Reactions of Inorganic Compounds in Aqueous Solution

Key Concepts

Lewis Acids and Bases

• Lewis Acid: Electron pair acceptor.
• Lewis Base: Electron pair donor.
• Ligands are Lewis bases in complex ion formation because they donate electrons to the metal ion, which is a Lewis acid.

Metal-Aqua Ions

• Formed in aqueous solutions.
• Examples:
  • [M(H₂O)₆]²⁺: M could be Fe (green) or Cu (blue).
  • [M(H₂O)₆]³⁺: M could be Al (colourless) or Fe (violet).
• In solution, Fe(III) appears yellow/brown due to hydrolysis reactions.

Acidity or Hydrolysis Reactions

• [M(H₂O)₆]³⁺ is more acidic than [M(H₂O)₆]²⁺ due to higher charge density and polarising power.
• Greater polarising power weakens the O-H bond, making it easier to release H⁺ ions.

Reactions with OH⁻ and NH₃

• Limited OH⁻ and NH₃:
  • Form hydroxide precipitates through deprotonation acid-base reactions.
  • Common Precipitates:
    • Cu: Blue ppt
    • Fe(II): Green ppt
    • Fe(III): Brown ppt
    • Al: White ppt
• Excess OH⁻:
  • Al hydroxide dissolves to form [Al(OH)₄]⁻ (aq), a colourless solution.
  • Al hydroxide is amphoteric, meaning it reacts with both acids and bases.

Reaction with Excess NH₃

• Cu undergoes a ligand substitution reaction, dissolving to form a deep blue solution [Cu(NH₃)₄(H₂O)₂]²⁺.
• NH₃ acts as a Bronsted-Lowry base by accepting a proton and as a Lewis base by donating an electron pair.

Reactions with Carbonate Solution

• 2+ Ions:
  • Form MCO₃ ppt.
  • Cu forms blue/green ppt, Fe(II) forms green ppt.
• 3+ Ions:
  • Form M(OH)₃ ppt and CO₂ gas.
  • Greater polarising power of 3+ ions prevents M₂(CO₃)₃ formation.
  • Al forms white ppt, Fe(III) forms brown ppt.

Summary of Metal Ion Reactions

• [M(H₂O)₆]²⁺:
  • Cu: Blue solution, forms blue ppt with hydroxide.
  • Fe(II): Green solution, forms green ppt with hydroxide.
• [M(H₂O)₆]³⁺:
  • Al: Colourless solution, forms white ppt with hydroxide.
  • Fe(III): Yellow solution, forms brown ppt with hydroxide.

Reactions Summary with CO₃²⁻

• Cu and Fe(II) form carbonate precipitates.
• Al and Fe(III) form hydroxide precipitates with CO₂ release.
• Precipitates:
  • CuCO₃: Blue ppt
  • FeCO₃: Green ppt
  • Al(OH)₃(H₂O)₃: White ppt with CO₂ bubbles
  • Fe(OH)₃(H₂O)₃: Brown ppt with CO₂ bubbles