Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is correct? (a) \(\mathrm{Fe}^{3+}\) gives red colour with potassium ferrocyanide (b) \(\mathrm{Fe}^{3+}\) gives brown colour with potassium ferricyanide (c) \(\mathrm{Fe}^{2+}\) gives brown colour with ammonium thiocyanate (d) \(\mathrm{Fe}^{2+}\) gives blue colour with potassium ferricyanide

Step-by-Step Solution

Answer

Option (d) is correct: \(\mathrm{Fe}^{2+}\) gives blue colour with potassium ferricyanide.

Step 1: Understanding the Problem

The problem asks us to identify the correct statement about reactions involving iron ions (\(\mathrm{Fe}^{3+}\) and \(\mathrm{Fe}^{2+}\)) with certain chemicals: potassium ferrocyanide, potassium ferricyanide, and ammonium thiocyanate.

Step 2: Recall Key Chemical Reactions

Recall that \(\mathrm{Fe}^{3+} \) produces a blue color when it reacts with potassium ferrocyanide. \(\mathrm{Fe}^{3+} \) forms a red-brown complex with SCN-, but not specifically mentioned here. \(\mathrm{Fe}^{2+}\) reacts with potassium ferricyanide to yield a deep blue precipitate (known as Turnbull's blue). \(\mathrm{Fe}^{2+}\) does not form a colored product with ammonium thiocyanate.

Step 3: Eliminate Incorrect Statements

By using the information from Step 2, eliminate incorrect statements:- Option (a) is incorrect: \(\mathrm{Fe}^{3+}\) gives blue, not red, color with potassium ferrocyanide.- Option (b) is incorrect: \(\mathrm{Fe}^{3+}\) does not give a brown color with potassium ferricyanide.- Option (c) is incorrect: \(\mathrm{Fe}^{2+}\) does not give a brown color with ammonium thiocyanate.

Step 4: Verify the Correct Statement

Verify the remaining option, (d): \(\mathrm{Fe}^{2+}\) gives a blue color with potassium ferricyanide. This aligns with the known reaction where they form a deep blue color, confirming it as the correct choice.

Key Concepts

Potassium Ferrocyanide Reaction

Potassium ferrocyanide, K₄[Fe(CN)₆], is a well-known compound in the chemistry world, often used in qualitative analysis of iron ions. It forms stable complex reactions with iron ions, most notably with \(\mathrm{Fe}^{3+}\).When \(\mathrm{Fe}^{3+}\) ions are introduced to a solution containing potassium ferrocyanide, a famous reaction occurs, resulting in the formation of Prussian blue, a deeply colored pigment. The chemical reaction can be shown as:\[4 \mathrm{Fe}^{3+} + 3 \text{K}_4[\mathrm{Fe}( ext{CN})_6] \rightarrow \text{Fe}_4[\mathrm{Fe}( ext{CN})_6]_3 + 12 \mathrm{K}^+\]This results in a characteristic blue color, contrary to the red color some might expect. This reaction isn't just flashy; it finds real-world applications in pigments and dyes, showcasing the utility of chemistry in everyday materials.
This concept is vital in understanding some of the incorrect options given, such as the idea that \(\mathrm{Fe}^{3+}\) giving a red color with potassium ferrocyanide instead of the correct blue.

Potassium Ferricyanide Reaction

Potassium ferricyanide, K₃[Fe(CN)₆], is another key reagent in iron complex reactions. Unlike its cousin, potassium ferrocyanide, this compound reacts with \(\mathrm{Fe}^{2+}\) ions to produce a color that is significant in identifying the presence of certain ions.When solutions containing \(\mathrm{Fe}^{2+}\) react with potassium ferricyanide, the resultant product is known as Turnbull's blue. This reaction can be represented by the equation:\[3 \mathrm{Fe}^{2+} + 2 \text{K}_3[\mathrm{Fe}( ext{CN})_6] \rightarrow \text{Fe}_3[\mathrm{Fe}( ext{CN})_6]_2 + 6 \mathrm{K}^+\]The deep blue color produced in this reaction is often used in chemical tests to indicate the presence of \(\mathrm{Fe}^{2+}\) ions. This concept is crucial for understanding which chemistry choice in the problem is correct. For instance, dismiss the incorrect statement of brown color formation between \(\mathrm{Fe}^{3+}\) and potassium ferricyanide. It's the ferrous iron (\(\mathrm{Fe}^{2+}\)), not ferric (\(\mathrm{Fe}^{3+}\)), that couples with potassium ferricyanide to yield a blue precipitate.

Ammonium Thiocyanate Reaction

Ammonium thiocyanate, NH₄SCN, is commonly used in chemistry to test for the presence of iron ions, specifically \(\mathrm{Fe}^{3+}\). The testing relies on its ability to form a distinct complex with ferric iron.The reaction between \(\mathrm{Fe}^{3+}\) ions and thiocyanate ions (SCN⁻) leads to the formation of a complex with a red-brown coloration as follows:\[\mathrm{Fe}^{3+} + 3 \mathrm{SCN}^- \rightarrow \mathrm{Fe(SCN)}_3\]This red-brown color is unique to this reaction and is particularly useful in iron detection. This concept clarifies why the statement that \(\mathrm{Fe}^{2+}\) gives a brown color with ammonium thiocyanate is incorrect.
Indeed, \(\mathrm{Fe}^{2+}\) don’t react with thiocyanate to produce a colored product. This makes ammonium thiocyanate an effective qualitative test to differentiate between ferrous and ferric iron in solutions.