Question

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 \(\mathrm{HAuCl}_{4}\) and \(\mathrm{NO}_{2} .\) ) (b) What is the function of \(\mathrm{HCl} ?\)

Short answer

(a) \(\text{Au} + 4\text{HCl} + \text{HNO}_3 \rightarrow \text{HAuCl}_4 + \text{NO}_2 + 2\text{H}_2\text{O}\). (b) \(\text{HCl}\) provides \(\text{Cl}^-\) ions for complex formation.

Step-by-step solution

Determine Reactants and Products

In the reaction involving aqua regia, solid gold (\(\text{Au}\)) is reacting with the mixture of hydrochloric acid (\(\text{HCl}\)) and nitric acid (\(\text{HNO}_3\)). The products given are \(\text{HAuCl}_4\) and \(\text{NO}_2\).

Write the Unbalanced Equation

Start by writing the unbalanced reaction: \[\text{Au} + \text{HCl} + \text{HNO}_3 \rightarrow \text{HAuCl}_4 + \text{NO}_2 + \text{H}_2\text{O}\]

Balance the Equation

To balance the equation, ensure the number of atoms of each element is equal on both sides. The balanced equation is: \[\text{Au} + 4\text{HCl} + \text{HNO}_3 \rightarrow \text{HAuCl}_4 + \text{NO}_2 + 2\text{H}_2\text{O}\]

Describe the Function of HCl

In this reaction, \(\text{HCl}\) provides chloride ions (\(\text{Cl}^-\)) that facilitate the formation of the chloroauric acid complex \(\text{HAuCl}_4\). This complex formation is crucial for dissolving gold.

Key concepts

Gold Dissolution

Gold is a very stable metal and typically does not react with individual acids such as nitric acid (HNO₃) or hydrochloric acid (HCl) alone. However, when these two acids are combined in specific proportions to form aqua regia, they can dissolve gold. Aqua regia is a potent combination that can break down the robust atomic structure of gold.

This reaction is unique because it involves both oxidation and coordination processes. The nitric acid serves as an oxidizing agent, helping to oxidize the surface of gold. This is where the gold atoms are converted to gold ions. Meanwhile, hydrochloric acid provides chloride ions, which are critical for forming a stable complex with these gold ions. This process effectively dissolves gold into the solution. The interaction of these processes results in the dissolution of gold, enabling it to convert into a soluble form.

Chemical Reaction

A chemical reaction describes how substances interact to form new substances. In the case of gold dissolution by aqua regia, a chemical reaction occurs between the acids and gold. During this interaction, the gold transitions from a metallic state to a dissolved ionic state.

The reaction involves the transformation of reactants into products. Initially, solid gold interacts with the nitric and hydrochloric acids. Over time, these reactants undergo chemical changes that yield products such as chloroauric acid (HAuCl₄) and nitrogen dioxide (NO₂), a gas. This reaction exemplifies a real-world application of chemical principles, illustrating how even resistant metals like gold can participate in a chemical reaction when suitable conditions are met.

• Gold is oxidized by nitric acid.
• Chloride ions from hydrochloric acid stabilize the gold ions.
• Products formed are chloroauric acid and nitrogen dioxide.

Balanced Equation

Balancing a chemical equation is essential to represent a chemical reaction accurately. It ensures that the number of atoms for each element is conserved throughout the reaction. For the reaction of gold with aqua regia, it starts with gold and the acids and produces chloroauric acid, nitrogen dioxide, and water:

\[\text{Au} + 4\text{HCl} + \text{HNO}_3 \rightarrow \text{HAuCl}_4 + \text{NO}_2 + 2\text{H}_2\text{O}\]

In this balanced equation, the reactants and products showcase that the number of gold, chloride, nitrogen, oxygen, and hydrogen atoms is the same on both sides. Balancing the equation helps understand how much of each reactant is needed and what quantities of products will form. This ensures that the law of conservation of mass is respected.

Chloroauric Acid

Chloroauric acid (HAuCl₄), also known as gold(III) chloride acid, is a vital product formed in the aqua regia reaction. This compound is a crucial intermediate in the process of dissolving gold.

The formation of chloroauric acid occurs as gold atoms interact with chloride ions in the presence of oxidizing nitric acid. Gold, upon losing electrons due to the action of nitric acid, combines with chloride ions provided by hydrochloric acid to form this complex salt.

• Chloroauric acid is soluble, which enables the gold to be in a dissolved state.
• It makes gold accessible for further chemical processes or refinement.
• The resulting acid is used in gold recovery, refining, and processing.

Chloroauric acid plays a significant role in various industrial and laboratory applications, including its use for gold plating and as a catalyst in various chemical processes.