Question

Aluminium is more reactive than iron because its standard reduction potential is higher. Still aluminium is less easily corroded 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)

Short answer

The correct option is (b): it has higher reducing power and forms a self-protective layer of \(\mathrm{Al}_{2} \mathrm{O}_{3}\).

Step-by-step solution

Understanding Reactivity

Aluminum (Al) is more reactive than iron (Fe) because its standard reduction potential is higher. This implies that Al has a greater tendency to be oxidized compared to Fe.

Formation of Protective Layer

Aluminum reacts with oxygen in the air to form a layer of aluminum oxide (\(\mathrm{Al}_{2} \mathrm{O}_{3}\)). This oxide layer is very stable and adheres tightly to the surface of the aluminum, protecting the underlying metal from further corrosion.

Assessing the Options

(a) incorrectly claims that aluminum reacts with carbon dioxide instead of oxygen. (b) correctly states that aluminum's higher reactivity leads to the formation of a protective layer of \(\mathrm{Al}_{2} \mathrm{O}_{3}\). (c) incorrectly suggests that aluminum does not react easily with oxygen, which is not true since it quickly forms a protective oxide layer. (d) is incorrect because option (a) is incorrect.

Conclusion

The correct choice is (b) because it accurately describes that aluminum's higher reactivity results in the formation of a protective self-defending layer of aluminum oxide.

Key concepts

Standard Reduction Potential

The standard reduction potential is a measure of the tendency of a substance to gain electrons and be reduced. It is represented as a voltage in electrochemical cells. A higher standard reduction potential indicates a greater ability of the substance to accept electrons.

In the context of metals, aluminum (Al) has a standard reduction potential of -1.66 V, which is higher compared to many other metals including iron (Fe), which is -0.44 V. This means aluminum is more eager to give away its electrons and oxidize.

Therefore, aluminum is considered more reactive than iron based on its standard reduction potential values, highlighting its greater reactivity to form compounds with other elements.

Protective Oxide Layer

When aluminum is exposed to air, it reacts with oxygen to form a thin layer of aluminum oxide ( Al2O3). This layer acts as a shield, preventing further oxidation of the aluminum beneath it.

The formation of this protective oxide layer is crucial because it effectively guards the aluminum metal underneath from further reactions with environmental elements. The layer is tightly adhered to the surface, effectively stopping further exposure to oxygen and moisture.

Due to this stable and adhering characteristic, aluminum exhibits resistance to corrosion despite its high reactivity. Without this oxide layer, aluminum would corrode much like other reactive metals do when exposed to environmental conditions.

Aluminium Oxidation

Oxidation is a chemical reaction that involves the loss of electrons. For aluminum, oxidation happens readily due to its high reactivity and standard reduction potential.

When aluminum comes in contact with oxygen, it undergoes oxidation forming a layer of aluminum oxide ( Al2O3). This process happens very quickly, providing instant protection.

This natural tendency to form an oxide layer upon oxidation is unique to aluminum and is what makes it less prone to corrode like other metals. In technical terms, this transformative reaction acts as aluminum’s self-defense mechanism against various forms of corrosion.

Reactivity of Metals

The reactivity of metals refers to how easily they can undergo chemical reactions, particularly oxidation. Reactivity is heavily influenced by both the metal's electron configuration and its standard reduction potential.

In terms of the periodic table, alkali and alkaline earth metals generally exhibit high reactivity. Aluminum is also quite reactive due to its ability to lose three electrons from its outer shell easily. Its greater tendency to oxidize compared to other metals is demonstrated by its position in the electrochemical series.

Despite its high reactivity, aluminum's ability to quickly form a protective oxide layer sets it apart from other reactive metals like iron, which do not form as effective protective barriers. This distinction ensures its wide usage in environments where exposure to the elements is prevalent.