CBSE Class 10 Answered

Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au³⁺). The hydrochloric acid provides a ready supply of chloride ions (Cl^?), which react with the gold ions to produce chloroaurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloroaurate anions (AuCl₄^?). This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloroauric salt

 

Au (s) + 3 NO₃^? (aq) + 6 H⁺ (aq) ? Au³⁺ (aq) + 3 NO₂ (g) + 3 H₂O (l) and

Au³⁺ (aq) + 4 Cl^? (aq) ? AuCl₄^? (aq).

Platinum being dissolved in aqua regia

Similar equations can be written for Platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product.

Pt (s) + 4 NO₃^? (aq) + 8 H⁺ (aq) ? Pt⁴⁺ (aq) + 4 NO₂ (g) + 4 H₂O (l)

3Pt (s) + 4 NO₃^? (aq) + 16 H⁺ (aq) ? 3Pt⁴⁺ (aq) + 4 NO (g) + 8 H₂O (l)

The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion.

Pt⁴⁺ (aq) + 6 Cl^? (aq) ? PtCl₆^2? (aq)