Problem 109 - A Complete Resource Book in Chemistry for JEE Main

Question

The mixture of which of the following can produce blue colouration? (a) Iron (III) chloride and \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (aq) (b) \(\mathrm{NH}_{4} \mathrm{OH}\) (aq) and \(\mathrm{CuSO}_{4}\) (aq) (c) Adding anhydrous \(\mathrm{CuSO}_{4}\) to water (d) \(\mathrm{ZnCl}_{2}\) (aq) and \(\mathrm{K}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right](\mathrm{aq})\)

Step-by-Step Solution

Answer

The mixture of Iron(III) chloride and potassium ferrocyanide (option a) produces a blue coloration known as Prussian Blue.

Step 1: Analyze Mixture (a)

The substances in mixture (a) are Iron(III) chloride and potassium ferrocyanide, \( \text{K}_4[\text{Fe(CN)}_6] \). When Iron(III) chloride (\( \text{FeCl}_3 \)) is mixed with potassium ferrocyanide in aqueous solution, a blue precipitate of Prussian Blue (\( \text{Fe}_4[\text{Fe(CN)}_6]_3 \)) is formed. This is because iron(III) ions react with ferrocyanide ions to form the blue, insoluble compound.

Step 2: Analyze Mixture (b)

The substances in mixture (b) are aqueous ammonia (\( \text{NH}_4\text{OH} \)) and copper sulfate (\( \text{CuSO}_4 \)). Mixing these produces a light blue complex known as tetraaminecopper(II) sulfate, but this is a light blue coloration, not the intense blue typical of Prussian Blue.

Step 3: Analyze Mixture (c)

Mixture (c) involves adding anhydrous copper(II) sulfate (\( \text{CuSO}_4 \)) to water. Copper(II) sulfate dissolves to form a blue solution, attributed to the presence of \( \text{Cu}^{2+} \) ions, but again, this is a light blue solution, not a deep blue precipitate like Prussian Blue.

Step 4: Analyze Mixture (d)

The substances in mixture (d) are zinc chloride (\( \text{ZnCl}_2 \)) and potassium ferricyanide (\( \text{K}[\text{Fe(CN)}_6] \)). Zinc ions generally do not produce a precipitate when reacted with ferricyanide ions; hence, no blue coloration is produced with this mixture.

Step 5: Identify the Correct Mixture

From the analyses, we know that Mixture (a) produces a significant blue coloration because of the formation of Prussian Blue. Mixtures (b) and (c) produce light blue colors, while mixture (d) does not produce any noticeable blue coloration.

Key Concepts

Understanding Iron(III) Chloride

Iron(III) chloride, often referred to as ferric chloride, is a chemical compound with the formula \( \text{FeCl}_3 \). In its anhydrous form, it appears as dark, glossy crystals, while in its hydrated form, it often appears as a yellow-brown liquid.
This compound is highly soluble in water, which means it easily dissolves to form a solution containing \( \text{Fe}^{3+} \) ions. These iron ions are critical in chemical reactions, especially those forming colored precipitates like Prussian Blue.

• Commonly used as a coagulant in water treatment processes.
• Acts as a powerful oxidizing agent.

When combined with solutions containing ferrocyanide ions, iron(III) ions react to form the deep blue pigment known as Prussian Blue, a notable feature due to its vibrant and intense color.

Role of Potassium Ferrocyanide

Potassium ferrocyanide, represented as \( \text{K}_4[\text{Fe(CN)}_6] \), is a complex salt formed from potassium and the ferrocyanide ion \( [\text{Fe(CN)}_6]^{4-} \).
This compound is a pale yellow, crystalline material that's highly soluble in water, which means it breaks down readily into its constituent ions in aqueous solutions.

• Commonly used in the cyanotype printing process.
• Safely employed in food processing, for instance, as an anti-caking agent.

When mixed with solutions containing iron(III) chloride, potassium ferrocyanide undergoes a reaction, leading to the formation of Prussian Blue.
This occurs because ferrocyanide ions strongly bind with iron(III) to create an intensely blue, insoluble complex.

Formation of Blue Precipitate

The blue precipitate formation in the reaction between iron(III) chloride and potassium ferrocyanide leads to the production of Prussian Blue, whose chemical name is iron(III) ferrocyanide.
Prussian Blue is characterized by its deep blue color and is formed when \( \text{Fe}^{3+} \) ions from iron(III) chloride react with \( [\text{Fe(CN)}_6]^{4-} \) ions from potassium ferrocyanide. This reaction can be represented by the chemical equation:
\[ 4\text{Fe}^{3+} + 3[\text{Fe(CN)}_6]^{4-} \rightarrow \text{Fe}_4[\text{Fe(CN)}_6]_3 \downarrow \]

• The downward arrow \( \downarrow \) signifies the formation of a solid precipitate from the solution.
• This precipitate has significant applications in painting, medicine, and analytical chemistry.

As soon as the two solutions are mixed, the distinct deep blue color emerges, showcasing the strong interaction between the iron ions and the ferrocyanide ions.