + H2O
H
3
O+ + CH3COO
–
Weak acid Strong conjugate base
Acid-1 Base-2 Acid-1 Base-2
H
O + NH
2
HCl + NH
HNO
HSO
NH
+
4
+ H2O H3O
3
–
+ H2O H3O
4
+ CH3COO
H2O + CO
+
NH
3
3
–
2–
3
4
+
NH
4
+
+
CH3COOH + NH
+
HCO
3
+ OH
+ Cl
+ NO
+ SO
+ OH
–
–
–
3
–
2
4
3
–
1.19.2 Relative Strength of Acids and Bases
while the strength of a base depends on its tendency to gain protons.
1. Let us consider hydrochloric acid in water.
−
Strong acid H+ + Weak conjugate base
3
3−3
1.19.3 Advantages of Bronsted–Lowry Concept Over Arrhenius Concept
233
group and hence were not bases according to arrhenius concept) on the basis that they accept
protons.
bases.
77
Structure and Properties
1.19.4 Limitations of Bronsted–Lowry Concept
ations also.
2
23
CaO + SO3 CaSO
4
33
donate.
1.20 LEWIS CONCEPT OF ACIDS AND BASES
an acid is a substance that can accept a pair of electrons to from a coordinate bond and a base is a substance
acid is an electron pair acceptor.
Examples of acid–base reactions
F
F
F
F
FF
H
H
H
H
H
H
B BNN
Coordinate compound
Lewis
base
Lewis
acid
+ :
e.g.
3
has multiple bonds. It is not an arrhenius or bronsted acid. It is a Lewis acid and neutralizes
bases.
Ca2+ + O2� + SO
23�
[SO
24�
]Ca
2+
Lewis
base
Lewis
acid
3++ act as Lewis acids because they have empty orbitals
in their outermost energy levels.
Learning Plus
If we apply Lewis theory, a need arises to modify
the term
neutralization
. It can no more be used
in the sense in which it has been hitherto used.
Since Lewis acid-base interaction invariably
results in the formation of a covalent bond, the
word coordination is more appropriate than
neutralization
. However, one might still use the
term neutralization.
78
SnCl4 + 2CI
–
Lewis
acid
Lewis
base
SnCl
6
2–
H
H
H
H
H
H
H
H
H
N
N : + O
Base Acid
OH
−
+
H
H
H
HH
H
H
H
H
H
H
H
H
N
N
N : + O
Base Acid
OH
−
or + OH
−
+
+
d 4 has empty
d
–
4
–
.
3
also capable of accepting a proton. but bronsted acids are not Lewis acids.
(does not accept electron pair).
1.20.1 Limitations of Lewis Concept
1. It is very general and includes all the coordination compounds and reactions. Protonic acids do not
form coordinate bond with bases.
+ ions. Lewis acids may not contain
24
Structure and Properties
Me3B < BH
3
< BF
3
Electron-releasing
group decreases
Lewis acid
strength
BH
H
H
Electron-
withdrawing group
increases Lewis
acid strength
Me
Me
Me
B
BF
F
F
1.20.2 Effect of Substituents on the Strength of Lewis Acids and Bases
1. Inductive effect
3. Steric effects of the substituents
Inductive effect of the substituents
act as Lewis acid will increase.
pair of the donation becomes less and this decreases its basic strength.
electron density on the donor atom and consequently its tendency to donate a pair of electrons (basic
character) increases.
3
its valence shell. It needs two more electrons to complete its octet and therefore behaves as a Lewis acid.
3 is a
33
333
79
80
Me3N > NH3 > NF
3
Electron-releasing
group decreases
Lewis acid
strength
Me
Me
Me
N
:
N
:
H
H
H
N
:
F
F
F
Electron-
withdrawing group
decreases Lewis
acid strength
3333) acts as a Lewis base
3
33
333
3
Resonance effect of the substituents
boron trihalides behave as Lewis acids because they have only six electrons in their outermost shells and
3
e
3
pp
back bonding.
BF3 > BCl3 > BBr
BF3 > BCl3 > BBr
3
3