Gold metal will not dissolve in either concentrated nitric...

Gold metal will not dissolve in either concentrated nitric acid or concentrated hydrochloric acid. It will dissolve, however, in aqua regia, a mixture of the two concentrated acids. The products of the reaction are the AuCl, $^{-}$ ion and gaseous NO. Write a balanced equation for the dissolution of gold in aqua regia.

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is fundamental in chemistry to ensure that mass and charge are conserved in every reaction. In the context of the dissolution of gold in aqua regia, careful balancing accounts for the atoms of each element and the electrons transferred during the redox process, ensuring that the overall reaction properly reflects the conservation laws.

Complex Ion Formation

After its oxidation, the gold ion (Au3+) readily forms a stable complex with chloride ions present in the solution, resulting in the chloroaurate ion (AuCl4–). This complex ion formation is crucial as it stabilizes the oxidized gold in solution, allowing the reaction to proceed and preventing the gold ion from precipitating out.

Redox Reaction

The dissolution of gold in aqua regia is an example of a redox process. In this reaction, gold is oxidized from its elemental form (0 oxidation state) to a higher oxidation state (Au3+), while the nitric acid is reduced, producing nitric oxide gas. This interplay of oxidation and reduction is key to understanding how relatively inert substances can be transformed chemically.

Aqua Regia

Aqua regia is a potent mixture of concentrated nitric acid and hydrochloric acid. Its unique combination produces both a strong oxidizing agent (from nitric acid) and a source of chloride ions (from hydrochloric acid), enabling the dissolution of noble metals like gold that are otherwise resistant to single acids.

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