acid base PUC 2023.pptx

Bronsted – Lowry Concept
ACID:
Proton donor
BASE:
Proton acceptor

ACID BASE

ACID BASE
AMPHOTERIC SUBSTANCES

Relative strength of Bronsted acids and
Bronsted bases in aqueous medium
•Reaction of HCl with water
•HCl is a stronger acid than H3O+
•Water is a strong base than Cl-
•Reaction of acetic acid with water
•H3O+ is a stronger acid than acetic acid
•Acetate ion is stronger base than water

Relative strengths of acids and bases
Charge distribution on the ion
Charge or oxidation state
Effect of solvent
Effect of substituent groups
• Nature of the groups
• Steric and solvation effects
Electronegativity of the central atom
Back bonding
Resonance effects

BINARY HYDRIDE IONS – BRONSTED BASES

HYDRIDE IONS OF THE NON METALS
OF THE SAME PERIOD
CH3
- NH2
- OH- F-
ACIDIC STRENGTH OF THEIR CONJUGATE ACIDS
CH4 < NH3 < H2O < HF

THE BASICITY
CH3
- NH2
- OH- F-
PH2
- SH- Cl-
SbH2
- TeH- I-
AsH2
- SeH- Br-
DECREASES
DECREASES

Charge distribution on the ion
Charge or oxidation state
Effect of solvent
Effect of substituent groups
• Nature of the groups
• Steric and solvation effects
Electronegativity of the central atom
Resonance effects

OXIDATION STATE OF CENTRAL ATOM IN THE OXY ACIDS
Higher the oxidation state greater is the acidic strength
HClO4 HClO3 HClO2 HClO
HBrO4 HBrO3 HBrO2 HBrO
HIO4 HIO3 HIO2 HIO
HMnO4 H2MnO4
H2SO3 H2SO4
HNO2 HNO3

ACIDIC STRENGTH OF OXYACIDS OF
PHOSPOROUS
P OH
O
H
H P OH
O
OH
H P OH
O
OH
O
H

Levelling Effect:
All strong acids in aqueous solution appear equally
strong. Their relative strengths in aqueous solution
can not be compared. This phenomenon is known
as levelling effect.
The solvent is known as levelling solvent. Therefore
H2O is a levelling solvent.

LEWIS ACID BASE THEORY(1930)
• ACID – accepts pair of electrons
1. Molecule with central atom with vacant p
orbitals
– BeX2 BF3 BX3
– BeF2 + 2F- 
– BH3 + H- 
– BH3 + NH3 
– BF3 + OH2 
– BF3 + O(C2H5) 2

LEWIS ACID BASE THEORY
– BF3 + OH- 
– BF3 + F- 
– BF3 + PCl3 
– BMe3 + NH2 -NH2 

2. Molecule with central atom with vacant d
orbitals
– AlCl3 AlF3 GaCl3 SiX4 GeX4 SnX4
– PX3 PX5 AsX5 SbF3 SbF5 XeF6
– AlCl3 + COCl2 
– SnCl3 + Cl- 
– SiF4 + 2F- 

3. Molecule with central atom linked to more
electronegative atom by double bond
C O
O
S
O O
OH-
O-2
OH-
O-2
S
O O
O
OH-
O-2

4. Simple cations / metal cations which form
coordination compounds
H+
Ag+
Cu+2
Cd+2

5. Elements with sextet in their valence shell
S + SO3
2- 
O + SO3
2- 

Variation Lewis acid strength
1. SIZE OF THE CATION
– Smaller the size greater is the acidity
– H+ > Na + > K +
– Li+ < Be +2

2. OXIDATION STATE OF THE CATION
– Higher the oxidation state greater is the acidity
– Fe+3 > Fe +2

3. BACK BONDING
Trihalides of boron BF3 BCl3 BBr3 BI3

4. AVAILABILITY OF VACAT d ORBITALS
Tetra halides of carbon do not behave as
Lewis acids whereas tetra halides of other
elements of group 14 show acidic nature

HARD-SOFT ACID-BASE THEORY
Hard acids
• Small
• Non-polarisable
• High charge
density
Soft acids
• Large
• More polarisable
• Low charge

Hard and Soft Acids and Bases.

Acids
Hard Acids Borderline Soft Acids
H+, Li+, Na+, K+
Be2+, Mg2+, Ca2+
BF3, BCl3, B(OR)3 BBr3,B(CH3)3 BH3,Tl+, Tl(CH3)3
Al3+,Al(CH3)3,AlCl3,AlH3
Cr3+,Mn2+, Fe3+, Co3+ Fe2+,Co2+,Ni2+ Cu+,Ag+, Au+,
Cu2+,Zn2+,Rh3+ Cd2+,Hg2
2+,
Ir3+, Ru3+, Os2+ Hg2+, Pd2+,Pt2+,
SO3 SO2 Pt4+

Acids – Effect of Oxidation
Hard Acids Borderline Soft Acids
H+, Li+, Na+, K+
Be2+, Mg2+, Ca2+
BF3, BCl3, B(OR)3 BBr3,B(CH3)3 BH3,Tl+, Tl(CH3)3
Al3+,Al(CH3)3,AlCl3,AlH3
Cr3+,Mn2+, Fe3+, Co3+ Fe2+,Co2+,Ni2+ Cu+,Ag+, Au+,
Cu2+,Zn2+,Rh3+ Cd2+,Hg2
2+,
Ir3+, Ru3+, Os2+ Hg2+, Pd2+,Pt2+,
SO3 SO2 Pt4+

