Chemistry Revision of Acids, Bases And Salts For NDA

Bases

The taste of bases are bitter. Bases give a slippery feeling; for example, soaps, which contain bases, exhibit this property. Bases cause color changes in plant dyes; for example, they change the color of litmus from red to blue.

`=>` According to Arrhenius, a base is a compound that releases `OH^-` ions in water.

e.g. `undersettext {Sodium hydroxide (base) } ( NaOH (aq)) ⇌ Na^+ (aq) +undersettext (hydroxyl ion ) (O barH (aq) ) `

Similarly, ` undersettext (potassium hydroxide ) (KOH(aq)) ⇌ K^+ (aq) + undersettext (Hydroxylion) (O bar H (aq ) )`

`=>` According to Bronsted-Lowry concept, a base is any molecule or ion that can accept a proton.

e.g. `undersettext (Base) ( NH_3) + undersettext (proton) ( H^+ ) ⇌ undersettext ( conjugate acid) (NH_(4)^+)`

Similarly , ` undersettext (base) (H_2O) + undersettext (proton ) (H^+) ⇌ undersettext { Hydronium ion ( conjugate acid) } ( H_3 O^+ )`

`=>` According to Lewis concept, a base is an electron-pair donor. e.g. `overset(• •)NH_3 , R overset(• •)OH , R_2 overset(• •)O`, pyridine etc.

Note : Conjugate acid of a weak base is strong or vice-versa.

`text(Base) + H^+ → text(Conjugate acid)`

Types of Bases

On the basis of degree of dissociation bases are of two types

• Strong base : These bases dissociate completely in concentrated solutions. e.g. `KOH, NaOH` etc.

• Weak base : These bases only partially dissociate in water. e.g. `Mg(OH)_2 , NH_4 OH, NaHCO_3 , Ca(OH)_2` etc.

Properties of Bases

Bases have the following general properties

(i) Bases form salt with metals with the evolution of hydrogen gas.

` undersettext (base) (2 NaOH) +Zn -> undersettext {sodium zincate (salt) } (Na_2 ZnO_2 + H_2 ↑)`

Note : This reaction is not given by all bases.

(ii) The reaction of bases with acids is known as neutralisation reaction, which leads to the formation of corresponding salt and water.

(iii) Bases form salt and water with non-metallic oxides.

`undersettext { sodoum hydroxide (Base) } (2 NaOH) + undersettext {sulphur trioxide (acid) } (SO_3) -> undersettext {Sodium sulphate (salt) } (Na SO_4) + undersettext (water) (H_2O)`

Above reaction is similar to neutralisation reaction (acid-base reaction). 'Therefore nature of non-metallic oxides is acidic.

(iv) Bases on dissolving in water give hydroxide ions (`OH^-` ions)

e.g. `undersettext (sodium hydroxide) (Na OH (s) ) oversettext (water) -> Na^+ (aq) + OH^- (aq)`.

Note : All the bases do not dissolve in water. The bases which dissolve in water are called alkali e.g. `NaOH, KOH`, `Ca(OH)_2`, `NH_4 OH` etc. Hence, all alkalis are bases but all the bases are not alkalis.

Acidity of Base

The number of replaceable hydroxyl ions present in a base is known as acidity of the base.

• Monoacidic bases : produces only one hydroxide ions in aqueous solutions. e.g. `KOH, NH_4 OH , NaOH`.

• Diacidic bases : produces two hydroxide ions in aqueous solutions. e.g. `Ca(OH)_2, Fe(OH)_2 , Zn(OH)_2`.

• Triacidic bases : produces three hydroxide ions in aqueous solutions. e.g. `Fe(OH)_3 , Al(OH)_3`.

Uses of Some Important Bases

`=>` Sodium hydroxide (`NaOH`) is used in manufacture of soap, purification of bauxite and manufacture of rayon.

`=>` Calcium hydroxide is used in preparation of sodalime (mixture of calcium hydroxide and caustic soda), for white washing buildings and softening of hard water.

`=>` Potassium hvdroxide is used in the manufacture of soft soap. A suspension of magnesium hydroxide in water is used in medicines as an antacid. (milk of magnesia).