AIIMS 2005 Chemistry Corrosion Assertion Reason Question

To address the question about the assertion and reason regarding galvanised iron and its resistance to rusting, let's break it down step by step.

Assertion: Galvanised iron does not rust.

Explanation: Galvanised iron is iron that has been coated with a layer of zinc. This zinc layer serves as a protective barrier against corrosion. When iron is exposed to moisture and oxygen in the environment, it tends to rust (form iron oxides, mainly $Fe_2O_3$). However, galvanised iron does not rust because the zinc coating prevents moisture from reaching the underlying iron. Additionally, if the zinc layer is damaged, zinc is more reactive than iron and will corrode preferentially, thus protecting the iron underneath.

Reason: Zinc has a more negative electrode potential than iron.

Explanation: Electrode potential is a measure of the tendency of a chemical species to be reduced. Metals with a more negative electrode potential are more likely to oxidize. In electrochemical terms, zinc has a standard electrode potential of approximately $-0.76 \, \text{V}$ while iron has a standard electrode potential of about $-0.44 \, \text{V}$. This means that zinc is more reactive and will oxidize before iron does. Hence, when galvanised iron is exposed to corrosive environments, zinc will preferentially oxidize, protecting the iron from rusting.

Conclusion on Answer:

Since both the assertion and reason are true, and the reason correctly explains why galvanised iron does not rust due to the reactivity of zinc, the correct answer is:

A) If both assertion and reason are true and reason is the correct explanation of assertion.

Clarification of Incorrect Options:

• B) If both assertion and reason are true but reason is not the correct explanation of assertion.

  • This option is incorrect because the reason provided does indeed explain why galvanised iron does not rust.

• C) If assertion is true but reason is false.

  • This option is incorrect since both the assertion (galvanised iron does not rust) and the reason (zinc has a more negative electrode potential than iron) are true.

• D) If both assertion and reason are false.

  • This option is incorrect as both statements are true.

Summary:

In the context of the question, we have established that galvanised iron does indeed resist rusting due to the protective properties of zinc, which has a more negative electrode potential than iron, leading to zinc's preferential oxidation. This dual truth confirms that option A is the correct choice.