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

Question

Corrosion of iron is essentially an electrochemical 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) \(\mathrm{Fe}\) 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 option (a).

Step 1: Understanding Iron Corrosion Process

Corrosion of iron involves an electrochemical reaction where iron oxidizes and another substance is reduced. In a typical rusting process of iron, Fe is oxidized to Fe^{2+} ions. This takes place in presence of water and dissolved oxygen.

Step 2: Identify the Oxidation Reaction

The oxidation reaction in the corrosion of iron is when iron (Fe) loses electrons to form ferrous ions (Fe^{2+}). This is represented by:\( \mathrm{Fe} \rightarrow \mathrm{Fe}^{2+} + 2e^- \)

Step 3: Identify the Reduction Reaction

The reduction reaction accompanies the oxidation of iron. Dissolved oxygen in water accepts electrons and forms hydroxide ions (\mathrm{OH}^{-}) as per the equation:\( \mathrm{O}_2 + 4e^- + 2\mathrm{H}_2\mathrm{O} \rightarrow 4\mathrm{OH}^- \)

Step 4: Evaluating the Given Options

Compare the given options to identify which one represents the correct reactions that occur during the electrochemical corrosion of iron. The expected cell reactions involved are iron oxidizing to \( \mathrm{Fe}^{2+} \) and dissolved oxygen reducing to \( \mathrm{OH}^- \).

Step 5: Selecting the Correct Answer

Option (a) matches the assigned reaction where \( \mathrm{Fe} \) is oxidized to \( \mathrm{Fe}^{2+} \) and dissolved oxygen in water is reduced into \( \mathrm{OH}^{-} \). This is consistent with the typical electrochemical reactions involved in iron corrosion.

Key Concepts

Electrochemical Reaction

An electrochemical reaction is a type of chemical process where electrical energy is produced from a chemical reaction or chemical energy is used to bring about a chemical change. In corrosion of iron, these reactions play a crucial role. Here, the iron surface acts like an electrochemical cell where different parts of the iron surface serve as electrodes.
This process involves both oxidation and reduction reactions happening simultaneously, which is why it is also called a redox reaction.

• Oxidation: This occurs at the anode site on the iron surface, where iron atoms lose electrons and form ions.
• Reduction: Occurs at the cathode site, typically where oxygen dissolved in water gains electrons to form other products like hydroxide ions.

Understanding this dual nature of the reaction helps explain how rust spreads, since both parts need to occur for corrosion to continue.

Oxidation and Reduction

Oxidation and reduction are core parts of a redox reaction, which is essential for understanding the corrosion of iron.

• Oxidation: This is the process where iron loses electrons. In the chemical equation, \( \mathrm{Fe} \rightarrow \mathrm{Fe}^{2+} + 2e^- \), iron is oxidized to iron ions (Fe²⁺).
• Reduction: This happens when another chemical species, like oxygen, gains the electrons lost by iron. The reaction is \( \mathrm{O}_2 + 4e^- + 2\mathrm{H}_2\mathrm{O} \rightarrow 4\mathrm{OH}^- \), where dissolved oxygen in water is reduced.

These two reactions together drive the rusting process of iron. Without the transfer of electrons from iron to oxygen, corrosion can't proceed.

Rusting Process of Iron

The rusting process is a common illustration of iron corrosion and involves an electrochemical reaction that leads to the formation of rust.
It typically occurs in the presence of moisture and air. This is why you often notice rust forming on iron that's left exposed to the elements.

• First, the iron in contact with air and water forms ferrous ions through oxidation.
• Then, the electrons from this reaction are transferred to dissolved oxygen in water, reducing it to hydroxide ions.
• These ions combine to form iron(III) hydroxide, which dehydrates to form ferric oxide or rust.

The chemical interaction between these components is what not only causes the iron to weaken but also expands the area affected by rust, making it a progressive issue.