Chemistry – Experimental techniques and chemical analysis - Identification of ions and gases | e-Consult

(a) Aluminium (Al³⁺)

Aqueous sodium hydroxide (NaOH) reacts with aluminium(III) ions to form a soluble complex ion, [Al(OH)₄]^-, which is pale blue. Aqueous ammonia (NH₃) also reacts with aluminium(III) ions to form a similar complex ion, [Al(OH)₄]^-, resulting in a pale blue solution. The formation of the complex ion is the key observation.

Reaction with NaOH: Al³⁺(aq) + 3OH^-(aq) ⇌ [Al(OH)₄]^-(aq)

Reaction with NH₃: Al³⁺(aq) + 3NH₃(aq) ⇌ [Al(OH)₄]^-(aq) + 3NH₄⁺(aq)

(b) Ammonium (NH₄⁺)

Aqueous ammonia (NH₃) readily reacts with ammonium ions to form a soluble complex ion, [NH₄(OH)₂]⁺. This complex ion is formed in excess, leading to a clear solution. Aqueous sodium hydroxide (NaOH) will not cause a noticeable change.

Reaction: NH₄⁺(aq) + OH^-(aq) ⇌ [NH₄(OH)₂]⁺(aq)

(c) Calcium (Ca²⁺)

Aqueous sodium hydroxide (NaOH) reacts with calcium(II) ions to form a white precipitate of calcium hydroxide, Ca(OH)₂. Aqueous ammonia (NH₃) does not react with calcium(II) ions to form a precipitate.

Reaction: Ca²⁺(aq) + 2OH^-(aq) → Ca(OH)₂(s)

(d) Chromium(III) (Cr³⁺)

Aqueous sodium hydroxide (NaOH) reacts with chromium(III) ions to form a green precipitate of chromium(III) hydroxide, Cr(OH)_{3). Aqueous ammonia (NH3) reacts with chromium(III) ions to form a green complex ion, [Cr(OH)4]^2-. The solution will initially turn green and then become a deeper green.}

Reaction with NaOH: Cr³⁺(aq) + 3OH^-(aq) → Cr(OH)₃(s)

Reaction with NH₃: Cr³⁺(aq) + 6NH₃(aq) ⇌ [Cr(OH)₄]^2-(aq) + 6NH₄⁺(aq)

(e) Copper(II) (Cu²⁺)

Aqueous sodium hydroxide (NaOH) reacts with copper(II) ions to form a blue-green precipitate of copper(II) hydroxide, Cu(OH)₂. Aqueous ammonia (NH₃) reacts with copper(II) ions to form a blue-violet complex ion, [Cu(NH₃)₄]²⁺. The solution will initially turn blue-green and then blue-violet.

Reaction with NaOH: Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s)

Reaction with NH₃: Cu²⁺(aq) + 4NH₃(aq) ⇌ [Cu(NH₃)₄]²⁺(aq)

(f) Iron(II) (Fe²⁺)

Aqueous sodium hydroxide (NaOH) reacts with iron(II) ions to form a pale blue-green precipitate of iron(II) hydroxide, Fe(OH)₂. Aqueous ammonia (NH₃) reacts with iron(II) ions to form a pale blue-green solution of the tetraammine iron(II) complex ion, [Fe(NH₃)₄]²⁺.

Reaction with NaOH: Fe²⁺(aq) + 2OH^-(aq) → Fe(OH)₂(s)

Reaction with NH₃: Fe²⁺(aq) + 4NH₃(aq) ⇌ [Fe(NH₃)₄]²⁺(aq)

(g) Iron(III) (Fe³⁺)

Aqueous sodium hydroxide (NaOH) reacts with iron(III) ions to form a reddish-brown precipitate of iron(III) hydroxide, Fe(OH)₃. Aqueous ammonia (NH₃) reacts with iron(III) ions to form a reddish-brown complex ion, [Fe(OH)₃]^3-. The solution will initially turn reddish-brown and then remain reddish-brown.

Reaction with NaOH: Fe³⁺(aq) + 3OH^-(aq) → Fe(OH)₃(s)

Reaction with NH₃: Fe³⁺(aq) + 3NH₃(aq) ⇌ [Fe(OH)₃]^3-(aq) + 3NH₄⁺(aq)

(h) Zinc (Zn²⁺)

Aqueous sodium hydroxide (NaOH) does not react with zinc(II) ions. Aqueous ammonia (NH₃) reacts with zinc(II) ions to form a white precipitate of zinc(II) hydroxide, Zn(OH)₂.

Reaction: Zn²⁺(aq) + 2OH^-(aq) → Zn(OH)₂(s)