Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Iron is rendered passive by treatment with concentrated (a) \(\mathrm{HCl}\) (b) \(\mathrm{HNO}_{3}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (d) \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

Iron is rendered passive by treatment with concentrated \\( \mathrm{HNO}_{3} \\\).

Step 1: Understanding Passivation

Passivation is a process where a metal becomes less reactive by forming a thin film that protects it from further reaction. This is often achieved with the help of oxidizing agents.

Step 2: Analyzing the Options

Evaluate each of the given acids to determine which one can cause passivation of iron. Compare their properties to see which is a strong oxidizing agent capable of forming a protective oxide layer on iron.

Step 3: Identifying the Strong Oxidizing Agent

Among the options, concentrated \( \mathrm{HNO}_{3} \) is a strong oxidizing acid. It can cause the oxidation of iron, forming a passive oxide layer on its surface, thereby passivating the iron.

Step 4: Eliminating Incorrect Options

Concentrated \( \mathrm{HCl} \), \( \mathrm{H}_{3} \mathrm{PO}_{4} \), and \( \mathrm{H}_{2} \mathrm{SO}_{4} \) are not known to passivate iron through the formation of a protective layer. Thus, they are not the correct choices for this particular property.

Key Concepts

Oxidizing Agents

Oxidizing agents play a crucial role in the chemistry of metals, especially when discussing passivation. These chemicals can accept electrons in a chemical reaction, effectively oxidizing other substances. When it comes to iron, oxidizing agents are instrumental in creating a protective layer.
A strong oxidizing agent, such as concentrated nitric acid (\(\text{HNO}_3\)), can cause the surface of iron to oxidize. This results in the formation of a thin oxide film that prevents further corrosion or reaction with the environment.

• Concentrated \(\text{HNO}_3\) acts by taking electrons from iron, which leads to the formation of iron oxide.
• This oxide serves as a barrier, safeguarding the underlying metal from air and moisture, and thus minimizing reactivity.

Using oxidizing agents effectively is essential when one needs to protect metals like iron from corrosive elements.

Iron Chemistry

The chemical behavior of iron is fascinating, especially when considering its reactions with various substances. Iron is a metal that can react with numerous agents, leading to different outcomes.
Concentrated nitric acid (\(\text{HNO}_3\)) is particularly interesting in iron chemistry due to its ability to passivate the metal. When iron is exposed to this strong acid, a complex chemical interaction occurs.

• The interaction of iron with the acid oxidizes the iron surface, leading to a transformation.
• As a result, a protective oxide layer forms, which is less reactive and very stable.
• This layer effectively shields iron against further interaction with other potentially harmful substances.

This passivation phenomenon showcases the delicate balance in iron chemistry between reactivity and protection, highlighting why nitric acid is a preferred agent for passivating iron.

Protective Oxide Layer

The formation of a protective oxide layer is key in safeguarding metals from their environment. This layer is a thin film, typically resulting from a chemical reaction between a metal and an oxidizing agent.
In the case of iron, exposure to strong oxidizing agents like \(\text{HNO}_3\) leads to the creation of this protective layer. The oxide layer effectively reduces the metal's reactivity and offers substantial protection against corrosion.

• An iron oxide layer forms when iron reacts with concentrated nitric acid, covering the surface with a hard barrier.
• It prevents scratches, impacts, and penetration by various environmental elements like water and oxygen, which can cause rust.
• This property is crucial in many applications, ensuring durability and longevity of iron-based materials.

Understanding the dynamics of this protective layer can help in designing better materials and protecting existing ones from degradation.