Salts, Double Salts & Complex Salts – Comprehensive Study Notes

Neutralisation & Definition of Salts

  • Neutralisation is the reaction between an acid and a base to give salt and water.
    • Typical example: \text{HCl}{(aq)} + \text{NaOH}{(aq)} \to \text{NaCl}{(aq)} + \text{H}2\text{O}_{(l)}
    • The product ‘salt’ is defined as the neutralisation product of an acid and a base.

Classification of Salts

  • Two broad ways to classify salts in this lecture:
    1. By composition/stoichiometry:
    • Simple salts
    • Addition compounds ➔ subdivided into Double salts & Complex salts
    1. By behaviour/stability (only for complexes):
    • Perfect complexes (high stability)
    • Imperfect complexes (low stability)
  • Key remark: “Complex salt” is synonymous with “Coordination compound.”

Simple Salts

  • Contain only one type of cation and one type of anion.
  • Completely ionise in aqueous solution and give positive qualitative test for each ion present.
  • Examples:
    • \text{KCl},\; \text{NaCl},\; \text{K}2\text{SO}4,\; \text{Al}2(\text{SO}4)_3

Addition Compounds

  • Formed when two or more stable compounds combine in fixed (stoichiometric) ratios.
  • General formula written as a dot-product: \text{(compound 1)}\,\cdot\,\text{(compound 2)}
  • Two types:
    1. Double salts
    2. Complex salts

Double Salts

  • Definition: Addition compounds stable only in solid state; in water they dissociate completely into their constituent ions.
  • Give individual qualitative tests for every ion present.
  • Regarded as “imperfect” or “low-formation-constant” complexes.
  • Key examples (all prepared industrially by crystallisation):
    • Carnallite: \text{KCl}\,\cdot\,\text{MgCl}2\,\cdot 6\text{H}2\text{O}
    • Dissociation: \text{K}^+,\; \text{Mg}^{2+},\; 3\text{Cl}^-
    • Mohr’s salt: \text{FeSO}4\,\cdot (\text{NH}4)2\text{SO}4\,\cdot 6\text{H}_2\text{O}
    • Dissociation: \text{Fe}^{2+},\;\text{SO}4^{2-},\;\text{NH}4^{+}
    • Potash alum: \text{K}2\text{SO}4\,\cdot \text{Al}2(\text{SO}4)3\,\cdot 24\text{H}2\text{O}
    • Dissociation: \text{K}^+,\; \text{SO}_4^{2-},\; \text{Al}^{3+}
  • Solubility notes:
    • "All nitrate salts are soluble" (basic solubility rule).
    • \text{AgNO}3 used for halide tests; \text{BaCl}2 used for sulphate tests.

Complex Salts

  • Definition: Addition compounds in which some ions/molecules lose their separate identity; do not dissociate completely into their individual ions in water.
  • Constituent giving up identity forms a central ‘complex ion’ with attached ligands.
  • Qualitative tests for all constituent ions cannot be obtained from their aqueous solutions.
  • Example formation & dissociation (Potassium ferrocyanide):
    • Preparation: 4\text{KCN}{(aq)} + \text{Fe(CN)}2{(aq)} \xrightarrow[\text{Cryst.}]{\text{H}2\text{O}} 4\text{KCN}\,\cdot\,\text{Fe(CN)}2 \to \text{K}4[\text{Fe(CN)}_6]
    • In solution: \text{K}4[\text{Fe(CN)}6] \rightleftharpoons 4\text{K}^+ + [\text{Fe(CN)}_6]^{4-} (only feeble further dissociation)

Formation & Dissociation Constants (Kf & Kd)

  • Complex formation equilibrium: \text{Fe}^{2+} + 6\text{CN}^- \rightleftharpoons [\text{Fe(CN)}_6]^{4-}
    • Formation constant: Kf = \dfrac{[\,[\text{Fe(CN)}6]^{4-}\,]}{[\text{Fe}^{2+}]\,[\text{CN}^-]^6}
    • Dissociation constant: Kd = \dfrac{1}{Kf}
  • Stability criterion: larger K_f ⇒ more stable complex.
  • Chelate effect: Chelating (multidentate) ligands give higher K_f due to favourable entropy.

Stability: Perfect vs Imperfect Complexes

  • Perfect complex
    • High K_f, negligible dissociation; complex ion is stable.
    • Example: \text{K}4[\text{Fe(CN)}6] behaves as perfect complex in water.
  • Imperfect complex
    • Lower K_f, appreciable reversible dissociation; individual ion tests partly observed.
    • Example: \text{K}2[\text{Cd(CN)}4] \rightleftharpoons 2\text{K}^+ + [\text{Cd(CN)}_4]^{2-} \rightleftharpoons \text{Cd}^{2+} + 4\text{CN}^-
    • Extremely unstable imperfect complexes can dissociate completely and are essentially double salts.
  • No absolute numeric boundary; stability is often judged relative to a specific reagent.
    • [\text{Ag(NH}3)2]^+ is:
    • Perfect w.r.t. \text{KBr} (no precipitation of \text{AgBr})
    • Imperfect w.r.t. \text{KI} (yellow \text{AgI} ppt. forms)
    • [\text{Cu(CN)}4]^{3-} perfect vs \text{H}2\text{S}; [\text{Cd(CN)}4]^{2-} imperfect vs \text{H}2\text{S} producing yellow \text{CdS}.

Alternative Classification of Complexes

  • On the basis of ligand variety:
    • Homoleptic complex: only one kind of ligand present.
    • Heteroleptic complex: two or more ligand types.
  • On the basis of the charge on the complex ion:
    • Cationic complex (positively charged)
    • Anionic complex (negatively charged)
    • Neutral complex (overall neutral)
  • On the basis of stability: perfect vs imperfect (already covered).

Analytical Applications & Precipitation Tests

  • Precipitation rules applied throughout qualitative analysis:
    • \text{AgNO}_3 tests for halide ions:
    • \text{F}^- ➔ \text{AgF} (soluble, no ppt.)
    • \text{Cl}^- ➔ \text{AgCl} (white ppt.)
    • \text{Br}^- ➔ \text{AgBr} (pale yellow ppt.)
    • \text{I}^- ➔ \text{AgI} (yellow ppt.)
    • \text{BaCl}_2 tests for sulphate ions:
    • \text{SO}4^{2-} + \text{Ba}^{2+} \to \text{BaSO}4 (white ppt.)
  • Knowledge of complex stability is critical in designing selective reagents for ion detection or removal.
    • Example: Formation of [\text{Ag(NH}3)2]^+ keeps (\text{Ag}^+) in solution, preventing unwanted precipitation until a more strongly binding anion appears.