Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

Aluminium is more reactive than iron because its standard reduction potential is higher. Still aluminium is less easily corrdoed than iron because (a) Al reacts with atmospheric carbon dioxide to form a self protective layer of \(\mathrm{Al}_{2} \mathrm{O}_{3}\) (b) it has higher reducing power and forms a self protective layer of \(\mathrm{Al}_{2} \mathrm{O}_{3}\) (c) it has higher reducing power and does not react with oxygen so easily (d) both (a) and (b)

Step-by-Step Solution

Answer

The correct answer is (d): both (a) and (b).

Step 1: Understanding Aluminium's Reactivity

Aluminium is more reactive than iron because it has a higher standard reduction potential, indicating that it is more likely to lose electrons and react in chemical processes.

Step 2: Examination of Aluminium's Corrosion Resistance

Even though aluminium is highly reactive, it is less prone to corrosion. This is because aluminium can react with oxygen in the air to form a thin, protective layer of \(Al_{2}O_{3}\), also known as aluminium oxide.

Step 3: Comparing Options to Identify Correct Statements

We need to determine which statements accurately describe why aluminium is less easily corroded than iron. Option (a) mentions the \(Al_{2}O_{3}\) protective layer formation, while option (b) highlights aluminium's reducing power and the formation of this layer. Since aluminium's reducing power and the formation of the \(Al_{2}O_{3}\) layer make it corrosion resistant, both (a) and (b) are correct.

Step 4: Selecting the Right Answer

Since both (a) and (b) properly describe the formation of the protective aluminium oxide layer that prevents corrosion, the answer aligns with option (d), which acknowledges both points.

Key Concepts

Standard Reduction Potential

In chemistry, the standard reduction potential is a measure of how easily an element can gain electrons during a chemical reaction. It is denoted by the symbol \(E^{\circ}\) and is usually measured in volts. Elements with a higher standard reduction potential are more likely to gain electrons, meaning they are better oxidizing agents.
This concept is key to understanding why aluminium is more reactive than iron. Despite having a higher standard reduction potential, aluminium tends to lose electrons readily, which makes it more reactive.
Here's how it works:

• Standard reduction potentials are used to predict the direction of redox reactions.
• A higher \(E^{\circ}\) value indicates a greater tendency to accept electrons.
• Aluminium's high reactivity is due to its propensity to lose electrons even though it has a more positive reduction potential compared to iron.

Thus, while aluminium is indeed more reactive, its high standard reduction potential enhances its ability to form protective layers that resist corrosion.

Corrosion Resistance

Corrosion is the gradual destruction of materials (usually metals) due to chemical reactions with the environment. Metals like iron are highly susceptible to corrosion, especially when exposed to moisture and oxygen, resulting in rust formation.
On the other hand, aluminium has excellent corrosion resistance even though it is highly reactive. This characteristic can be attributed to its ability to form a protective layer of aluminium oxide (\(\mathrm{Al}_2\mathrm{O}_3\)) that shields the underlying metal from further reaction.
This process is also known as passivation, where a naturally occurring thin protective layer forms on the surface of the metal.

• The formation of the \(\mathrm{Al}_2\mathrm{O}_3\) layer happens almost instantly when aluminium is exposed to oxygen.
• This layer adheres strongly to the metal surface, preventing further oxidation and corrosion.

With this self-protecting mechanism, aluminium can withstand harsh environments, thus offering enhanced durability over other metals like iron.

Aluminium Oxide Layer

The aluminium oxide layer plays a crucial role in the corrosion resistance of aluminium. This layer is essentially a thin, dense coat of \(\mathrm{Al}_2\mathrm{O}_3\) that forms when aluminium comes into contact with air. Despite being only a few nanometers thick, this layer is highly effective at preventing further oxidation of the aluminium underneath.
Let's explore why the aluminium oxide layer is so important:

• The layer is impermeable, blocking oxygen and water from reaching the metal surface.
• It adheres strongly to the aluminium, making it difficult to remove and thus maintaining protection.
• This oxide layer is stable and does not crack easily, which is why aluminium can resist corrosion over prolonged periods.

This auto-protective feature makes aluminium a favorable choice for many industrial applications, especially where longevity and reliability are crucial.