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 the process of making a material less reactive by forming a protective oxide layer over its surface. For iron, this protective layer is often formed by reactions with certain chemicals that prevent further corrosion.

Step 2: Identifying the Correct Acid

Concentrated acids that can cause iron to become passive are noted for their ability to form a protective oxide layer. One of the most well-known acids for rendering iron passive due to the formation of a protective oxide layer is concentrated nitric acid, \(\mathrm{HNO}_3\).

Step 3: Checking Other Options

While other concentrated acids, such as \(\mathrm{HCl}\), \(\mathrm{H}_3\mathrm{PO}_4\), and \(\mathrm{H}_2\mathrm{SO}_4\), can react with iron, they do not typically result in passivation by forming a stable protective layer. Instead, these acids usually continue to react with and corrode the iron.

Key Concepts

Protective Oxide Layer

A protective oxide layer acts like a shield on the surface of metals, preventing them from undergoing further reactions. In the context of iron, this layer is essential in protecting it from environmental factors that may cause rust or deterioration.
When iron reacts with specific agents, such as acids, a thin, non-reactive oxide layer forms. This oxide film creates a barrier that stops air and moisture from reacting with the iron beneath it.
The formation of this protective layer is an excellent example of passivation, which helps enhance the durability and lifespan of metal objects.

• The protective oxide layer prevents further chemical reactions.
• It serves as a barrier against corrosion.
• It is critical for maintaining the integrity of iron surfaces.

Concentrated Nitric Acid

Concentrated nitric acid, represented as \(\mathrm{HNO}_3\), is a potent oxidizing agent that plays a significant role in the passivation of iron. When iron is treated with concentrated nitric acid, a chemical reaction occurs, leading to the formation of a thin but stable oxide layer on the iron's surface.
This reaction is unique because, instead of constantly reacting with the metal like other acids might, concentrated nitric acid quickly forms an inert layer, thereby restricting further chemical activity. Thus, iron becomes 'passive' and less prone to further corrosion.
A few key points about concentrated nitric acid include:

• It acts as a strong oxidizer.
• It helps form a protective layer on metals like iron.
• It inhibits further metallic reactions by creating a non-reactive surface.

Reactivity of Acids with Iron

Different acids interact with iron in varied ways, affecting how well they can form protective layers. While concentrated nitric acid is known for making iron passive, other acids do not produce the same effect.
For example, hydrochloric acid ( HCl), phosphoric acid ( H_3PO_4), and sulfuric acid ( H_2SO_4) usually continue to react with iron without forming a stable oxide layer. These reactions can lead to continuous corrosion, as the acids do not transform into a protective film.
Here are some insights on the interaction of these acids with iron:

• HCl, H_3PO_4, and H_2SO_4 do not promote passivation.
• They often lead to further degradation of iron.
• They lack the ability to create an inert, stable surface on iron.

Understanding these reactions helps in selecting the right acid to protect iron components effectively.