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Double salt vs complex salt; Werner's theory

Short answer

Double salts dissociate into constituent ions in solution, e.g., Mohr's salt. Complex salts retain identity through coordinate bonds, e.g., K₄[Fe(CN)₆]. Werner's theory explains primary and secondary valency, coordination number, conductivity, isomerism, and why ions inside coordination sphere do not precipitate with AgNO₃.

Differentiate between double salt and complex salt. Write the application of Werner's theory of co-ordination compound.

Double salt vs complex salt

BasisDouble saltComplex salt
FormationTwo simple salts crystallise togetherMetal ion combines with ligands by coordinate bonds
ExistenceExists only in solid; breaks up in waterRetains identity in solid and solution
Ions in waterGives the ions of both saltsGives a complex ion and counter ions
Test for metalMetal ions respond to normal testsCentral metal ion does not respond to usual tests
ExampleMohr's salt FeSO₄·(NH₄)₂SO₄·6H₂O; potash alumK₄[Fe(CN)₆], [Cu(NH₃)₄]SO₄

Applications of Werner's theory

  • Explains why metals have two types of valency, primary (ionisable) and secondary (non-ionisable), and shows that the secondary valency equals the coordination number.
  • Explains structure: octahedral for CN 6, square planar or tetrahedral for CN 4.
  • Explains the conductivity of complexes (from the number of ions formed).
  • Explains isomerism (geometrical and optical) in complexes.
  • Explains why some chlorides do not precipitate with AgNO₃: they are inside the coordination sphere. For example, all 3 Cl⁻ of [Co(NH₃)₆]Cl₃ precipitate with AgNO₃, but only 2 Cl⁻ of [Co(NH₃)₅Cl]Cl₂ do.

✓ Answer checked by Claude Opus 5.5 (high effort)

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