Reactions of ions in aqueous solution (A Level) flashcards

Topic 18: Reactions of ions in aqueous solution

Metal-aqua ion

A metal ion with six water ligands, each bonded by a co-ordinate bond from oxygen, for example [Cu(H₂O)₆]²⁺.

Why are [M(H₂O)₆]³⁺ solutions more acidic than [M(H₂O)₆]²⁺ solutions?

M³⁺ has a higher charge and smaller radius, so a higher charge/size ratio. It is more polarising, weakening the O–H bonds in the water ligands, so H⁺ is released more easily.

Fe²⁺, Cu²⁺, Fe³⁺ and Al³⁺ with a little NaOH(aq): precipitate colours

Fe²⁺: green. Cu²⁺: pale blue. Fe³⁺: brown. Al³⁺: white.

Which hydroxide precipitate dissolves in excess NaOH, and which in excess NH₃?

Excess NaOH: aluminium hydroxide only (amphoteric), giving a colourless solution of [Al(H₂O)₂(OH)₄]⁻. Excess NH₃: copper hydroxide only, giving deep blue [Cu(NH₃)₄(H₂O)₂]²⁺.

Sodium carbonate with 2+ aqua ions vs 3+ aqua ions

2+ (Fe²⁺, Cu²⁺): a metal carbonate precipitates (FeCO₃, CuCO₃). 3+ (Fe³⁺, Al³⁺): the hydroxide precipitates and CO₂ is given off (effervescence).

Equation: [Fe(H₂O)₆]³⁺ with ammonia (a few drops)

[Fe(H₂O)₆]³⁺ + 3NH₃ → [Fe(H₂O)₃(OH)₃] + 3NH₄⁺ (brown precipitate).

Why does an iron(II) hydroxide precipitate darken on standing in air?

Iron(II) is oxidised to iron(III) by oxygen in the air, so the green precipitate turns brown.

Why do the neutral hydroxides precipitate?

They carry no charge (e.g. [Cu(H₂O)₄(OH)₂]), so they are not stabilised by water and come out of solution.