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 answer is (b).

Step 1: Introduction

We are asked to determine the reason why aluminum is less easily corroded than iron, despite being more reactive. The options given mention aluminum forming a protective layer.

Step 1: Understanding Reactivity

Aluminum is more reactive than iron because it has a higher standard reduction potential. This means that aluminum more readily loses electrons compared to iron.

Step 2: Formation of a Protective Layer

Aluminum reacts with atmospheric oxygen to form aluminum oxide (\(\mathrm{Al}_2\mathrm{O}_3\)) on its surface. This oxide layer is extremely thin yet very effective at protecting the aluminum beneath from further corrosion.

Step 3: Evaluating Options

Option (a) states that aluminum reacts with carbon dioxide, but aluminum primarily reacts with oxygen, not carbon dioxide. Option (b) correctly states that aluminum has higher reducing power and forms a protective aluminum oxide layer, which prevents further reaction with oxygen.

Step 5: Conclusion: Correct Option

Considering the steps above, option (b) correctly explains that aluminum uses its higher reducing power to form a protective layer of aluminum oxide which prevents further corrosion.

Key Concepts

Standard Reduction Potential

In the context of metals and corrosion, standard reduction potential is a measure of how easily an element gains electrons. It's a crucial concept when discussing reactivity. The higher an element's standard reduction potential, the more likely it is to be reduced, or gain electrons, during a chemical reaction.

For metals like aluminum and iron, standard reduction potential helps to explain their reactivity. Aluminum has a higher standard reduction potential than iron, meaning it loses electrons more readily in reactions. This makes aluminum more reactive overall compared to iron.

However, reactivity in terms of losing electrons does not necessarily correlate directly to the likelihood of corrosion. This illustrates that while aluminum is highly reactive, its corrosion resistance is a different story, owing to other factors we're about to explore.

Aluminum Oxide Layer

One key reason aluminum resists corrosion despite its high reactivity is the formation of an aluminum oxide layer. When exposed to the air, aluminum reacts quickly with oxygen, but not in a harmful way. Instead, it forms a thin, yet highly durable layer of aluminum oxide (\( \mathrm{Al}_2\mathrm{O}_3 \)).

This protective layer is incredibly effective at preventing further oxidation because it adheres tightly to the surface of the metal. Unlike rust on iron, which can flake off and expose fresh metal underneath, the aluminum oxide remains intact, safeguarding the underlying metal from additional exposure to corrosive elements.

This oxide layer acts as a barrier, making the material appear less reactive to environmental conditions. It essentially "seals" the metal, preventing air and moisture from inducing further reactions.

Reactivity of Metals

Metals vary widely in their reactivity. This essentially dictates how they interact with various elements and compounds. Highly reactive metals, like aluminum, engage in swift chemical changes under normal conditions, such as exposure to air or water. Interestingly, while this often leads to corrosion in other metals, aluminum's unique feature saves it.

The reactivity series of metals is a pager that lists elements in order of their ability to lose electrons easily. Aluminum ranks above iron in this series, signifying that it is more eager to participate in oxidation-reduction reactions.

Despite its high rank in reactivity, the same does not directly predict the rate of corrosion. Aluminum's quick formation of the protective aluminum oxide layer distinguishes it from other metals, as other reactive metals do not form such protective layers as naturally or swiftly.

• Reactivity depends on standard reduction potentials.
• Protective oxide layers can curb the corrosion process.
• Each metal's unique properties affect how quickly it might corrode.