Q.i) Write two differences with example between double salt and complex salt. ii) What is meant by Chelating effect? Draw the structure of chelate complex formed during determination of hardness of water using chelating ligand.
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Start your 14-day free trial to unlock the full solution →Double salts break up entirely into simple ions in solution; complex salts retain a stable complex ion. The chelate effect is the enhanced stability from ring-forming polydentate ligands, exemplified by the octahedral Ca-EDTA / Mg-EDTA chelate used in hardness determination.
(i) Double salt vs complex salt (two differences with examples)
- On dissolving in water: a double salt dissociates completely into all its constituent simple ions and loses its own identity; a complex salt ionises to give a complex ion that does not break down into simple ions and keeps its identity.
- Tests for constituent ions: a double salt gives the normal tests of all its ions (e.g. Fe2+, NH4+, SO4^2- for Mohr's salt); a complex salt does not give the ordinary tests of the metal/ligand locked in the complex ion.
Examples - double salt: Mohr's salt FeSO4.(NH4)2SO4.6H2O (or potash alum). Complex salt: potassium ferrocyanide K4[Fe(CN)6], which in water gives 4K+ + [Fe(CN)6]^4- (no free Fe2+ or CN-).
(ii) Chelate effect
When a polydentate (multidentate) ligand binds a metal ion through two or more donor atoms, it forms one or more rings; such complexes are far more stable than comparable complexes of monodentate ligands. This extra stability from ring formation is the chelate effect (largely entropy-driven).
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