AIIMS 2000 Chemistry Oxidation States MCQ Question

To analyze the assertion and reason given in the question, let's break down both components carefully.

Assertion: Pb⁴⁺ can be reduced easily to Pb²⁺.

This assertion is true. The reduction of lead from a +4 oxidation state (Pb⁴⁺) to a +2 oxidation state (Pb²⁺) is thermodynamically favorable and occurs readily in various chemical environments. The reason for this is that Pb⁴⁺ is a relatively unstable oxidation state, and it tends to gain electrons to achieve a more stable electronic configuration represented by Pb²⁺.

In terms of electron configuration, Pb⁴⁺ has lost four electrons compared to neutral lead (Pb), leading to the configuration of [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁰. When it is reduced to Pb²⁺, it gains two electrons, resulting in a more stable configuration of [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p².

Reason: Pb²⁺ is paramagnetic.

This reason is also true. The term "paramagnetic" refers to species that have unpaired electrons in their atomic or molecular orbitals, which leads to a net magnetic moment.

For Pb²⁺, the electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p². In this configuration, the two electrons in the 6p subshell are unpaired, making Pb²⁺ paramagnetic. This is an important property to note, as it can influence the reactivity and behavior of the ion in chemical reactions.

Conclusion on the Options:

1. Option A: Both A and R are true and R is the correct explanation of A.

   • Incorrect. While both the assertion and reason are true, the reason (paramagnetism) does not explain why Pb⁴⁺ can be reduced to Pb²⁺. The reduction is more about the stability of the oxidation states rather than their magnetic properties.

2. Option B: Both A and R are true but R is not the correct explanation of A.

   • Correct. This option is accurate because while both the assertion and reason are true, the reason does not provide a direct explanation for the reduction of Pb⁴⁺ to Pb²⁺.

3. Option C: A is true but R is false.

   • Incorrect. The assertion is true, and the reason is also true, making this option invalid.

4. Option D: A is false but R is true.

   • Incorrect. The assertion is true, so this option cannot be valid.

Summary:

The correct answer is B because both the assertion that Pb⁴⁺ can be reduced to Pb²⁺ and the statement that Pb²⁺ is paramagnetic are true, but the paramagnetism of Pb²⁺ does not explain the reduction process. Understanding the stability of oxidation states and the electronic configurations is crucial in explaining such redox reactions, rather than magnetic properties.