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: $\text{HCl}<em>{(aq)} + \text{NaOH}</em>{(aq)} \to \text{NaCl}<em>{(aq)} + \text{H}</em>2\text{O}_{(l)}$
  • 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:
  • $\text{KCl},\; \text{NaCl},\; \text{K}<em>2\text{SO}</em>4,\; \text{Al}<em>2(\text{SO}</em>4)_3$

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: $\text{KCl}\,\cdot\,\text{MgCl}<em>2\,\cdot 6\text{H}</em>2\text{O}$
  • Dissociation: $\text{K}^+,\; \text{Mg}^{2+},\; 3\text{Cl}^-$
  • Mohr’s salt: $\text{FeSO}<em>4\,\cdot (\text{NH}</em>4)<em>2\text{SO}</em>4\,\cdot 6\text{H}_2\text{O}$
  • Dissociation: $\text{Fe}^{2+},\;\text{SO}<em>4^{2-},\;\text{NH}</em>4^{+}$
  • Potash alum: $\text{K}<em>2\text{SO}</em>4\,\cdot \text{Al}<em>2(\text{SO}</em>4)<em>3\,\cdot 24\text{H}</em>2\text{O}$
  • Dissociation: $\text{K}^+,\; \text{SO}_4^{2-},\; \text{Al}^{3+}$
• Solubility notes:
  • "All nitrate salts are soluble" (basic solubility rule).
  • $\text{AgNO}<em>3$ used for halide tests; $\text{BaCl}</em>2$ 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: $4\text{KCN}<em>{(aq)} + \text{Fe(CN)}</em>2<em>{(aq)} \xrightarrow[\text{Cryst.}]{\text{H}</em>2\text{O}} 4\text{KCN}\,\cdot\,\text{Fe(CN)}<em>2 \to \text{K}</em>4[\text{Fe(CN)}_6]$
  • In solution: $\text{K}<em>4[\text{Fe(CN)}</em>6] \rightleftharpoons 4\text{K}^+ + [\text{Fe(CN)}_6]^{4-}$ (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: $K<em>f = \dfrac{[\,[\text{Fe(CN)}</em>6]^{4-}\,]}{[\text{Fe}^{2+}]\,[\text{CN}^-]^6}$
  • Dissociation constant: $K<em>d = \dfrac{1}{K</em>f}$
• 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: $\text{K}<em>4[\text{Fe(CN)}</em>6]$ behaves as perfect complex in water.
• Imperfect complex
  • Lower $K_f$, appreciable reversible dissociation; individual ion tests partly observed.
  • Example: $\text{K}<em>2[\text{Cd(CN)}</em>4] \rightleftharpoons 2\text{K}^+ + [\text{Cd(CN)}_4]^{2-} \rightleftharpoons \text{Cd}^{2+} + 4\text{CN}^-$
  • 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.
  • $[\text{Ag(NH}<em>3)</em>2]^+$ is:
  • Perfect w.r.t. $\text{KBr}$ (no precipitation of $\text{AgBr}$)
  • Imperfect w.r.t. $\text{KI}$ (yellow $\text{AgI}$ ppt. forms)
  • $[\text{Cu(CN)}<em>4]^{3-}$ perfect vs $\text{H}</em>2\text{S}$; $[\text{Cd(CN)}<em>4]^{2-}$ imperfect vs $\text{H}</em>2\text{S}$ 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:
  • $\text{SO}<em>4^{2-} + \text{Ba}^{2+} \to \text{BaSO}</em>4$ (white ppt.)
• Knowledge of complex stability is critical in designing selective reagents for ion detection or removal.
  • Example: Formation of $[\text{Ag(NH}<em>3)</em>2]^+$ keeps (\text{Ag}^+) in solution, preventing unwanted precipitation until a more strongly binding anion appears.