Question

Aluminium is more reactive than iron. However, aluminium is less easily corroded than iron because (1) Aluminium is a metal. (2) Iron undergoes reaction easily with water. (3) Oxygen forms a protective layer. (4) Iron forms mono- and divalent ions.

Short answer

Option (3): Oxygen forms a protective layer.

Step-by-step solution

Identify the Core Reason for Aluminium's Corrosion Resistance

The main reason why aluminium is less easily corroded than iron must be identified. Since the properties of metals can involve reactivity with concepts like protective layers, the solution lies in why aluminium retains a protective layer.

Analyze Provided Options

(1) Aluminium is a metal. While true, this property alone does not explain its resistance to corrosion. ewline (2) Iron undergoes reaction easily with water. Although true, this is more about iron's behavior, not aluminium's resistance. ewline (3) Oxygen forms a protective layer. Aluminium forms a layer of aluminium oxide that protects it from further corrosion. ewline (4) Iron forms mono- and divalent ions. This option relates to iron's chemical behavior and not to aluminium’s corrosion resistance.

Select the Correct Reason

From the analysis, the option that correctly explains why aluminium is less easily corroded than iron is (3) Oxygen forms a protective layer. Aluminium reacts with oxygen to form aluminium oxide, which acts as a barrier against further corrosion.

Key concepts

Aluminium Oxide Layer

Aluminium, despite being highly reactive, is well-known for its excellent corrosion resistance. This is largely due to the formation of a protective aluminium oxide layer on its surface. When aluminium is exposed to air, it reacts with oxygen to form a thin, but very effective, layer of aluminium oxide (Al₂O₃). This oxide layer is incredibly stable and adheres strongly to the aluminium surface. It acts as a shield, preventing further interaction between the metal and atmospheric elements.

This protective layer is formed almost instantly and regenerates itself if damaged. The rapid formation and self-healing properties of the aluminium oxide layer are key reasons why aluminium doesn't corrode as easily as some other metals, like iron. This incredible efficiency means that under normal conditions, aluminium maintains its integrity and appearance for a long time.

Reactivity of Metals

The reactivity of a metal refers to its tendency to lose electrons and form positive ions. Metals such as aluminium and iron are highly reactive, meaning they easily participate in chemical reactions. However, the products of these reactions can differ greatly with significant implications.

Aluminium reacts with oxygen in the air to quickly form a stable and protective aluminium oxide layer, as previously discussed. This self-limiting reaction means that once the oxide layer covers the surface, the aluminium underneath is protected from further corrosion.

On the other hand, iron, although less reactive in some respects, does not form such a protective layer. Instead, iron reacts with oxygen and moisture to form rust (iron oxide), which is porous and flakes off. This process exposes more iron to the environment, leading to progressive and extensive corrosion.

Thus, the difference in the end products of these reactions – a stable oxide layer for aluminium versus a flaky rust for iron – explains why aluminium is better protected against corrosion and lasts longer in various environments.

Protective Oxide Layers

Protective oxide layers play a crucial role in enhancing the durability of metals. Some metals react with oxygen to form these beneficial layers, which significantly reduce the rate of further corrosion. Aluminium's ability to develop an aluminium oxide layer is a prime example.

These oxide layers act as barriers, preventing the metal underneath from direct exposure to environmental factors such as air, water, and various chemicals. This is what makes some metals appear less reactive over time. The thickness and stability of these layers determine how effective they are in protecting the metal.

For example, the aluminium oxide layer is both thick enough to block further reactions and strong enough to stay adhered to the metal underneath. It's like a natural shield that constantly protects the metal, even if it's scratched or damaged.

In contrast, iron does not benefit from such a protective mechanism. Iron oxide (rust) does not form a continuous protective layer, leading to ongoing corrosion and material degradation. Thus, understanding the formation and properties of these protective oxide layers helps explain why some metals, like aluminium, offer superior corrosion resistance compared to others, such as iron.