Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

In nitroprusside ion, the iron and NO exist as \(\mathrm{Fe}^{\mathrm{ll}}\) and NO \(^{+}\)rather than \(\mathrm{Fe}^{\mathrm{m}}\) and NO. These forms can be differentiated by (a) estimating the concentration of iron (b) measuring the concentration of \(\mathrm{CN}^{-}\) (c) measuring the solid state magnetic moment (d) thermally decomposing the compound

Step-by-Step Solution

Answer

The solid state magnetic moment (option c) distinguishes the forms.

Step 1: Analyze the Ion Forms

In the nitroprusside ion, \( \text{Fe}^{\text{II}} \) and NO^+ are two oxidation states and species involved. This affects the electron configuration and magnetic properties of iron, as \( \text{Fe}^{\text{III}} \) would result in a different electron configuration.

Step 2: Evaluate the Detection Methods

Each option represents a method that targets a specific property of the compound: (a) concentration measurements of iron and cyanide do not distinguish between the oxidation states of iron, which are the same on the atomic level for a given species until complexation or chemistry affects them differently.(b) Similarly, measuring \( \text{CN}^{-} \) concentration does not indicate the oxidation state directly.(c) Magnetic moments indicate unpaired electrons: \( \text{Fe}^{\text{II}} \) in low-spin states caused by strong field ligands like \( \text{NO}^+ \) is diamagnetic, while \( \text{Fe}^{\text{III}} \) is typically paramagnetic.(d) Thermal decomposition does not specifically target iron's oxidation state or nitrosyl group.

Step 3: Determine the Correct Method

The magnetic moment is directly influenced by the electron configuration, which changes with oxidation state. \( \text{Fe}^{\text{II}} \) in a low-spin nitroprusside has no unpaired electrons (diamagnetic), while \( \text{Fe}^{\text{III}} \) would have some (paramagnetic). Therefore, measuring the magnetic moment reveals useful information.

Key Concepts

Oxidation States

In chemistry, an oxidation state, also known as oxidation number, refers to the degree of oxidation (loss of electrons) of an atom in a chemical compound. For the nitroprusside ion, it is crucial to understand the oxidation states of both iron (Fe) and the nitrosyl group (NO).

- **Iron Oxidation States**: In the nitroprusside ion, iron is found in the oxidation state of +2, denoted as \( \text{Fe}^{\text{II}} \). This indicates that iron has lost two electrons. Compare this with the +3 oxidation state \( \text{Fe}^{\text{III}} \), where iron would have lost three electrons. The different oxidation states affect the electron configuration and thus the chemical properties of iron significantly.

- **Nitrosyl Group**: The nitrosyl group is in the form of \( \text{NO}^+ \) which implies that it has a positive charge indicating a net electron loss compared to the neutral molecule \( \text{NO} \). Understanding these states is crucial for predicting the compound's behavior in chemical reactions.

Magnetic Properties

Magnetic properties of a compound are directly linked to the electron configuration, which is influenced by the oxidation states of its elements. Examining the nitroprusside ion reveals interesting magnetic behavior due to its low-spin configuration.

- **No Unpaired Electrons**: In \( \text{Fe}^{\text{II}} \), when paired with strong field ligands like \( \text{NO}^+ \), the electrons tend to pair up, resulting in no unpaired electrons, a condition known as "diamagnetic." This means that the compound is not attracted to a magnetic field, which differs from paramagnetic substances that have unpaired electrons and shows attractions to magnetic fields.

- **Low-Spin vs. High-Spin**: The presence of ligands can lead to either low-spin or high-spin electron arrangements. In a low-spin complex like the nitroprusside ion, electrons are forced into pairing due to the significant splitting of energy levels, leading to diamagnetism. In contrast, in a high-spin complex, unpaired electrons cause the chemical to be paramagnetic, characteristics typical of \( \text{Fe}^{\text{III}} \). This differentiation helps in using magnetic measurements to infer the state of iron in such compounds.

Detection Methods

For complex ions like nitroprusside, particular detection methods are essential to understand and identify various properties such as oxidation states.

- **Concentration Measurements**: While measuring the concentrations of elements (like iron or cyanide ions) is helpful in stoichiometry, it doesn't directly tell us about the oxidation states. The atomic configuration isn't confirmed solely by these concentrations.

- **Magnetic Moment Measurements**: Measuring the magnetic moment is an important technique. It indicates the presence of unpaired electrons, hence revealing the oxidation state. As noted, \( \text{Fe}^{\text{II}} \) in a low-spin state is diamagnetic, while \( \text{Fe}^{\text{III}} \) would exhibit paramagnetic behaviors due to unpaired electrons. This method is particularly useful because it directly correlates with the electron configuration elucidating on the oxidation states.

- **Thermal Decomposition**: It's a method involving breaking down a compound with heat. While it can provide some element-level information, it does not specifically target changes in oxidation states or differentiate between the presence of \( \text{Fe}^{\text{II}} \) and \( \text{Fe}^{\text{III}} \). Hence, it is not the preferred method when the goal is to specifically assess redox states in complex ions like nitroprusside.