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 the Concept of Passivation

Passivation is a process by which a metal becomes resistant to corrosion and surface tarnishing. For iron, passivation occurs under certain conditions that create a protective oxide layer on the surface.

Step 2: Identifying the Reactants

The question asks which concentrated acid will render iron passive. The options provided are concentrated hydrochloric acid (\( \mathrm{HCl} \)), nitric acid (\( \mathrm{HNO}_{3} \)), phosphoric acid (\( \mathrm{H}_{3} \mathrm{PO}_{4} \)), and sulfuric acid (\( \mathrm{H}_{2} \mathrm{SO}_{4} \)).

Step 3: Analyzing the Role of Each Acid

\( \mathrm{HCl} \): Chloride ions can often lead to the corrosion of metals rather than passivation. \( \mathrm{H}_{2} \mathrm{SO}_{4} \): Known to corrode iron without making it passive. \( \mathrm{H}_{3} \mathrm{PO}_{4} \): Used as a rust remover, but does not directly passivate. \( \mathrm{HNO}_{3} \): Concentrated \( \mathrm{HNO}_{3} \) forms a protective oxide layer on iron making it passive.

Step 4: Choosing the Correct Answer

Based on the analysis, concentrated nitric acid (\( \mathrm{HNO}_{3} \)) renders iron passive by forming a protective layer, hence it is the correct answer.

Key Concepts

Protective Oxide Layer

A protective oxide layer is a thin film that forms on the surface of certain metals, acting as a shield against environmental factors that cause corrosion. When metals like iron come into contact with specific substances, they can develop this layer. This layer is incredibly thin yet robust, providing a barrier that protects the underlying metal from reacting with water, air, and various chemicals.
The formation of this layer is crucial for enhancing the durability and life span of metals. By preventing further oxidation, it ensures that the metal remains resistant to corrosion. In the context of concentrated nitric acid, iron develops an oxide layer that prevents further degradation.

Corrosion Resistance

Corrosion resistance refers to the ability of a material, especially a metal, to withstand damage caused by oxidizing agents or corrosive environments. Metals have varying degrees of corrosion resistance based on their nature and the environment they are exposed to.
Common strategies to enhance corrosion resistance include the application of coatings and treatments that form a protective barrier. Passivation is a well-known technique where metals are treated to form a non-reactive surface. This process is vital in industries that require long-lasting metal infrastructure.

Acids and Metals Reaction

When metals interact with acids, they tend to undergo a chemical reaction. Acids typically donate protons (H+ ions) which react with metals to produce salts and hydrogen gas. However, the nature and concentration of the acid can significantly affect the interaction.
For instance, dilute acids may cause metal to corrode, whereas concentrated acids can sometimes passivate certain metals by forming a protective layer. For iron, concentrated nitric acid can actually form an oxide layer making the iron resistant to further reaction.

Iron Passivation

Iron passivation occurs when iron is treated with substances that create a protective barrier on its surface. This process makes iron resistant to oxidation and corrosion. Iron, when untreated, is prone to rusting due to its exposure to oxygen and moisture.
To prevent this, passivation is employed, often using specific substances like concentrated nitric acid. This acid helps in forming a layer of iron oxide on the metal's surface, keeping it passive and protected. This process is crucial for maintaining the structural integrity of iron in various applications.

Concentrated Nitric Acid

Concentrated nitric acid is a potent chemical known for its strong oxidizing properties and ability to render metals like iron passive. When iron is introduced to concentrated \(\mathrm{HNO}_{3}\), the acid facilitates the formation of a temporary yet effective protective oxide layer on the iron's surface. This inhibits further reaction with typical oxidizing agents.

This layer formation is what sets concentrated nitric acid apart from other acids which often cause increased corrosion. Its application in creating passive states for metals like iron is crucial in various industrial processes, ensuring the longevity and functionality of metal components.