Problem 53 - General Chemistry: Principles and Modern Applications

Question

Although Au reacts with and dissolves in aqua regia (3 parts \(\mathrm{HCl}+1\) part \(\mathrm{HNO}_{3}\) ), Ag does not dissolve. What is (are) the likely reason(s) for this difference?

Step-by-Step Solution

Answer

The likely reasons for the difference in reactivity between gold (Au) and silver (Ag) with aqua regia lie in the different reactivities of the metals, the stability of their ions, and the solubility of their chloride complexes. Gold can be oxidized by nitric acid and react with chloride to form a soluble complex, while silver forms insoluble silver chloride that reduces further reaction.

Step 1: Understand the Role of Aqua Regia

Aqua regia, a mixture of hydrochloric acid (\(\mathrm{HCl}\)) and nitric acid (\(\mathrm{HNO}_{3}\), is known for its ability to dissolve gold, a metal that does not react with a single acid alone. Nitric acid is a strong oxidizing agent that can oxidize gold to a stable +3 oxidation state. The chloride ions from the hydrochloric acid then complex with the gold ions, creating soluble complexes that can dissolve in the solution.

Step 2: Consider the Reactivity of Silver

Silver, while lower in the reactivity series compared to many metals, does not readily form a stable oxidation state like gold does. Additionally, silver chloride formed in the reaction with hydrochloric acid is insoluble and forms a protective layer around the silver metal, reducing further reaction.

Step 3: Compare Gold and Silver

The main reason for the difference in reaction between gold and silver in aqua regia lies in their different reactivities, the stability of their ions, and the solubility of their chloride complexes. Gold forms a stable ion that reacts with chloride to form a soluble complex, while silver cannot form a similar soluble complex and is protected from further reaction by the formation of insoluble silver chloride.

Key Concepts

Gold Reactivity

Aqua regia is a unique blend of hydrochloric acid and nitric acid, known for its ability to dissolve gold, a generally inert metal. This mixture is particularly effective for gold because it combines the strengths of its components in a synergistic way. Here's how it works:

• Nitric Acid Role: Nitric acid serves as an oxidizing agent, converting elemental gold (\[\text{Au} \]) into gold ions (\[\text{Au}^{3+}\]) by oxidizing its atoms. This oxidation is crucial because gold itself is very resistant to oxidation when exposed to oxygen or simple acids alone.
• Chloride Ion Importance: Once oxidized, these gold ions interact with chloride ions from the hydrochloric acid to form chloroauric acid. The resulting compound is soluble, which allows the gold to dissolve in the solution.

Gold's reactivity in aqua regia is exceptional due to this dual attack from both acid components. It's a dance between oxidation and complex formation that gold accommodates successfully, unlike many other precious metals.

Silver Reactivity

Silver behaves differently when exposed to aqua regia. While silver is not as reactive as some other metals, its interaction with aqua regia highlights specific interesting characteristics:

• Resistance to Nitric Acid Alone: Silver doesn't undergo significant oxidation with nitric acid alone. It does react to form silver ions, but not to the extent needed for solubility in this context.
• Formation of Silver Chloride: When hydrochloric acid is introduced, it reacts with silver rather than with nitric acid. This reaction leads to the formation of silver chloride (\[\text{AgCl}\]), which is insoluble and forms a barrier over the silver metal, preventing further dissolution.

Due to these factors, silver is protected from the corrosive action of aqua regia, unlike gold. This makes aqua regia a less suitable medium for dissolving silver.

Oxidation State

Understanding oxidation states is crucial in predicting how substances will interact chemically, especially in reactions involving metal dissolution.Oxidation states refer to the hypothetical charge of an atom if all its bonds were ionic. Here's how it applies in these reactions:

• Gold's Oxidation to +3 State: For gold, nitric acid oxidizes the metal. The oxidation results in gold achieving a +3 oxidation state, which is stable. This state is essential for the formation of soluble chloroauric acid (\[\text{HAuCl}_4\]) in aqua regia.
• Silver's Reluctance to Change State: Silver, on the other hand, does not easily attain such a stabilizing higher oxidation state in the presence of nitric acid. This reluctance limits its ability to dissolve readily as the protective \[\text{AgCl}\] forms a shielding layer.

In essence, the oxidation states of these metals play a pivotal role in their reactivity, especially in the context of dissolution in acids like aqua regia. Gold's ability to achieve a reactive state under these conditions enables its solubility, unlike silver, which remains largely unaffected.