Acid Base Concept

acid base concept

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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^–

(aq.)

CH₃COOH <—->CH₃COO^–+H⁺

(aq.)

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.

H₂O+H^+–>H₃O⁺

CO3^–– +H⁺ –>HCO3^–

CN^–+H⁺ –>HCN

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”

Example.1

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.

Example.2:

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^–

H₂SO_{4                             HSO4^–}

HNO_{3                             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 : NH_{3 ,} :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

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

Example.1

+1                       +3                +5         +7

HClO         < HClO2   <    HClO3  < HClO4

very weak     weak             strong       very strong

Example.2

+4            +6

H2SO3 < H2SO4

5. 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

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

H₂SiO₄<H₃PO₄<H₂SO₄<HClO₄

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

Ex.NH₃>H₂O >HF

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