Question

Corrosion of metals costs us billions of dollars annually, slowly destroying cars, bridges, and buildings. Corrosion of a metal involves the oxidation of the metal by the oxygen in the air, typically in the presence of moisture. Write a balanced equation for the reaction of each of the following metals with \(\mathrm{O}_{2}: \mathrm{Zn}, \mathrm{Al}, \mathrm{Fe}, \mathrm{Cr},\) and \(\mathrm{Ni}\).

Short answer

The balanced equations for the reaction of each metal with oxygen are as follows: 1. Zn: \(2 \mathrm{Zn} + \mathrm{O}_{2} \rightarrow 2 \mathrm{ZnO}\) 2. Al: \(4 \mathrm{Al} + 3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Al}_{2}\mathrm{O}_{3}\) 3. Fe: \(4 \mathrm{Fe} + 3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Fe}_{2}\mathrm{O}_{3}\) 4. Cr: \(4 \mathrm{Cr} + 3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Cr}_{2}\mathrm{O}_{3}\) 5. Ni: \(2 \mathrm{Ni} + \mathrm{O}_{2} \rightarrow 2 \mathrm{NiO}\)

Step-by-step solution

1. Reaction of zinc (Zn) with oxygen (O2)

Zinc has an oxidation state of +2 and oxygen has an oxidation state of -2. When zinc reacts with oxygen, zinc oxide (ZnO) is formed. The balanced equation for this reaction is: \[2 \mathrm{Zn} + \mathrm{O}_{2} \rightarrow 2 \mathrm{ZnO}\]

2. Reaction of aluminum (Al) with oxygen (O2)

Aluminum has an oxidation state of +3, and oxygen has an oxidation state of -2. When aluminum reacts with oxygen, aluminum oxide (Al2O3) is formed. The balanced equation for this reaction is: \[4 \mathrm{Al} + 3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Al}_{2}\mathrm{O}_{3}\]

3. Reaction of iron (Fe) with oxygen (O2)

Iron can have oxidation states of +2 or +3, and oxygen has an oxidation state of -2. We'll focus on the reaction with iron(III) oxide (Fe2O3) as the product, since it's a more common product of corrosion. The balanced equation for this reaction is: \[4 \mathrm{Fe} + 3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Fe}_{2}\mathrm{O}_{3}\]

4. Reaction of chromium (Cr) with oxygen (O2)

Chromium has an oxidation state of +6, and oxygen has an oxidation state of -2. When chromium reacts with oxygen, chromium oxide (Cr2O3) is formed. The balanced equation for this reaction is: \[4 \mathrm{Cr} + 3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Cr}_{2}\mathrm{O}_{3}\]

5. Reaction of nickel (Ni) with oxygen (O2)

Nickel has an oxidation state of +2, and oxygen has an oxidation state of -2. When nickel reacts with oxygen, nickel oxide (NiO) is formed. The balanced equation for this reaction is: \[2 \mathrm{Ni} + \mathrm{O}_{2} \rightarrow 2 \mathrm{NiO}\]

Key concepts

Oxidation Reactions

Oxidation reactions are essential chemical processes where a substance loses electrons. For metals, these reactions are crucial because they are prone to losing electrons when they come in contact with oxygen. This is fundamental in the concept of corrosion. When oxygen interacts with metals like iron, zinc, or aluminum, they undergo an oxidation reaction. This reaction involves metal atoms losing electrons and oxygen molecules gaining those electrons. As a result, metal oxides are formed.

For example, when zinc (\( \mathrm{Zn} \)) reacts with oxygen (\( \mathrm{O}_{2} \)), zinc oxide (\( \mathrm{ZnO} \)) is produced:

• Zinc atoms lose electrons (get oxidized)
• Oxygen molecules gain electrons (get reduced)

This exchange of electrons is characteristic of oxidation reactions, and understanding it is crucial to grasping how corrosion occurs.

Balanced Chemical Equations

A balanced chemical equation represents a chemical reaction with equal numbers of each type of atom on both sides of the equation. This balance is necessary because, according to the law of conservation of mass, atoms are neither created nor destroyed during a chemical reaction.

To balance equations, we often adjust the coefficients, which are numbers placed before the compounds or elements in the reaction. For example, the balanced equation for the reaction of aluminum with oxygen is:

• \[ 4 \mathrm{Al} + 3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Al}_{2}\mathrm{O}_{3} \]
• This equation indicates that 4 aluminum atoms react with 3 oxygen molecules to form 2 units of aluminum oxide.

Balancing equations helps us understand the proportion of reactants used and the amount of products formed, which is vital in practical applications like preventing excessive corrosion.

Corrosion Prevention

Preventing corrosion is crucial because it can lead to significant structural damage, which is costly to repair. There are several methods to prevent corrosion, focusing mainly on protecting the metal surface:

• Coatings and Paints: These act as a physical barrier between the metal and the environment, preventing oxygen and moisture from reacting with the metal.
• Galvanization: Involves coating iron or steel with a layer of zinc, which corrodes preferentially, protecting the underlying metal.
• Alloys: Mixing metals with other elements can enhance corrosion resistance; for example, stainless steel contains chromium that helps resist rusting.
• Cathodic Protection: This technique uses a sacrificial metal less noble than the protected one to corrode first, sparing the latter.

Understanding and applying these strategies can significantly extend the lifespan of metal structures and reduce maintenance costs.

Transition Metals

Transition metals are a group of metals located in the d-block of the periodic table. These metals are characterized by having partially filled d orbitals, which lend them unique properties, making them crucial in industrial processes and everyday life.

Some common transition metals include iron (Fe), chromium (Cr), and nickel (Ni). They possess properties such as:

• Variable Oxidation States: This allows them to form various types of compounds, enabling diverse reactions such as the formation of different metal oxides.
• Ability to form Complex Ions: Transition metals can interact with a variety of ligands to form complexes used in catalysts and biological systems.
• Metallic Luster and High Melting Points: These properties make them suitable for use in construction and materials that require durability.

The properties of transition metals make them both essential and complex to work with, especially concerning their susceptibility to corrosion. Understanding their chemistry assists in developing effective methods to combat corrosion.