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 HNO_{3} .

Step 1: Understanding the Reaction Context

When aluminium reacts with concentrated nitric acid ( HNO_{3} ), unique reactions occur because nitric acid is a strong oxidizing agent. Instead of a typical displacement reaction, something different happens due to the chemistry of aluminium and the passivation effect of nitric acid.

Step 2: Effect of Concentrated HNO_{3} on Aluminium

Concentrated HNO_{3} causes aluminium to form a protective layer of aluminium oxide on its surface. This layer prevents further reaction with the acid, a phenomenon known as passivation.

Step 3: Analyzing the Passivation Effect

Due to this passivation, no typical reaction products (like liberation of gases such as O_{2} or N_{2} ) are evolved from the reaction. The aluminium is essentially rendered inactive or 'passive' to further reaction by the acid.

Step 4: Identifying the Correct Statement

Given the passivation effect, the correct outcome of the reaction between aluminium and concentrated HNO_{3} is the formation of a passive layer, making option c) 'becomes passive', the accurate choice.

Key Concepts

Passivation and Its Role in Chemical Reactions

Passivation is an essential concept in the field of chemistry, particularly when dealing with reactive metals like aluminium. In simple terms, passivation refers to the process where a material, typically a metal, becomes less susceptible to further reaction after an initial interaction with a specific chemical. This happens because a thin, protective oxide layer forms on the metal's surface. This oxide layer acts as a barrier, preventing the metal underneath from easily reacting with its surroundings. When aluminium is exposed to concentrated nitric acid ( HNO_{3} ), it doesn't undergo a typical metal-acid reaction. Instead, it becomes passive due to this protective oxide layer. The passivation process is vital for preventing further corrosion or reaction, meaning the metal becomes essentially inactive when exposed to the acid. In the case of aluminium, the passivation is significant because it protects the structural integrity of the metal, which is why it's commonly used in environments where corrosion resistance is critical. This phenomenon can be observed in various applications, such as in the production of aluminium cookware or components that endure harsh environmental conditions.

Understanding Oxidizing Agents

Oxidizing agents are substances that can accept electrons from another substance. In this process, they cause the oxidation of that substance while being reduced themselves. Nitric acid ( HNO_{3} ) is a strong oxidizing agent, meaning it has a high capacity to accept electrons. When aluminium interacts with concentrated nitric acid, we might expect a violent reaction, given how reactive aluminium can be. However, due to nitric acid's nature as an oxidizing agent, instead of a fierce reaction, a thin aluminium oxide layer forms, passivating the metal. This is due to the acidic nature and high oxidizing potential of HNO_{3} . The unique nature of oxidizing agents not only changes the expected outcome of reactions but also plays a critical role in industries where controlled oxidations are essential, including metallurgy and chemical manufacturing.

The Formation of Protective Layers

The formation of protective layers on metals is a key process in ensuring material longevity and resistance to degradation. When discussing aluminium's interaction with nitric acid, the formation of a protective aluminium oxide layer is the primary reason why aluminium doesn't readily dissolve in the acid. This thin oxide layer, which results from the initial reaction with nitric acid, is incredibly stable and bonds tightly to the surface of aluminium. This protective layer has several benefits:

• Corrosion Resistance: It prevents further reaction of the metal with other substances that may cause corrosion.
• Enhanced Durability: The protective layer enhances the durability of the metal by shielding it from external environmental factors.
• Environmental Stability: Aluminium coated with an oxide layer is often used in construction and spacecraft due to its ability to resist changes in various environmental conditions.

The formation of such a layer is crucial in many industrial applications, ensuring that materials remain intact and functional over time despite exposure to potentially harmful substances.