Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Aqua regia is used for dissolving noble metals (Au, Pt, etc.). The gas evolved in this process is: [Main Sep. 03, 2020 (I)] (a) NO (b) \(\mathrm{N}_{2} \mathrm{O}_{5}\) (c) \(\mathrm{N}_{2}\) (d) \(\mathrm{N}_{2} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

The gas evolved in aqua regia is nitric oxide (NO).

Step 1: Understanding Aqua Regia

Aqua regia is a mixture of concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO₃) in a 3:1 ratio. It is used for dissolving noble metals such as gold (Au) and platinum (Pt). In this process, gases are evolved as by-products during the reaction.

Step 2: Chemical Reaction in Aqua Regia

The key reaction in aqua regia involves the oxidation of metals and the reduction of nitric acid. Simplifying these reactions, the interaction of ext{HNO}_3 and ext{HCl} generates nascent chlorine (Cl) and nitrosyl chloride (NOCl). However, the critical step involves the evolution of a nitrogen oxide gas.

Step 3: Identifying the Evolved Gas

By further examining the reactions in aqua regia, the decomposition of nitric acid ( ext{HNO}_3) leads to the formation of nitrogen oxides. Specifically, aqua regia produces ext{NO} gas (nitric oxide) as one of the significant gas products.

Key Concepts

Noble Metals

Noble metals are a group of metals that are highly resistant to corrosion and oxidation, due to their stable electronic structure. This makes them "noble," much like nobles in society who are resistant to change. Common noble metals include gold (Au), platinum (Pt), and silver (Ag). These metals are often found in elemental form in nature, and their intrinsic inertness makes them difficult to dissolve using ordinary acids.
A unique feature of noble metals is their ability to maintain a shiny appearance and their resistance to tarnishing. This property is why they are often used in jewelry, electronics, and high-value industrial applications. To dissolve them, special mixtures like aqua regia are required since normal acids do not work effectively. Understanding the properties of noble metals helps in grasping why certain chemical mixtures like aqua regia are necessary for processes involving these precious metals.

Chemical Reaction

Chemical reactions involve the transformation of substances through breaking and forming chemical bonds. In the context of aqua regia, the chemical reaction encompasses the interaction between hydrochloric acid (HCl) and nitric acid (HNO₃). Aqua regia's effectiveness in dissolving noble metals hinges on its unique chemical reactions.
When the two acids are mixed, they produce nascent (fresh and highly reactive) chlorine and nitrosyl chloride (NOCl). However, the critical part of the reaction involves the reduction of nitric acid, which consequently oxidizes noble metals like gold and platinum. This oxidation-reduction mechanism allows aqua regia to break down these otherwise inert metals.

• The hydrochloric acid provides chloride ions that bind with metal ions, forming soluble complex ions.
• Nitric acid acts as an oxidizing agent that enables metal dissolution by attacking the electrons of the metal atoms.
• This collaboration between the acids results in the metal getting dissolved and the evolution of gases.

By understanding these reactions, one can see how different roles of acids contribute to the power of aqua regia.

Nitric Oxide

Nitric Oxide (NO) is a colorless gas that plays a crucial part in the reactions involved in aqua regia. It is produced during the breakdown of nitric acid (HNO₃), when the acid interacts with noble metals in the mixture. The generation of nitric oxide is key to the dissolution process as it signifies the reaction's progression.
Nitric oxide is part of a group of nitrogen oxides, collectively referred to as NOx. While it occurs naturally in small quantities in the Earth's atmosphere, it can also be released in chemical processes such as combustion and acid reactions. In the specific context of aqua regia:

• The decomposition of nitric acid in the reactive mixture releases nitric oxide.
• It is a major by-product that evolves as a gaseous component when noble metals are dissolved.
• Its presence helps in the ongoing chemical reactions by maintaining the oxidation state necessary for continuous dissolution.

Understanding the role of nitric oxide gives insight into not only the chemical behavior of acids but also the processes involved in industrial applications that utilize aqua regia for refining metals.