Transcript Slide 1

In the name of GOD
Theories of
ACIDS and BASES
By : A.Rouhollahi
The Chemistry of Acids
and Bases
OUTLINES
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Introduction
Primary definition
Arrhenius concept
The Bronsted-Lowery concept
The Lewis concept
Introduction
• Throughout the history of chemistry
various acid-base concepts have been
proposed and used
1.Primary definition (based on observed
properties of aq.soln)
2.Arrhenius concept (1887)
3.The Bronsted-Lowery concept (1923)
4.The Lewis concept(1923-1938)
Primary definition
ACIDS
 Taste sour
 Turn litmus
 React with active
metals – Fe, Zn
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


BASES
Taste bitter
Turn litmus
Feel soapy or slippery
(react with fats to
make soap)
React with acids
 React with bases
blue to red
red to blue
Arrhenius concept
• Arrhenius published a “chemical theory
of electrolytes” in 1887 (electrolyte
dissociates into iones in water solution)
Acids: compounds that produce H+(aq)
ions in water
Bases:compounds that produces OH- (aq)
ions in water
Strength of an acid or a bases is
determined by the extent that the
compound dissociates in water
Acids:
1.Strong acids: completely dissociates:
HClO4, HCl, HNO3
2.Weak acids :partially dissociates: HF,
HCO2H, H3PO3-
The net ionic equation for a neutralization is
H+(aq) + OH-(aq)
H2O
Oxides of many nonmetals react with water to
form acids and are therefore called “acidic
oxides or acidic anhydrides”
Na2O5(s) +H2O
2H+(aq) + 2NO3-(aq)
• Many oxides of metals dissolve in water to
form hydroxides
• Metal oxides are called “basic oxides or base
anhydrides”
Na2O(s) + H2O
2Na+ (aq) + 2OH-(aq)
• Arrehnious concept limitation :it’s emphasis
on water and reaction in aqueous solution
Bronsted-Lowery concept
• In 1923 Johannes Bronsted and Thomas
Lowery independently proposed a broader
concept of acids and bases
Acids: a substance that can donate H+
Base: a substance that can accept H+
The reaction of an acid with a base is a proton
transfer
• When an acid release a proton ,it converted to
its conjugated base
• When a base accept a proton ,it converted to its
conjugated acid
• Loss or gain of a proton = conjugated pair
• H2O is the conjugated base of H3O+
• H3O+ is the conjugate acid of H2O
Acid(1) + Base(2)
Acid(2) + Base(1)
+H+
-H+
CH3CO2H + H2O
H3O+ + CH3 CO-2
+H+
-H+
NH4++ OH-
H2O + NH3
+H+
-H+
H2O is an amphiprotic substance
Strength of acid and base is depend on extent of
its proton transfer during the reaction with
solvent
HA + SH = SH2+ + A-
Ki = aA-. aSH2+ /aHA .aSH =(Ka)HA .(Kb )SH
SH + H+
SH2+
Kb = aSH+2 /aSH .aH+
The important properties
of solvents:
• Dielectric constants (electrostatic interaction)
• Solvent acidity or basicity
• Solvation ability of solvent
• HA + SH
SH2+A-
• SH + B
BH+S
SH2+ + ABH+ + S-
Acids increase the possitive species of solvent.
Bases increase the negative species of solvent.
Solvent excert a “leveling effect” on solute acids
and bases.
HClO4 (aq)+H2O
HCl(aq)+H2O
HNO3(aq)+H2O
H3O+(aq)+ClO4-(aq)
H3O+(aq)+Cl-(aq)
H3O+(aq)+NO3-
Thermodynamic Of Acid / Base
Reaction
The role of solvent on shape
of titration curve
Neutralization reaction during the titration of
a weak acid by standard solution of a strong
base.
OH-+HA
H2O+AKeq=[A-]/[OH-][HA] . [H+]/[H+]=Ka/Kw
And in general in solvent SH
S-+HA
SH+AKeq=[A-]/[S-][HA] . [H+]/[H+]=Ka/Ks
Keq
Sharper
titration curve
Acid Strength And Molecular
Structure
ACIDS
1.Covalent hydrides
2.Oxyacids
3.Other acids(especially organic acids)
Hydrides:
Two factor influence the acid strength of
hydride
1.The electronegativity
2.Atomic size
A . Hydrides of element of a period
(the electronegativity factor is important)
Electronegativity N < O < F
Acidity strength NH3 < H2O < HF
B . Hydrides of the element of a group
(the element size is important)
H2O < H2S < H2Se < H2Te
HF < HCl < HBr < HI
Oxyacids
(HO)mZOn
The simplest oxyacids: H-O-Z
H-O-Cl
(Z is an atom of nonmetal)
Acid strength: HOI<HOBr<HOCl
In the same order of the electronegativity of z:
I<Br<Cl
In the same molecules additional O atoms bonded
to Z ,for example
H-O-ZO2
H-O-Cl<H-O-Cl-O<H-O-Cl(O)2<H-O-Cl(O)3
HOCl
Formal charge of Cl:
0
Oxidation number of Cl: 1+
HOClO
1+
3+
HOClO2
2+
5+
Which of them is a reliable indicators,
formal charges or oxidation number?
HOClO3
3+
7+
The oxyacids of phosphorus are all weak acids
-about of equal strength:
HO-POH2 HO-PO2H2 HO-PO3H2
Formal charge of P:
Oxidation number of P:
1+
1+
1+
3+
1+
5+
“Then the formal charge is more reliable factor”
Acid strength:
HNO2<HNO3
H2SO3<H2SO4
In General:
The Strength of (HO)mZO n Can Be
Related to The Value of n :
a. If n=0 ,the acid is very weak;HOCl,(HO)3B
b.If n=1 ,the acid is weak; HOCl,HONO,
(HO)3PO
c.If n=2 ,the acid is strong;HOClO2,HONO2
(HO)2SO2
d.If n=3 ,the acid is very strong;HOClO3, HOIO3
Organic Acids:
The Effect of Electron-withdrawing
Groups In Organic Acids
Acidity:
HO-CH2-CH3<<HO-CO-CH3<HO-CO-CH2-Cl<HO-CO-CH(Cl)2<HO-CO-C(Cl)3
PKa
16.5
4.76
2.86
1.29
0.65
4.The Lewis concept (1923-1938)
Gilbert N.Lewis proposed a broader concept of acids
and bases which liberated acid-bases phenomena
from the proton.
A Lewis Base: a subestance that can form a
covalent bond with an atom ,
molecule or ion.
A Lewis Acid: a substance that can form a covalent
bond accepting an electron pair
from a bases.
BF3 + :NH3
Acid
F3B:NH3
Base
A Lewis acid must have an empty orbital capable
of receiving an electron pair of the base.
Lewis acids:
1 . Molecules or atoms that have incomplete octet:
BF3+F-
BF4-
2 . Many simple cations
Cu2++4 NH3
Cu(NH3)42+
3.Some metal atoms
Ni +4 CO
Ni(CO)4
4. Compounds that have central atoms capable
of expanding their valence shells:
SnCl4+2Cl-
SnCl62-
5.Some compounds that have multiple bonds:
O
O
S
O
_
:O H-
S
O
O
H
Lewis Acid and base intraction