Problem 34 - Chemistry An Atoms First Approach

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 \(\mathrm{AuCl}_{4}^{-}\) ion and gaseous NO. Write a balanced equation for the dissolution of gold in aqua regia.

Step-by-Step Solution

Answer

The balanced equation for the dissolution of gold in aqua regia is: \( Au + 2HNO_3 + 4HCl \rightarrow AuCl_{4}^{-} + 2NO (g) \).

Step 1: Write the unbalanced chemical equation

We know that gold metal will dissolve in a mixture of concentrated nitric acid (HNO3) and concentrated hydrochloric acid (HCl) to form AuCl4- ions and gaseous NO. So, the unbalanced equation can be represented as: \[ Au + HNO_3 + HCl \rightarrow AuCl_{4}^{-} + NO (g) \]

Step 2: Balance the chemical equation

Now, we need to balance the equation. To do this, we will apply the law of conservation of mass which states that the count of individual atoms must be equal on both sides of the equation. 1. Gold (Au): On both sides of the equation, we have one gold atom, so this component is already balanced. 2. Nitrogen (N): On the left side, we have 1 nitrogen atom and on the right side, we have 1 nitrogen atom, so this component is also balanced. 3. Hydrogen (H): On the left side, there's a total of 1+1 = 2 hydrogen atoms, and on the right side, there's a total of 4 hydrogen atoms. To balance the hydrogen, we need to multiply the HCl by 4: \[ Au + HNO_3 + 4HCl \rightarrow AuCl_{4}^{-} + NO (g) \] 4. Oxygen (O): On the left side, we have a total of 3+ (4×0) = 3 oxygen atoms, and on the right side, we have a total of 2 oxygen atoms. We can balance the oxygen atoms by multiplying the HNO3 by 2: \[ Au + 2HNO_3 + 4HCl \rightarrow AuCl_{4}^{-} + 2NO (g) \] 5. Chlorine (Cl): On the left side, we have 4 chlorine atoms, and on the right side, we also have 4 chlorine atoms. So, the chlorine component is balanced. Finally, the balanced chemical equation for the dissolution of gold in aqua regia is: \[ Au + 2HNO_3 + 4HCl \rightarrow AuCl_{4}^{-} + 2NO (g) \]

Key Concepts

Aqua Regia

Aqua regia is a highly corrosive and fuming liquid made by mixing together concentrated nitric acid (HNO₃) and hydrochloric acid (HCl) in a molar ratio typically of 1:3. It is called "regia," meaning kingly or royal, because it is capable of dissolving noble metals such as gold and platinum. Individually, nitric acid and hydrochloric acid will not dissolve gold, but when combined, their chemical interaction generates nascent chlorine and nitrosyl chloride, highly reactive species that attack the metal, leading to its dissolution.

This powerful solvent works effectively due to the presence of both oxidizing agents that break metallic bonds and chloride ions that form stable metal chlorides. If you ever need to identify or refine precious metals, aqua regia can be essential. However, it requires careful handling due to its corrosive nature.

Gold Dissolution

Gold dissolution in aqua regia involves a chemical reaction where gold atoms are transformed into ions. When gold comes into contact with aqua regia, it reacts to form chloroauric acid, creating the \( AuCl_{4}^{-} \) ion, while producing nitrogen monoxide gas (NO) as a byproduct. This reaction demonstrates the ability of aqua regia to dissolve otherwise inert metals, primarily due to its generation of reactive species within the mixture.

Gold dissolution is essential in various applications such as metallurgy and the purification of gold. The key to this process is the role of chloride ions, which stabilize the gold ions after its bonds are broken, allowing the gold to become a soluble ion in the solution.

Law of Conservation of Mass

The law of conservation of mass is a fundamental principle in chemistry stating that mass is neither created nor destroyed during a chemical reaction. Instead, the amount of matter stays constant, and this principle is crucial when balancing chemical equations.

In the context of aqua regia dissolving gold, the equation must account for all atoms on both sides. For example, we include all nitrogen, hydrogen, oxygen, and chlorine atoms consistently, preserving the law by ensuring the total count of each individual element's atoms is the same before and after the reaction.

• It helps chemists predict the outcome of reactions.
• Ensures that all reactants and products are accounted in a reaction.

This law is essential not only academically but also practically, ensuring safe chemical experiments and accurate production yields.

Chemical Reactions

Chemical reactions involve rearrangements of atoms resulting in the formation of new substances. When substances like gold and aqua regia interact, their atoms form new chemical bonds, leading to the creation of products such as \( AuCl_{4}^{-} \) and NO gas.

These reactions underline many processes in chemistry, from industrial synthesis to biological metabolism. For gold dissolution, the reaction shows the transformation of solid gold into its ionic form within a solution.

• Chemical reactions often involve changes in energy.
• They can result in physical evident changes like color shifts or gas production.

Understanding chemical reactions is pivotal for predicting behavior and controlling outcomes in scientific work.