Reactions of ions in aqueous solution (A Level) flashcards

3.2.6.1 Reactions of ions in aqueous solution

Aqua ion

A metal ion surrounded by six water ligands, each joined through a co-ordinate bond from oxygen.

Hydrolysis (of an aqua ion)

Transfer of a proton from a water ligand to a solvent water molecule, producing H₃O⁺.

Why is [Fe(H₂O)₆]³⁺ more acidic than [Fe(H₂O)₆]²⁺?

Fe³⁺ has a higher charge and a smaller radius, so a higher charge/size ratio. It is more polarising, weakening the O–H bonds in its water ligands, so H⁺ is released more easily.

Fe²⁺(aq) and Cu²⁺(aq) with NaOH(aq) or a little NH₃(aq)

Fe²⁺: green precipitate [Fe(H₂O)₄(OH)₂], browns on standing in air. Cu²⁺: blue precipitate [Cu(H₂O)₄(OH)₂]. Neither dissolves in excess NaOH.

Cu²⁺(aq) with excess ammonia

The blue precipitate dissolves to give a deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺.

Fe³⁺(aq) and Al³⁺(aq) with NaOH(aq), then excess

Fe³⁺: orange-brown precipitate [Fe(H₂O)₃(OH)₃], no change in excess. Al³⁺: white precipitate [Al(H₂O)₃(OH)₃], dissolves in excess to colourless [Al(H₂O)₂(OH)₄]⁻.

Amphoteric (aluminium hydroxide)

Reacts with both acids and bases: [Al(H₂O)₃(OH)₃] + 3H⁺ → [Al(H₂O)₆]³⁺ [Al(H₂O)₃(OH)₃] + OH⁻ → [Al(H₂O)₂(OH)₄]⁻ + H₂O

Metal aqua ions with sodium carbonate: 2+ ions vs 3+ ions

2+ (Fe²⁺, Cu²⁺): metal carbonate precipitate (FeCO₃ green, CuCO₃ blue-green), no gas. 3+ (Fe³⁺, Al³⁺): hydroxide precipitate and effervescence of CO₂. The 3+ aqua ion is acidic enough to protonate carbonate.

Equation for a 3+ aqua ion with carbonate ions

2[M(H₂O)₆]³⁺ + 3CO₃²⁻ → 2[M(H₂O)₃(OH)₃] + 3CO₂ + 3H₂O