Problem 96 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Concentrated \\(\mathrm{HNO}_{3}\\) passivates iron by forming a protective oxide layer.

Step 1: Identifying Passivation

Passivation involves making a metal less reactive or inhibiting its corrosion. In this context, we need to determine which acid renders iron passive by forming a protective oxide layer on its surface.

Step 2: Understanding Iron's Reaction with Concentrated Acids

Concentrated acids react differently with iron. For iron passivation, the acid should form a thin oxide layer rather than extensively corrode the metal.

Step 3: Evaluating Each Acid

Concentrated \(\mathrm{HCl}\) and \(\mathrm{H}_{2} \mathrm{SO}_{4}\) do not effectively render iron passive as they tend to corrode or etch the surface. \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is not known for preventing iron corrosion. However, concentrated \(\mathrm{HNO}_{3}\) forms a thin protective oxide layer on iron, making it passive.

Step 4: Selection of Correct Option

Following the evaluation, \(\mathrm{HNO}_{3}\) is the acid that effectively passivates iron by forming a protective layer.

Key Concepts

Iron Corrosion

Iron is a widely used metal, but it has a natural tendency to corrode when exposed to certain environmental conditions. Corrosion occurs because iron reacts with oxygen and moisture in the air, forming iron oxide, commonly known as rust. This is a slow, progressive process that deteriorates the iron over time. To control corrosion, it's important to understand what causes it and how it can be prevented or mitigated.

• Corrosive Agents: The main culprits responsible for initiating iron corrosion include water, oxygen, acids, and certain salts.
• Consequences of Corrosion: The deterioration due to corrosion affects the mechanical strength and structural integrity of iron-based materials.

Thus, preventing corrosion is essential to prolong the lifespan of iron and maintain its functionality.

Protective Oxide Layer

The formation of a protective oxide layer on the surface of iron is a key concept in preventing iron corrosion. This layer acts as a barrier, shielding the underlying iron from further exposure to corrosive elements.

• Passivation: Passivation is a process through which metals like iron become less chemically reactive after an oxide layer forms.
• Nitric Acid (HNO3): Among various concentrated acids, concentrated nitric acid (HNO3) is effective in forming a passive protective oxide layer on iron. Instead of a corrosive attack, this process leads to the creation of a thin, inert layer.
• Mechanism: The oxide layer formed is stable and non-reactive, preventing contact of the iron surface with moisture and oxygen.

This layer plays a critical preventative role, reducing the rate of corrosion significantly.

Concentrated Acids

Concentrated acids have distinctive effects on metals, and their interactions can lead to different outcomes for the surface of the iron. Understanding these reactions helps us select the right acid to induce passivation.

• Hydrochloric Acid (HCl): When concentrated hydrochloric acid is applied to iron, it tends to corrode rather than protect the surface.
• Sulfuric Acid (H2SO4): Like hydrochloric acid, concentrated sulfuric acid also leads to corrosion and does not aid in forming a protective layer.
• Phosphoric Acid (H3PO4): Though it finds various applications, phosphoric acid is not particularly effective in passivating iron.
• Nitric Acid (HNO3): As noted, concentrated nitric acid is unique for its ability to oxidize and thereby passivate the iron surface via a protective layer formation.

This selection and understanding of reactions highlight the importance of choosing the right acid for achieving desired passivation results.