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 given metals reacting with \(\mathrm{O}_{2}\) are as follows: 1. Zinc: \(2 \,\mathrm{Zn} + \mathrm{O}_{2} \rightarrow 2 \,\mathrm{ZnO}\) 2. Aluminum: \(4 \,\mathrm{Al} + 3 \,\mathrm{O}_{2} \rightarrow 2 \,\mathrm{Al}_{2}\mathrm{O}_{3}\) 3. Iron: \(\mathrm{Fe} + \mathrm{O}_{2} + 3 \,\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Fe}_{2}\mathrm{O}_{3} \cdot 3 \,\mathrm{H}_{2}\mathrm{O}\) 4. Chromium: \(2 \,\mathrm{Cr} + 3 \,\mathrm{O}_{2} \rightarrow \mathrm{Cr}_{2}\mathrm{O}_{3}\) 5. Nickel: \(2 \,\mathrm{Ni} + \mathrm{O}_{2} \rightarrow 2 \,\mathrm{NiO}\)

Step-by-step solution

1. Zinc and Oxygen

When zinc reacts with oxygen, zinc oxide is formed. The balanced equation for this reaction is: \[2 \,\mathrm{Zn} + \mathrm{O}_{2} \rightarrow 2 \,\mathrm{ZnO}\]

2. Aluminum and Oxygen

When aluminum reacts with oxygen, aluminum oxide 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. Iron and Oxygen

When iron reacts with oxygen and water, hydrated iron (III) oxide (also known as rust) is formed. The balanced equation for this reaction is: \[\mathrm{Fe} + \mathrm{O}_{2} + 3 \,\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Fe}_{2}\mathrm{O}_{3} \cdot 3 \,\mathrm{H}_{2}\mathrm{O}\]

4. Chromium and Oxygen

When chromium reacts with oxygen, chromium (III) oxide is formed. The balanced equation for this reaction is: \[2 \,\mathrm{Cr} + 3 \,\mathrm{O}_{2} \rightarrow \mathrm{Cr}_{2}\mathrm{O}_{3}\]

5. Nickel and Oxygen

When nickel reacts with oxygen, nickel (II) oxide is formed. The balanced equation for this reaction is: \[2 \,\mathrm{Ni} + \mathrm{O}_{2} \rightarrow 2 \,\mathrm{NiO}\]

Key concepts

Oxidation

Oxidation is a fundamental concept in chemistry and is crucial in understanding how corrosion affects metals. It refers to a process where an atom, ion, or molecule loses electrons. During oxidation, the substance undergoing this process is said to be 'oxidized'. This often involves gaining oxygen or losing hydrogen. For metals, oxidation typically occurs when they are exposed to oxygen in the presence of moisture, which can lead to the formation of oxides. For example, when zinc ( Zn ) is oxidized, it loses electrons to form zinc oxide (ZnO). Similarly, other metals like aluminum ( Al ), iron ( Fe ), chromium ( Cr ), and nickel ( Ni ) react with oxygen to form their respective oxides.

Balanced Chemical Equation

A balanced chemical equation is essential in accurately representing a chemical reaction. It ensures that the number of atoms for each element is the same on both sides of the equation, complying with the Law of Conservation of Mass. This law states that matter cannot be created or destroyed in a chemical reaction.

When balancing chemical equations:

• Identify the reactants and products.
• Count the number of atoms of each element in the reactants and products.
• Use coefficients to balance the atoms on both sides.

For instance, in the reaction for aluminum and oxygen:\[4 \, \mathrm{Al} + 3 \, \mathrm{O}_{2} \rightarrow 2 \, \mathrm{Al}_{2}\mathrm{O}_{3}\]There are 4 aluminum atoms and 3 oxygen molecules on the reactant side and an equal number on the product side, ensuring a balanced equation.

Metal Reactions

Metal reactions with oxygen are significant because they result in corrosion, impacting their longevity and utility. Metals react with oxygen to form metal oxides. This process involves the transfer of electrons from the metal to oxygen, leading to oxidation.

• Zinc: Forms zinc oxide by reacting with oxygen \(2 \, \mathrm{Zn} + \mathrm{O}_{2} \rightarrow 2 \, \mathrm{ZnO}\).
• Aluminum: Produces aluminum oxide when it reacts with oxygen \(4 \, \mathrm{Al} + 3 \, \mathrm{O}_{2} \rightarrow 2 \, \mathrm{Al}_{2}\mathrm{O}_{3}\).
• Iron: Corrodes to form rust, represented by hydrated iron (III) oxide \(\mathrm{Fe} + \mathrm{O}_{2} + 3 \, \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Fe}_{2}\mathrm{O}_{3} \cdot 3 \, \mathrm{H}_{2}\mathrm{O}\).
• Chromium and Nickel: Form chromium (III) oxide and nickel (II) oxide, respectively.

These reactions highlight the reactive nature of metals and their tendency to combine with oxygen, especially when exposed to moisture.

Chemical Reactions

Chemical reactions are processes where substances undergo chemical changes to form new substances. They occur everywhere in our world, from digestion in our bodies to the corrosion of metal structures.

• Reactants and Products: In a chemical reaction, reactants are transformed into products. For instance, in the reaction between chromium and oxygen, chromium serves as a reactant, and chromium (III) oxide is the product.
• Types of Chemical Reactions: Reactions can be classified into different types, such as oxidation-reduction, acid-base, synthesis, and decomposition reactions.
• Law of Conservation of Mass: This fundamental law dictates that the mass of reactants equals the mass of products, underscoring the importance of balanced chemical equations.

Understanding these aspects allows us to predict the outcomes of chemical interactions and helps in designing materials to resist undesired reactions, such as corrosion.