Problem 73 - A Complete Resource Book in Chemistry for JEE Main

Question

Aluminium reacts with concentrated \(\mathrm{HNO}_{3}\) and (a) liberates \(\mathrm{O}_{2}\) (b) liberates \(\mathrm{N}_{2}\) (c) becomes passive (d) forms AIN

Step-by-Step Solution

Answer

Aluminium becomes passive when reacting with concentrated \(\mathrm{HNO}_3\).

Step 1: Identify Reaction Type

Aluminium is a metal which reacts with acids, but when it comes to reacting with concentrated \(\mathrm{HNO}_3\), a concentrated oxidizing acid, this reaction has special properties. Highly concentrated \(\mathrm{HNO}_3\) is known for making some metals passivate, which means it forms a thin layer of inert oxide preventing further reaction.

Step 2: Understanding Metal Passivation

The passivation process occurs due to the formation of an oxide layer on aluminium's surface. For aluminium, this layer is generally composed of aluminum oxide (\(\mathrm{Al}_2\mathrm{O}_3\)), which acts as a protective coating, stopping further reactions with the acid and making the metal passive.

Step 3: Conclusion on Reaction Outcome

Given aluminium's reaction with concentrated \(\mathrm{HNO}_3\), it becomes passive due to this oxide layer formation. The metal ceases to react actively with the acid, as any further chemical interaction is inhibited by this protective layer.

Key Concepts

Metal Passivation

When certain metals, like aluminium, come in contact with strong acids such as concentrated nitric acid (\(\mathrm{HNO}_{3}\)), something intriguing happens. Instead of continuing to react aggressively, they enter a state called passivation.
This occurs because the metal surface starts to form a protective layer.
Think of it like a shield protecting the metal from further attack by exterior chemicals.

• Passivation essentially means the metal becomes unreactive or passive.
• This process helps prevent wear and tear, ensuring the metal remains intact.

For many metals, including aluminium, the layer that forms is usually an oxide-based layer.
Passivation acts like a natural defense mechanism, keeping the metal safe from corrosion and other destructive processes.

Oxide Layer Formation

The magical transformation that helps aluminium to passivate involves the formation of an oxide layer.
This layer typically consists of compounds like aluminum oxide (\(\mathrm{Al}_2\mathrm{O}_3\)) when reacting with concentrated nitric acid.
Aluminum oxide is tough and doesn't break down easily in many conditions.

• The oxide layer acts as a barrier, preventing further reactions with the environment.
• It stops aluminum from reacting with acids and other corrosive elements.

Due to this protective layer, aluminium can withstand harsh environments that would otherwise cause rapid degradation.
Think of this oxide shell as a protective armor that guards the metal underneath, keeping it in pristine condition.

Aluminum Oxide

Aluminum oxide (\(\mathrm{Al}_2\mathrm{O}_3\)) is a crucial player in the passivation game. What's fascinating about this compound is its ability to form quickly and adapt to the surrounding conditions.
When aluminium reacts with oxygen or specific acids like concentrated nitric acid, this sturdy compound emerges.

• Aluminum oxide is highly resistant to further chemical attacks.
• Its formation is rapid when exposed to air or certain chemicals.
• It ensures that the underlying aluminium metal remains untouched by corrosive forces.

This oxide not only shields the metal but also enhances its durability.
By naturally occurring on the surface, aluminum oxide creates a stable and enduring layer, vital for keeping aluminium intact during its numerous applications.