Gold will not dissolve in either concentrated nitric acid or...

Gold will not dissolve in either concentrated nitric acid or concentrated hydrochloric acid. However, the metal does dissolve in a mixture of the acids (one part $\mathrm{HNO}_{3}$ and three parts $\mathrm{HCl}$ by volume called aqua regia. (a) Write a balanced equation for this reaction. (Hint: Among the products are HAuCl $_{4}$ and $\mathrm{NO}_{2}$.) (b) What is the function of HCl?

Key Concepts

Redox Reactions

Redox reactions involve the transfer of electrons between species. In the dissolution of a noble metal like gold by aqua regia, gold is oxidized from its metallic state while an oxidizing agent (derived from nitric acid) is reduced. This interplay of oxidation and reduction is critical for converting a highly inert metal into soluble ionic species.

Complex Ion Formation

Complex ion formation occurs when metal ions interact with ligands to create coordination complexes. In the case of gold dissolving in aqua regia, chloride ions from hydrochloric acid coordinate with the gold ions to form a stable, soluble complex ion. This process not only stabilizes the oxidized gold but also drives the dissolution reaction by shifting the equilibrium.

Role of Acid Mixtures in Reaction Mechanisms

Acid mixtures like aqua regia combine acids with complementary chemical properties to enable reactions that neither could accomplish alone. Nitric acid provides strong oxidizing power to oxidize gold, while hydrochloric acid supplies chloride ions that form complex ions with the oxidized gold, thereby enhancing its solubility. The synergy between oxidation and complexation is essential to dissolve metals that are otherwise resistant to attack by single acids.

Chemical Equation Balancing

Balancing chemical equations is fundamental in chemistry to ensure conservation of mass and charge. It involves adjusting coefficients to equate the number of atoms of each element on both sides of the reaction. This process is critical for accurately representing the stoichiometry of the reaction, particularly when multiple reactants and products, such as gases and complex ions, are involved.

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