Problem 47 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

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

Step 1: Understand Passivation of Iron

Passivation of iron is a process where a protective oxide layer is formed on the surface of the iron, preventing further corrosion. This typically occurs with strong oxidizing agents.

Step 2: Identify Strong Oxidizers

To determine which concentrated acid causes passivation, we need to identify the acids that are strong oxidizers. Among the provided options, concentrated \( \mathrm{HNO}_{3} \)is a strong oxidizing agent.

Step 3: Apply Knowledge of Oxidizing Agents

Concentrated nitric acid \( \mathrm{HNO}_{3} \) is well-known for its ability to oxidize metals and cause passivation by forming a stable oxide layer. This makes concentrated \( \mathrm{HNO}_{3} \) the correct answer here.

Key Concepts

Oxidizing Agents

An oxidizing agent is a substance that can accept electrons from another substance, thus causing oxidation. In simple terms, oxidizing agents facilitate the transfer of electrons during a chemical reaction. They are essential in various industrial and chemical processes.
- **Properties of Oxidizing Agents:** - They gain electrons. - They undergo reduction themselves. - They can be identified through their role in electron transfer.
A common characteristic of strong oxidizing agents is their ability to oxidize other substances in a chemical process. In the context of passivation, strong oxidizing agents can form an oxide layer on metals leading to the phenomenon of passivation. For instance, concentrated nitric acid (\(\mathrm{HNO}_{3}\)) is such an agent because it oxidizes iron to form a protective layer on its surface. This layer prevents further reactions on the iron's surface, safeguarding it against corrosion.

Corrosion Prevention

Corrosion is a natural process that deteriorates metal materials due to environmental interactions. However, corrosion prevention techniques can be employed to prolong the lifespan of metal structures. Passivation is one such technique.
- **Methods of Corrosion Prevention:** - **Passivation:** Involves forming a thin, protective film on metals to shield them from rust. - **Coatings:** Applying paint or other materials to prevent exposure to moisture. - **Cathodic Protection:** Using electrical currents to prevent rusting.
The essence of passivation in corrosion prevention lies in its ability to form a non-reactive oxide layer on the metal's surface. Iron, for example, becomes resistant to ongoing corrosion when treated with certain oxidizing agents like concentrated \(\mathrm{HNO}_{3}\), which forms a passive oxide film. This method is widely used in industries to protect metal surfaces and components.

Iron Chemistry

Iron is a versatile metal with unique chemical properties that make it essential for structures and tools. However, its susceptibility to corrosion calls for a detailed understanding of its chemistry to mitigate this issue.
- **Properties of Iron:** - Pure iron is soft, grayish in appearance, and has a high melting point. - It readily reacts with oxygen in the air to form rust when not protected.
The chemistry of iron is crucial for developing different methods for its protection. The passivation of iron, for example, involves reacting it with oxidizers to form a protective oxide layer. Concentrated \(\mathrm{HNO}_{3}\) is particularly effective at achieving this, as it transforms iron into a state where its surface resists further oxidation and environmental damage.
Understanding iron's reactivity allows metallurgists and engineers to devise protective measures that maximize the metal's utility while minimizing its exposure to corrosive elements.