Bases – effect of Oxidation
Hard Bases Borderline Soft Bases
F-, Cl- Br- H-, I-
H2O, OH-,O2- H2S, HS-, S2-
ROH, RO-, R2O, CH3CO2
- RSH, RS-, R2S
NO3
-, ClO4
- NO2
-, N3
- , N2 SCN-, CN-,RNC, CO
CO3
2-,SO4
2-, PO4
3- SO3
2- S2O3
2-
NH3, RNH2 C6H5NH2, pyr R3P, C6H6

Distribution of Hard and Soft Bases by donor
atom in the periodic Table:
Figure 2. Distribution of hardness and softness for potential donor atoms
for ligands in the Periodic Table.
As Se Br
P S Cl
I
C N O F

HSAB
PRINCIPLE
Hard Acids prefer to
bond with Hard Bases
Soft Acids prefer to
bond with Soft Bases
Pearson’s Principle of Hard and Soft Acids and Bases

Stability of the complex compounds
Prediction of coordination in complexes of ambidentate ligands
Stability of a complex compound having different ligands
Symbiosis
Solubility of the compound
Occurrence of the metals in nature
Predicting feasibility of a reaction
APPLICATIONS OF HSAB CONCEPT

Symbiosis
AgI2
- is stable whereas
AgF2
- does not exists

_________________________________________________
Log K1 F- Cl- Br- I- classification
_________________________________________________
Ag+ 0.4 3.3 4.7 6.6 soft
Pb2+ 1.3 0.9 1.1 1.3 intermediate
Fe3+ 6.0 1.4 0.5 - hard
_________________________________________________
hard soft
hard-hard interaction
soft-soft interaction
CoF6
-3
CoI6
-3
Formation constants with halide ions for a representative
hard, soft, and intermediate metal ion .

Symbiosis
Thiocyanate, an ambidentate ligand
[Pd(SCN)4]-2 and [Co(NCS)4] -2
• With Au(I) and Fe(III)
• [Au(SCN)2]- and [Fe(NCS)6]3-

Alkyl thiocyanate ligand with
IODINE & PHENOL
SOFT HARD
RNCS +
Phenol
RSCN +
Phenol
RNCS +
Iodine
RSCN +
Iodine
L M M L

Symbiosis
[Co(NH3)5F]+2 [Co(NH3)5I]+2
[Co(CN)5F] -3 [Co(CN)5I]-3

• Soft ligands have a tendency to combine with
a centre which is already associated with soft
ligands.
• Hard ligands have a tendency to combine with
a centre which is already associated with hard
ligands.
Eg: BF3 + F- BF4
-
BH3 + H- BH4
-
CH3F + CF3H
Symbiosis

•soft-soft interactions are
more covalent
•hard-hard interactions
are ionic.

Aqueous Solubility - Silver Halides
Compound solubility product
AgF 205
AgCl 1.8 x 10-10
AgBr 5.2 x 10-13
AgI 8.3 x 10-17
AgX(s) + H2O(l) ↔ Ag+(aq) + X-(aq)

Consider the reaction between ZnO and
LiC4H9.
ZnO + 2 LiC4H9↔ Zn(C4H9)2 + Li2O
soft -hard hard -soft soft -soft hard -hard

LUX FLOOD THEORY
Acid is an oxide acceptor
and base is an oxide donor.

Examples
• CaO donates oxygen - basic oxide.
• SiO2 is an oxide acceptor - acidic oxide.

• Superacid is an acid with an acidity greater than
that of 100% pure sulfuric acid
• Superacid is a medium, in which the chemical
potential of the proton is higher than in pure
sulfuric acid.
• Concentrated sulphuric acid has a Hammett acidity
function (H0) of −12. Hence Hammett acidity
function (H0) for superacids should be more
negative than -12

Hammett acidity function
• The Hammett acidity function (H0) is a measure of acidity that
is used for very concentrated solutions of strong acids,
including superacids.
• It was proposed by the physical organic chemist Louis Plack
Hammett
• Acidity function used to extend the measure of acidity
beyond the dilute aqueous solutions for which the pH scale is
useful.
• In highly concentrated solutions, simple approximations such
as the Henderson-Hasselbalch equation are no longer valid
due to the variations of the activity coefficients.

• The Hammett acidity function, H0, can replace the
pH in concentrated solutions.
• It is defined using an equation analogous to the
Henderson-Hasselbalch equation
ACID H0
Sulfuric acid −12.0
Fluoroantimonic acid −31.3
Magic acid −19.2
Carborane superacid −18.0
Fluorosulfuric acid −15.1
Triflic acid −14.1
Indicator pKBH + (in H2SO4)
m-nitroaniline +2.5
p-nitroaniline +0.99
o-nitroaniline -0.29
2,4-dinitroaniline -4.53
3-methyl-2,4,6-trinitroaniline -8.22
2,4,6-trinitroaniline -10.10

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