Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

Corrosion of iron is essentially an electro-chemical phenomenon where the cell reactions are (a) \(\mathrm{Fe}\) is oxidized to \(\mathrm{Fe}^{2+}\) and dissolved oxygen in water is reduced to \(\mathrm{OH}\) (b) \(\mathrm{Fe}\) is oxidized to \(\mathrm{Fe}^{3+}\) and \(\mathrm{H}_{2} \mathrm{O}\) is reduced to \(\mathrm{O}_{2}^{2}\) (c) \(\mathrm{Fe}\) is oxidized to \(\mathrm{Fe}^{2+}\) and \(\mathrm{H}_{2} \mathrm{O}\) is reduced to \(\mathrm{O}_{2}^{-}\) (d) Fe is oxidized to \(\mathrm{Fe}^{2+}\) and \(\mathrm{H}_{2} \mathrm{O}\) is reduced to \(\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (a): \(\mathrm{Fe}\) is oxidized to \(\mathrm{Fe}^{2+}\) and dissolved oxygen in water is reduced to \(\mathrm{OH}^-\).

Step 1: Identify the Main Reaction Components

Corrosion of iron involves the oxidation of iron and reduction of oxygen. We need to find which option features the correct oxidation and reduction processes involving iron and oxygen.

Step 2: Understand Oxidation of Iron

Iron (Fe) in corrosion is typically oxidized from the elemental form to Fe\(^{2+}\) ions, not Fe\(^{3+}\) ions. This rules out option (b) which involves oxidation to Fe\(^{3+}\).

Step 3: Analyze the Reduction Process

For the reduction in this electrochemical process, dissolved oxygen in water gets reduced to produce hydroxide ions (OH\(^-\)). This corresponds to the reduction process mentioned in option (a). Other options mention reduction to other forms like \(\mathrm{O}_{2}^{2-}\) or \(\mathrm{O}_{2}^{-}\), which are not typical in this context.

Step 4: Cross-verify with Chemical Reactions

Check if other options (c) and (d) align with any standard or common reactions known for iron corrosion. Since they involve atypical reduced products, they can be ruled out.

Key Concepts

Electrochemical Reactions

Corrosion of iron is primarily driven by electrochemical reactions. These reactions occur when chemical changes are driven by the flow of electric charge at the surface of metal. This process involves the iron interacting with environmental elements like moisture and oxygen.
Instead of being a direct chemical reaction, it involves the movement of electrons between substances, creating a battery-like scenario. In corrosion, iron (Fe) acts as an anode where it loses electrons, and oxygen acts as the cathode where it gains electrons.
Both the given oxidation and reduction processes are essential to understand how corrosion happens:

• Oxidation of iron (loss of electrons)
• Reduction of oxygen (gain of electrons)

Oxidation of Iron

The oxidation of iron is a fundamental step in the corrosion process. This involves iron (Fe) being converted into ferrous ion (\( \mathrm{Fe}^{2+} \)) by losing two electrons. This can be represented by the following reaction:

\[\mathrm{Fe} \rightarrow \mathrm{Fe}^{2+} + 2 \mathrm{e}^- \]
When iron loses electrons, they become available for the reduction of another substance—in most corrosion cases, this substance is oxygen. Here, iron giving up electrons releases them for oxygen to use, establishing an electrochemical balance.
Oxidation doesn't only weaken the iron physically; it contributes to rust formation when these ions further react with oxygen and water to form iron oxides.

Reduction of Oxygen

Once iron has been oxidized, oxygen from the environment comes into play. Oxygen undergoes a reduction process, which involves gaining electrons. During corrosion, oxygen molecules in an aqueous environment are reduced to form hydroxide ions. This change can be summarized in the reaction:

\[\mathrm{O}_2 + 4 \mathrm{e}^- + 2 \mathrm{H}_2\mathrm{O} \rightarrow 4 \mathrm{OH}^- \]
This step is crucial as it creates hydroxide ions that will eventually react with the ferrous ions (\( \mathrm{Fe}^{2+} \)) to create rust, a mixture of iron oxides and hydroxides. The reduction of oxygen is influenced by the availability of moisture, which is essential for carrying electrons in the solution, making it a dynamic process in different environmental conditions.

Electrochemical Cells

Understanding the corrosion of iron as an electrochemical cell helps in visualizing the process. An electrochemical cell consists of an anode and a cathode where oxidation and reduction occur. These cells are often seen as batteries, but in the case of iron corrosion, they form naturally in environment.
At the anode, iron gets oxidized, losing electrons, creating ferrous ions. Meanwhile, at the cathode, oxygen is reduced as it gains electrons. Usually, the surface imperfections or stress points on the iron become the anodic zones, and locations with higher oxygen availability become cathodic.

• Typical electrolyte in corrosion: moisture or thin layers of water
• Anodic reaction: iron oxidation
• Cathodic reaction: oxygen reduction

This diffusion of electrons converts the iron into a "cell," completing the electrochemical circuit necessary for corrosion to proceed.