JEE Main 2024 — Electrochemistry Question with Solution

The statement that is not correct about the rusting of iron is Option C. Let's examine each option one by one:

Option A: Rusting of iron is envisaged as setting up of an electrochemical cell on the surface of iron object. This statement is correct. Rusting of iron is an electrochemical process that occurs when iron (Fe) reacts with water (H₂O) and oxygen (O₂) to form hydrated iron(III) oxide, which we commonly know as rust. Iron acts as the anode, the part where oxidation occurs as iron loses electrons. Oxygen, often dissolved in water, acts as the cathode where reduction occurs, completing the electrochemical cell.

Option B: Dissolved acidic oxides such as $${\mathrm{SO}}_2$$ and $${\mathrm{NO}}_2$$ in water act as catalyst in the process of rusting. This statement is correct. Acidic oxides like $${\mathrm{SO}}_2$$ and $${\mathrm{NO}}_2$$ can dissolve in water to form acidic solutions which can lead to the acceleration of the rate of rusting as they may lower the pH of water, thereby increasing the availability of hydrogen ions (H⁺) which may facilitate the electrochemical reactions involved in rusting.

Option C: Coating of iron surface by tin prevents rusting, even if the tin coating is peeling off. This statement is incorrect. Tin (Sn) is used as a protective coating for iron to prevent it from rusting; however, if the tin coating starts to peel off, it exposes the iron underneath to the atmosphere, which will then be susceptible to rusting. Once the protective layer is compromised, the iron is at risk, especially if the exposed parts are more anodic than tin. Tin as a more cathodic metal can even act to accelerate the corrosion of the exposed iron—this is known as a bimetallic corrosion or galvanic corrosion.

Option D: When $$\mathrm{pH}$$ lies above 9 or 10, rusting of iron does not take place. This statement is generally correct. Rusting of iron is lessened in alkaline conditions, i.e., when the pH is above 9 or 10 because the increased availability of hydroxide ions (OH^-) can reduce the solubility of iron(II) and iron(III) ions, leading to their precipitation as hydroxides rather than participating in the rusting process.

Therefore, the statement from the given options that is not correct about the rusting of iron is Option C, which falsely claims that a peeling tin surface can still prevent rusting of iron.