Question

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.

Short answer

The balanced chemical equation for the dissolution of gold in aqua regia is: \( Au (s) + 3 HNO_3 (aq) + 4 HCl (aq) \rightarrow AuCl_4^{-} (aq) + 3 NO (g) + 3 H_2O (l) \)

Step-by-step solution

Write the unbalanced equation

First, we will write the unbalanced equation representing the reaction of gold with aqua regia, which contains nitric acid and hydrochloric acid: Au (s) + HNO₃ (aq) + HCl (aq) → AuCl₄⁻ (aq) + NO (g) Next, we need to balance this equation.

Balance the gold atoms

There is already one gold atom on both sides of the equation, so it is already balanced with respect to gold: Au (s) + HNO₃ (aq) + HCl (aq) → AuCl₄⁻ (aq) + NO (g)

Balance the chlorine atoms

There are 4 chlorine atoms in AuCl₄⁻, so we need to put a coefficient of 4 in front of HCl to balance the chlorine atoms: Au (s) + HNO₃ (aq) + 4 HCl (aq) → AuCl₄⁻ (aq) + NO (g)

Balance the nitrogen atoms

There is one nitrogen atom in the reactant side (HNO₃) and one in the product side (NO), so the nitrogen atoms are already balanced: Au (s) + HNO₃ (aq) + 4 HCl (aq) → AuCl₄⁻ (aq) + NO (g)

Balance the hydrogen and oxygen atoms

From the reactants' side, we have one hydrogen atom from HNO₃ and four from the 4 HCl molecules, giving 5 hydrogen atoms in total. There are none in the products side, so we need to add water (H₂O) as a product to balance the H atoms. We add 3 H₂O molecules which contain a total of 6 H atoms and 3 O atoms. Also, to balance the O atoms, HNO₃ should be changed to 3 HNO₃ since it has 3 O atoms with 3 NO: Au (s) + 3 HNO₃ (aq) + 4 HCl (aq) → AuCl₄⁻ (aq) + 3 NO (g) + 3 H₂O (l) Now, we have a balanced chemical equation for the dissolution of gold in aqua regia: Au (s) + 3 HNO₃ (aq) + 4 HCl (aq) → AuCl₄⁻ (aq) + 3 NO (g) + 3 H₂O (l)

Key concepts

Aqua Regia

Aqua regia is a fascinating chemical mixture that is renowned for its ability to dissolve gold, a metal known for its resistance to corrosion and reactions. This potent mixture consists of two strong acids: concentrated nitric acid (HNO₃) and concentrated hydrochloric acid (HCl). What makes aqua regia particularly unique is the combination ratio, where typically three parts of HCl are mixed with one part of HNO₃.
This combination results in a powerful oxidizing solution that produces an array of reactive chemical species that contribute to its reactivity. One of the critical aspects of aqua regia is its ability to generate nitrosyl chloride and chlorine gas, which are crucial in the dissolution process of metals, especially gold.

• Aqua regia is named for its ability to "dissolve" noble metals like gold, with the name translating from Latin to "royal water."
• This mixture is often used in laboratories and industries involving the purification and extraction of precious metals.
• It should always be handled with extreme care due to its corrosive nature and release of toxic gases.

Gold Dissolution

Gold dissolution in aqua regia is a noteworthy chemical reaction because of gold's intrinsic resistance to both acids in their concentrated forms. This resistance is overcome when these acids are mixed, forming aqua regia, which transforms gold into soluble ions.
This chemical process involves multiple reactions where nitric acid acts as a powerful oxidizing agent. It oxidizes gold by converting it into gold ions, typically forming AuCl₄⁻ ions in solution.

• Hydrochloric acid contributes chloride ions (Cl⁻), which stabilize these gold ions, allowing gold to be solubilized.
• The dissolving of gold is usually accompanied by the evolution of nitrogen dioxide (NO₂) and nitrosyl chloride (NOCl), which are products of nitrosyl group reactions.

This ability to dissolve gold makes aqua regia essential in processes like gold refining and recycling, where it plays a critical role in extracting pure metal from its alloys and impurities.

Balancing Equations

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is observed. In the context of gold dissolution in aqua regia, balancing is essential to accurately depict the chemical changes occurring.
When balancing an equation for the dissolution of gold, such as \[ Au (s) + HNO₃ (aq) + 4 HCl (aq) \rightarrow AuCl_4^- (aq) + NO (g) + H_2O (l) \], it is critical to account for all atoms on both sides of the equation.

• Start by counting atoms for each element present in reactants and products.
• Match the number of gold, chlorine, nitrogen, hydrogen, and oxygen atoms on both sides of the equation.
• Adjust coefficients as needed to equilibrate the numbers, maintaining the proportions of reactants and products.

At its heart, balancing ensures that all atoms are accounted for and that the equation reflects the true nature of the chemical reaction. Practice makes perfect, and mastering this skill is key for any aspiring chemist.