Problem 36 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following statement(s) is (are) correct with reference to the ferrous and ferric ions? (a) \(\mathrm{Fe}^{3+}\) gives brown colour with potassium ferricyanide. (b) \(\mathrm{Fe}^{2+}\) gives blue precipitate with potassium ferricyanide. (c) \(\mathrm{Fe}^{3+}\) gives red colour with potassium thiocyanate. (d) \(\mathrm{Fe}^{2+}\) gives brown colour with ammonium thiocyanate.

Step-by-Step Solution

Answer

Statements (a), (b), and (c) are correct.

Step 1: Analyze Option (a)

Potassium ferricyanide, \((K_3[Fe(CN)_6])\), reacts with \(\mathrm{Fe}^{3+}\) ions to give a brown color. This statement is correct as the reaction leads to a brown complex.

Step 2: Analyze Option (b)

Potassium ferricyanide reacts with \(\mathrm{Fe}^{2+}\) ions to form Turnbull's blue precipitate \(Fe_3[Fe(CN)_6]_2\). This statement is correct as the product formed is distinctly blue.

Step 3: Analyze Option (c)

Potassium thiocyanate, \(KSCN\), reacts with \(\mathrm{Fe}^{3+}\) ions to form a blood-red complex, \([Fe(SCN)]^{2+}\). This statement is correct because it accurately describes the color change.

Step 4: Analyze Option (d)

Ammonium thiocyanate does not give any expected brown color with \(\mathrm{Fe}^{2+}\) ions. Generally, thiocyanate ions react with \(\mathrm{Fe}^{3+}\) to give a red color, making this statement incorrect.

Key Concepts

Potassium Ferricyanide

Potassium ferricyanide is a deep red, crystalline compound, often used in chemistry to detect the presence of specific iron ions. The chemical formula for potassium ferricyanide is \(K_3[Fe(CN)_6]\). It reacts differently with ferrous (\(Fe^{2+}\)) and ferric (\(Fe^{3+}\)) ions due to its distinct chemical composition.

• When potassium ferricyanide reacts with \(Fe^{3+}\) ions, it forms a brown-colored complex. This brown coloration occurs as a result of the change in the oxidation state and the coordination of ferric ions with the ferricyanide ion.
• On the other hand, when it interacts with \(Fe^{2+}\) ions, it leads to the formation of Turnbull's blue precipitate, a compound known as \(Fe_3[Fe(CN)_6]_2\). This blue color is due to the unique structural changes and bond formations between ferrous ions and ferricyanide.

Thiocyanate Reactions

Thiocyanate ions (\(SCN^{-}\)) are used in various chemical reactions to detect iron ions. These reactions are especially significant because they create distinctive color changes, helping in the identification and analysis of iron's oxidation states.
When thiocyanate ions react with \(Fe^{3+}\) ions, a deeply colored blood-red complex, \([Fe(SCN)]^{2+}\), is produced. This occurs because the thiocyanate ion acts as a ligand and binds to the ferric ion, causing the dramatic color change. This characteristic red color is often used as a qualitative indicator of ferric ions in solution.
On the contrary, \(Fe^{2+}\) ions do not react with thiocyanate ions to produce a noticeable color change like they do with ferric ions. This lack of interaction is essential in differentiating between the two oxidation states of iron in various compounds.

Colorimetric Analysis

Colorimetric analysis is a technique used to determine the concentration of colored compounds in solution. This method is instrumental in chemistry when dealing with solutions that involve color changes due to chemical reactions, such as those involving iron ions and different reagents.
For instance, in reactions with iron ions, colorimetric analysis can help identify the presence or concentration of \(Fe^{2+}\) or \(Fe^{3+}\) depending on the color of the solution or precipitate that forms.

• Brown color formation in reactions with potassium ferricyanide indicates the presence of ferric ions.
• Blue coloration signifies the presence of ferrous ions when reacting with the same reagent.
• A blood-red color seen with thiocyanate accurately indicates ferric ions in a solution.

This approach is not only used for identifying ions but also for quantifying them, as the intensity of the color can be measured and related to concentration.

Iron Complexes

Iron complexes are structures formed by the coordination of iron ions with surrounding ligands. The properties and colors of these complexes depend significantly on the oxidation state of iron and the types of ligands present.

• Ferrous ion complexes, like Turnbull's blue (formed with potassium ferricyanide), are generally formed by the interaction of \(Fe^{2+}\) with specific ligands, resulting in distinct colors such as blue.
• Ferric ion complexes, such as the blood-red \([Fe(SCN)]^{2+}\) formed with thiocyanate, are created when \(Fe^{3+}\) ions bond with ligands, often leading to intense color changes.

These complexes are vital not only in analytical chemistry but also in various biological and industrial processes where iron plays a crucial role. Understanding how iron forms complexes with different ligands reveals insights into their stability and reactivity, which can be further exploited for practical applications.