Problem 53 - 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) Fe is oxidized to \(\mathrm{Fe}^{3+}\) and \(\mathrm{H}_{2} \mathrm{O}\) is reduced to \(\mathrm{O}_{2}^{2-}\) (c) 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

(a) Fe is oxidized to Fe^{2+} and dissolved oxygen in water is reduced to OH^-.

Step 1: Understanding Corrosion of Iron

Corrosion of iron involves an electrochemical reaction. Corrosion occurs when iron reacts with oxygen in the presence of water, forming iron oxide. This process involves the oxidation of iron and the reduction of another substance or compound.

Step 2: Identifying Oxidation of Iron

Since corrosion involves iron ( Fe ) being oxidized, we focus on the oxidation state change. Iron is oxidized from 0 in Fe to +2 to form Fe^{2+} . This matches option (a), (c), and (d), but not option (b), where iron becomes Fe^{3+} .

Step 3: Understanding Reduction Reaction

During the corrosion process, the oxygen from water is reduced. The dissolved oxygen in the water is typically reduced to hydroxide ions, OH^- , during this process. This aligns best with option (a).

Step 4: Conclusion

From the above analysis, corrosion of iron consists of iron being oxidized to Fe^{2+} and dissolved oxygen in water being reduced to OH^- , correlating with option (a).

Key Concepts

Electrochemical Reaction

Understanding electrochemical reactions can help grasp why corrosion occurs. This type of reaction involves the direct transfer of electrons between substances.
In the case of corrosion, we deal with iron reacting with oxygen and water. These substances participate in an electrochemical process that leads to rust formation.
To put it simply, when iron is exposed to air and moisture, it starts losing electrons to the surrounding environment. This movement of electrons is the core of electrochemical reactions. The electrons flow from one part of the iron to another. In a broader sense:

• Ions and atoms exchange electrons.
• Electrons travel from the iron, which is oxidized, to dissolved oxygen in water, which is reduced.

This creates an electrochemical cell, where corrosion acts much like a small battery. Understanding this will help you see why certain environmental conditions speed up or slow down corrosion.

Oxidation-Reduction

Oxidation-reduction, or redox reactions, are essential in understanding rust formation on iron. A redox reaction involves two simultaneous processes: oxidation and reduction. Without knowing it, we encounter these reactions every day!
**Oxidation:** Here, the iron loses electrons. This happens when each iron atom loses two electrons, turning into iron ions (\(Fe^{2+}\)). That's oxidation in simple terms - losing electrons. It changes iron's oxidation state from 0 to +2.
**Reduction:** While iron loses electrons, something else gains them. In corrosion, it's the oxygen from water. The oxygen receives these electrons and turns into hydroxide ions (\(OH^-\)). Hence, reduction occurs.
These processes are directly linked.

• Iron oxidizes by losing electrons.
• Oxygen reduces by gaining electrons.

Remember, oxidation and reduction always occur together. If you master one, understanding the other becomes easier.

Iron Oxide Formation

Rust, scientifically known as iron oxide, is what remains when iron undergoes corrosion. The reddish-orange appearance on old nails or bridges results from iron oxide formation. But how exactly does this happen?
When iron is exposed to oxygen and moisture over time, the metal starts forming weak spots. In these spots:

• Iron oxidizes into \(Fe^{2+}\) ions.
• Simultaneously, oxygen undergoes reduction, creating hydroxide ions \(OH^-\).

The iron ions combine with hydroxide ions in the presence of water. This forms iron hydroxide, which is further oxidized to form iron oxide. Slowly, this iron oxide accumulates and we see it as rust. Understanding that rust is essentially iron oxide helps in realizing the significance of protective coatings like paint. These coatings can prevent exposure to moisture, slowing down rust formation.