acid base 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
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
a) Lewis Acid- Lewis Acid is electron pair acceptor
b) Lewis Base-Lewis base is electron pair donar
:NH3 + BF3 –> [H3N –> BF3]
(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. 2 Be(OH)2<Mg(OH)2<Ca(OH)2<Sr(OH)2< Ba(OH)2
- Mineral acids such as H2SO4, HCl&HNO3 are strong acids.
- All organic acids (except those having –SO3H gr. i.e. Sulphonic acids) are weak acids.
- 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
H2SO3 < H2SO4
- 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
- Strength of oxy acid increases from left to right in a period.
- The basicity of compound decreases from left to right among a period with increase in electronegativity.
- The basicity of compound decreases from top to bottom in a group with increase in size of atom.