Simple salt and Molecular (or) addition compounds

A salt is formed by the neutralisation of an acid by a base. There are different types of salts. They are:-

a) Simple salt

b) Molecular (or) addition compounds

a)    Simple salt

A simple salt is formed by the neutralisation of an acid by a base.

KOH   + HCl      ®   KCl + H2O

Normally, a simple salt ionises in water and produces ions in solution. The  solution of the simple salt exhibits the properties of its component ions.

b)    Molecular (or) addition compounds

i)     Double salts

These are molecular compounds which are formed by the evaporation of solution containing two (or) more salts in stoichiometric proportions. Hence the molecular compounds which dissociate in solution into its constituent ions are known as double salt. Double salts retain their properties only in solid state.

They are also called as lattice compounds.

Example

K2SO4 . Al2(SO4)3 . 24H2O     - Potash alum

FeSO4 . (NH4)2 SO4. 6H2O      - Mohr's salt

K2SO4 . Al2(SO4)3 . 24H2O ® 2K+ + 2Al3+ + 4SO42- + 24H2O

The double salts give the test of all their constituent ions in solution.

ii)    Coordination (or complex) compounds

Coordination compound is 'a compound formed from a Lewis acid and a Lewis base'. The molecular compounds, do not dissociate into its constituent ions in solution are called coordination compounds.

Example

Fe(CN)2 + 4KCN ®      Fe(CN)2 . 4KCN (or) K4[Fe(CN)6]

Ferrous cyanide

Fe(CN)2 . 4KCN          4K+ + [Fe(CN)6]4-

Complex anion

In K4[Fe (CN)6] the individual components lose their identity. The metal of the complex ion is not free in solution unlike metal in double salt in solution.

Nature of coordination (or) complex compounds

i)     An anionic complex compound contains a complex anion and simple cation.

K4 [Fe(CN)6]                 4K+     +     [Fe (CN)6 ]4-

simple cation    complex anion

ii)    A cationic complex contains complex cation and simple anion

[Co(NH3 )6] Cl3        [Co(NH3)6]3+ + 3Cl-

complex cation     simple anion

iii)    In the case of a complex compound, [Cr(NH3 )6] [Co(CN)6], it gives both

complex cation and complex anion

[Cr(NH3)6] [Co(CN)6]       [Cr(NH3)6]3+ + [Co(CN)6]3-

complex cation complex anion