Class 7 Science Acid Base and Salts Chapter Notes

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Acid, Base and Salts

Acids and Bases
1. Different workers gave different definitions for acids and bases from time to time.
i) Arrhenius Theory : Arrhenius defined acids are those compounds which give hydrogen ions in aqueous solution, and bases are those compounds which give hydroxide ions in solutions.e.g.

,The strength of an acid or base depends upon its tendency to furnish H⁺ or OH^– ions in solution.This definition can be applied only for those reactions which take place in aqueous solutions. Further this theory does not include substances that do not contain H⁺ and OH^– ions but still can neutralize acids and bases.

Basicity or protonicity of acids: - It is the number of H⁺ ions furnished by a molecule of an acid.

Monobasic acid : An acid furnishing one H⁺ ions e.g.HCl, HNO₃ etc.

Dibasic acid : An acid furnishing two H⁺ ions e.g. H₂SO₄, H₃PO₃ etc.

Tribasic acid : An acid furnishing three H⁺ ions e.g. H₃PO4.

Acidity or Hydroxicity of base : It may be defined as the number of OH^– ions furnished by amolecule of a base.

Monoacidic base : NaOH, KOH

Diacidic base : Ca(OH)₂, Ba(OH)₂

ii)Bronsted Lowry Theory : According to this theory acids are defined as those substances which give up proton and bases are substances which accepts proton.e.g.,

From the above observations we may conclude that every acid has a conjugate base and every base has a conjugate acid.In above equation 1, the conjugate base of acid HCl is Cl^– and The conjugate acid of water is H₃O+ (hydroxonium ion)

In the above equation 2, the conjugate acid of base NH₃ is NH4⁺ and the conjugate base of H₂O  is OH^–.
Thus according to this theory the following conclusions can be drawn.
a) A substance can act as an acid/base when another substance capable of accepting a proton and capable of donating a proton exists.

Classification of Solvents
i) Aprotic: Solvents which can neither accept nor donate a proton.e.g. CCl₄, CS₂, Benzene.
ii) Amphiprotic: Solvents which can both accept and donate a proton, e.g. H₂O, liquor Ammonia.
iii) Protophillic: Solvents which have a greater tendency to accept proton e.g. H₂O, R-OH, liquor NH3.

Levelling Effect
Water acts as a very strong base because it has a great tendency to accept proton from mineral acids. The ionization of strong acids in water may be represented as below :

Thus all strong acids (e.g. HNO₃, HCl, H₂SO₄, HClO₄) react almost completely to form H₃O+ ion. Therefore all strong acids in aqueous solution appear equally strong. i.e. Acidic levels for all acids are same. This is called levelling effect and water is the levelling solvent. Thus the relative acidic strengths or basic strengths in aqueous solutions cannot be compared.

When acetic acid is used as solvent instead of water it is found that the acidic strength of the acid lowers and now the given acid in acetic acid becomes weak and thereby different acids will dissociate to different extent in glacial acetic acid. (This is because of poor proton accepting character of acetic acid).

In this reaction the equalibrium does not lie very much to the right and hence strong acids are feebly dissociated. The degree of dissociations of a number of acids dissolved in glacial acetic acid has been determined and it is found that the acidic strength order is

Lewis Concept of Acids And Bases

According to Lewis, an acid is any species which is capable of accepting a pair of electrons and a base is a species that is having a lone pair of electrons for donations.

Lewis acids

i) Compounds where the central atom has incomplete octet. This means all electron deficient molecules can act as Lewis acids e.g.

BF₃, BBr₃, AlCl3, FeCl₃, GaCl₃ etc.

ii) Compounds in which the central atom has available vacant orbitals. e.g. SnF₄, SnCl₂, SnCl₄, PF₃, PF₄,SF₄, TiCl₄ etc.

iii) Simple cations like Ag⁺, Cu²⁺, Al³⁺ etc. e.g. Cu²⁺ + 4NH₃ [Cu (NH3)4]²⁺.

 

Lewis Bases

i) All simple anions are Lewis bases e.g. Cl–, NO₃ –, OH–.

ii) Molecules containing one or more unshared pairs of electrons (Lone pairs) e.g. H₂O, NH₃, ROH,NX₃, R₂S, etc. are Lewis bases.

iii) Multiple bonded compounds which can donate the pair of electrons. e.g. C = O, N = O,CH₂ = CH₂, CH ≡ CH.

 

SALT

A substance which ionizes in water to produce ions other than H+ and OH– is called a salt.

 

Types of Salts

Neutral Salts: Those salts whose aqueous solutions neither turn blue litmus red nor red litmus blue are called neutral salts. These are prepared by the neutralization of strong acid and strong base. e.g. NaCl, K₂SO₄,KNO₃ etc.

Acidic Salts: Those salts whose aqueous solutions turn blue litmus red are called Acidic salts. These are prepared by neutralisation of strong acid with weak base, e.g., NH₄NO₃, NH₄Cl.

Basic Salts: Those salts whose aqueous solutions turn red litmus blue are called basic salts. These are formed by the neutralization of strong bases with weak acids. e.g. Na₂CO₃, CH₃COONa

Mixed salts: Salts formed by the neutralization of one acid by two bases or one base by two acids are called mixed salts. e.g. CaOCl₂ 

Double Salts: A compound of two salts whose aqueous solution shows the tests for all constituent ions is called double salt e.g.

Mohr Salt FeSO₄. (NH₄)₂ SO₄. 6H₂O

Potash Alum K₂SO₄. Al2 (SO₄)₃. 24H₂O

Complex Salts: A compound whose solutions does not give tests for the constituent ions is called a complex salt. e.g.

 

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