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

The correct answer is (c) aluminium becomes passive.

Step 1: Identifying Reaction Characteristics

Aluminium is known to react with concentrated nitric acid. This reaction is unique because concentrated \( ext{HNO}_3\) often causes metals like aluminium to become passive. The passivation occurs due to the formation of a protective oxide layer on the metal surface, preventing further reaction. Therefore, understanding this characteristic of \( ext{HNO}_3\) with certain metals is crucial in predicting the outcome of this reaction.

Step 2: Determining the Effects of Passivation

Passivation in this reaction means that a protective oxide layer forms on the aluminium, which stops the metal from further reacting with nitric acid. The protective layer is an oxidized form of aluminium, typically \( ext{Al}_2 ext{O}_3\), which is why no further gases like \( ext{O}_2\) or \( ext{N}_2\) are liberated.

Step 3: Concluding the Reaction Outcome

Based on the nature of the reaction between aluminium and concentrated \( ext{HNO}_3\), it can be concluded that aluminium becomes passive due to this process. The protective layer prevents any liberation of gases or further reaction.

Key Concepts

Aluminium Reactions Explained

Aluminium, a silvery-white metal, has unique reactions with certain chemicals due to its reactive nature. When aluminium reacts, it often forms a thin layer of aluminium oxide on its surface, which can protect the metal underneath from further reaction. This is why aluminium is a popular choice for many everyday products. In particular, its interaction with concentrated acids can be especially interesting.

• Aluminium naturally forms a protective oxide layer, giving it resistance to corrosion.
• This oxide layer is dense and prevents many chemicals from reaching the metal beneath.
• This characteristic is crucial when aluminium is exposed to strong acids.

However, when concentrated nitric acid is involved, the reaction takes an unexpected turn. Aluminium is still protected by its oxide layer, which means the acid cannot penetrate to react completely with the metal underneath. The end result is a process where the protective oxide layer stays intact, shielding the aluminium from further damage or reaction.

The Role of Concentrated Nitric Acid

Concentrated nitric acid ( ext{HNO}_3) is a powerful oxidizing agent. This means it has the ability to accept electrons and promote oxidation reactions. However, its reaction with certain metals, like aluminium, is quite fascinating. Despite being a strong acid, it doesn't always produce a typical acid-metal reaction.

• When aluminium comes into contact with concentrated ext{HNO}_3, a phenomenon called passivation occurs.
• Instead of reacting vigorously, the metal becomes coated with an oxide layer, stopping further reaction.
• This odd behavior results in no liberation of gases like oxygen ( ext{O}_2) or nitrogen ( ext{N}_2) which are typically expected in such reactions.

This effect is unique to concentrated nitric acid's ability to immediately form a protective layer on the aluminium surface. The chemical nature of ext{HNO}_3 thus prevents it from breaking down or interacting further with the aluminium metal, effectively freezing the reaction in its tracks.

Understanding the Formation of Oxide Layers

The formation of an oxide layer on aluminium is a classic example of passivation. This process plays a central role in the reaction of aluminium with concentrated nitric acid. The layer that forms is composed primarily of aluminium oxide ( ext{Al}_2 ext{O}_3), a compound that's particularly effective at protecting the underlying metal.

• The aluminium oxide layer acts as a physical barrier against further chemical attacks.
• This barrier is not only chemically resistant but also provides a measure of physical durability to the metal surface.
• Such a reaction is crucial for the protective properties that make aluminium useful in various industrial and everyday applications.

This aspect of aluminium is incredibly important in materials science and helps explain why the metal remains untarnished in many environments. The formation of this stable oxide layer underpins the concept of passivation, ensuring that no gas, either oxygen or nitrogen, is released during the reaction. This keeps the metal surface stable and non-reactive, ensuring longevity and durability.