Question

Although aluminum is a reactive metal, pure aluminum ordinarily does not corrode severely in air because a protective layer of __________ builds up on the metal's surface.

Short answer

The protective layer that builds up on the metal's surface is aluminum oxide (Al2O3).

Step-by-step solution

Understanding Corrosion and Aluminum Reactivity

When a metal corrodes, it is undergoing a chemical reaction with its environment, which can lead to the deterioration of the metal's physical properties. Aluminum is a reactive metal, meaning it can easily react with other elements, especially oxygen in the air.

Aluminum's Reaction with Oxygen in the Air

When aluminum comes into contact with oxygen, it reacts to form aluminum oxide (Al2O3). This reaction creates a thin layer of aluminum oxide on the surface of the aluminum.

Formation of the Protective Layer

The aluminum oxide that forms on the aluminum surface creates a thin, dense, and adherent layer, which acts as a barrier between the bulk aluminum and its environment. The aluminum oxide layer is stable and protects the aluminum from further corrosion.

Identifying the Protective Layer

Since we know that the protective layer is formed by the reaction between aluminum and oxygen, we can conclude that the protective layer that builds up on the pure aluminum surface is aluminum oxide (Al2O3). In conclusion, the blank in the exercise should be filled with "aluminum oxide" or "Al2O3".

Key concepts

Understanding Corrosion

Corrosion is a natural process that deteriorates metals over time due to their reaction with environmental elements, such as air or water. When metals corrode, they lose their structural integrity and strength. For example, rust is a common result of corrosion in iron, leading to the metal wearing away. Aluminum, however, tends not to corrode as severely as other metals due to its special properties. It's important to note that while most metals can corrode under the right conditions, the rate and severity of corrosion vary greatly.

• Corrosion can be accelerated by moisture and chemicals.
• Preventing corrosion is critical in various industries, like construction and transportation.
• Aluminum benefits from a natural defense against corrosion that many other metals lack.

Aluminum Oxide

Aluminum oxide, denoted as Al\(_2\)O\(_3\), plays a crucial role in protecting aluminum from corrosion. This compound forms when aluminum reacts with oxygen present in the air. The layer of aluminum oxide that results from this reaction is thin yet very effective.
Unlike other chemical reactions that might weaken or degrade the surface, the formation of aluminum oxide actually strengthens it.
Al\(_2\)O\(_3\) is recognized for its:

• High durability and hardness.
• Resistance to scratches and wears.
• Ability to form rapidly on the surface of aluminum, sealing it from further exposure.

This quick formation of aluminum oxide is what sets aluminum apart, as it quickly becomes self-protective in a natural way.

Reactivity of Aluminum

Aluminum is classified as a reactive metal, meaning it readily forms bonds with other elements, particularly oxygen. Despite being reactive, aluminum doesn’t rust like iron because of its rapid reaction with oxygen, producing aluminum oxide. This contradicts the expectation that a highly reactive metal would be vulnerable to corrosion.
The reactivity of aluminum ensures it quickly combines with oxygen to create the protective aluminum oxide layer, halting further reactions.
Key aspects of aluminum’s reactivity include:

• Its tendency to attract and hold onto oxygen atoms.
• Formation of an immediate protective coating upon exposure to air.
• Ensuring longevity and usability in environments where other reactive metals would corrode.

Function of the Protective Layer

The protective layer of aluminum oxide is essential to aluminum’s longevity and usefulness. By covering the metal surface, it acts as a barrier against further exposure to environmental elements and pollutants. This barrier function is highly effective, allowing aluminum to maintain its integrity and appearance over time.

• Protects the metal from air and moisture.
• Prevents further corrosion and chemical reactions.
• Contributes to aluminum's suitability for outdoor and industrial uses.

This natural protective layer minimizes the need for additional coatings or treatments, making aluminum a cost-effective material in many applications.

The Oxidation Reaction of Aluminum

The process where aluminum reacts with oxygen to form aluminum oxide is an oxidation reaction. Oxidation reactions involve the loss of electrons by a metal as it forms compounds with nonmetals, like oxygen. In aluminum’s case, this reaction results in the stable Al\(_2\)O\(_3\) layer.

• Occurs rapidly when aluminum is exposed to air.
• Produces energy in the form of heat but it’s usually not noticeable.
• Stops further oxidation by sealing off the metal surface with the aluminum oxide layer.

Understanding oxidation is key to grasping why aluminum is unique among reactive metals, as it transforms this potential vulnerability into a protective feature.