acid base concept

acid base concept

Arrhenius concept

a) Acid – An acid is a substance which gives hydrogen ion (H+) in aqueous solution i.e. when dissolved in water

Ex.       HCl  –> H++Cl




KHSO4(aq.) –> K++H++ SO4

b) Base – A base is a substance which gives hydroxide ion (OH) in aqueous solution.

Ex.       KOH(aq.) –> K++OH

NH4OH(aq.)    –> NH4+ +OH

c) According to arrhenius concept acids & bases which ionises completely in aqueous solution are called strong acids & strong bases respectively.

Strong Acid    HCl(aq.) –>   H++Cl

Weak Acid     CH3COOH(aq.)<—-> CH3COO+ H+

Strong Base    NaOH (aq)–> Na++OH

Weak Base     NH4OH(aq.) –> NH4+ +OH

d) Water is amphoteric because it gives both H+& OH in solution

H2O –>H++OH

Bronsted Lowry concept

a) Acid An acid is a substance (molecule or cation or anion) that gives proton i.e. H+ or acid is proton donor

HCl   –> H++Cl

HCO3–   –>      H++ CO3

[Al(H2O)6]3+   –> H++[Al(H2O)5OH)2+

b) Base-A base is a substance (molecule or cation or anion) that accepts proton i.e.H+ or base is proton acceptor.


CO3+H+ –>HCO3


c) Conjugate Acid-Base pair

Conjugate Acid- Chemical species, that is formed from the base by the gain of proton or H+

Base + H–>[Conjugate Acid]

Ex.       H2O+H+  –>H3O+ (Conjugate Acid)

Conjugate Base-Chemical species that is formed from the acid by the loss of proton i.e H+

Acid –>H++[Conjugate base]

HCl–>H++Cl (Conjugate base)

“A pair of acid & base which differ by a proton is called as conjugate Acid & base pair”


NH3(aq.) +H2O(l) <—-> NH4+(Aq)+OH

base1         acid2               acid1       base2

NH3(aq.) and NH4+(Aq) are conjugate acid base pair.

H2O(l) and OH are conjugate acid base pair.


HCl +    H2O <—-> H3O+ +  Cl

acid 1    base2        acid2      base1

HCl and   Cl are conjugate acid base pair.

H2O and H3O+ are conjugate acid base pair.

(d)       Relative Strength of conjugate Acid -Base Pair – The conjugate base of strong acid is weak & conjugate acid of strong base is weak
stronger acid +stronger base <—-> weaker acid+weaker base

stronger acid and weaker base are conjugate acid base pair.

stronger base and weaker acid are conjugate acid base pair.

Acid & Their Conjugate Base

Acid                            Conjugate Base

HCl                             Cl

H2SO4                             HSO4

HNO3                             NO3

HSO4–                          SO4

H3PO4                          H2PO4

HF                               F

H2CO3                          HCO3

HCO3–                        CO3

H2S                              HS

HS                              S––

H2PO4–                      HPO4

HPO4–                        PO4

H2O                             OH

Lewis Concept

a) Lewis Acid- Lewis Acid is electron pair acceptor

b) Lewis Base-Lewis base is electron pair donar

:NH3 +              BF3          –> [H3N –>  BF3]

e-pair               e-pair

donar               acceptors

(Lewis base)    (Lewis acid)

c) Conditions for Lewis Acid-

i) Simple cation such as H+, Ag+, Fe++, Fe3+

2NH3 +Ag+ –> [H3N –> Ag <– NH3]+

ii) Compounds whose central atom has incomplete octet

Ex- BF3, BCl3, AlCl3, RMgX, MgCl2, etc.

iii)        Compounds in which the Central atom has empty d-orbital or have orbitals of suitable energy & May extend their octet. Ex- SiF4, SiCl4

SiF4+2 F–    –>  [SiF6]2–

d) Conditions for Lewis base-

i) Molecules whose central atom has lone pair of electron

Such as : NH3 , :PH3,:PCl3, H2O:,ROH, ROR etc

ii) Negative ions or anions such as F,Cl,I, etc

Definition of Hydroacids : Acid in which acidic H-atom is attached directly to other elements is called Hydroacid.

Definition of Oxyacids – Acid in which acidic H-atom is attached to central atom via an oxygen, acid is called oxyacid.

Some Important features of Acid & Bases

The oxides & hydroxides of alkali & alkaline earth metals are strong base & rest all are weak base. Basic nature increases on moving down the group.

Example.1        LiOH<NaOH<KOH<RbOH<CsOH

Example. 2       Be(OH)2<Mg(OH)2<Ca(OH)2<Sr(OH)2< Ba(OH)2

  1. Mineral acids such as H2SO4, HCl&HNO3 are strong acids.
  2. All organic acids (except those having –SO3H gr. i.e. Sulphonic acids) are weak acids.
  3. Acidic strength of oxy acid having same central atom increases with increases in oxidation number of central atom


+1                       +3                +5         +7

HClO         < HClO2   <    HClO3  < HClO4

very weak     weak             strong       very strong


+4            +6

H2SO3 < H2SO4

  1. Strength of oxy acid having different central atom but same oxidation state & same configuration increases with increase in electronegativity of central atom.

Example. 1.   HOI+1<HOBr+1<HOCl+1

Example2.    HIO4 < HBrO4 <HClO4

Example3.   H3AsO4<H3PO4

  1. Strength of oxy acid increases from left to right in a period.


  1. The basicity of compound decreases from left to right among a period with increase in electronegativity.

Ex.NH3>H2O >HF

  1. The basicity of compound decreases from top to bottom in a group with increase in size of atom.