Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

Ferric ion forms a Prussian blue coloured precipitate due to (a) \(\mathrm{Fe}(\mathrm{OH})_{3}\) (b) \(\mathrm{Fe}_{4}[\mathrm{Fe}(\mathrm{CN})]_{3}\) (c) \(\mathrm{KMnO}_{4}\) (d) \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)

Step-by-Step Solution

Answer

Prussian blue precipitate forms due to \(\mathrm{K}_{4} [\mathrm{Fe} (\mathrm{CN})_{6}]\).

Step 1: Understanding the Reaction

Prussian blue is a dark blue pigment, chemically known as ferric ferrocyanide. The formation of Prussian blue involves the reaction of ferric ions \(\mathrm{Fe}^{3+}\) with ferrocyanide ions \(\ [\mathrm{Fe} (\mathrm{CN})_{6}]^{4-}\).

Step 2: Analyzing the Options

We analyze each option to determine which compound leads to the formation of Prussian blue. Option (b) \(\mathrm{Fe}_{4} [\mathrm{Fe} (\mathrm{CN})]_{3}\) and option (d) \(\mathrm{K}_{4} [\mathrm{Fe} (\mathrm{CN})_{6}]\) are related to ferrocyanide compounds. Others involve different compounds that do not form Prussian blue.

Step 3: Selecting the Correct Compound

Option (d) provides \(\mathrm{K}_{4} [\mathrm{Fe} (\mathrm{CN})_{6}]\), which contains the ferrocyanide ions \(\left[\mathrm{Fe} (\mathrm{CN})_{6}\right]^{4-}\). These ions react with \(\mathrm{Fe}^{3+}\) to form Prussian blue, \(\mathrm{Fe}_{4} [\mathrm{Fe} (\mathrm{CN})_{6}]_{3}\), in the reaction: \[3 \mathrm{Fe}^{3+} + 4 \left[\mathrm{Fe} (\mathrm{CN})_{6}\right]^{4-} \rightarrow \mathrm{Fe}_{4} [\mathrm{Fe} (\mathrm{CN})_{6}]_{3}\]

Key Concepts

Ferric Ion

In the context of chemical reactions, the term 'ferric ion' refers to the iron ion with a +3 charge, represented as \( \mathrm{Fe}^{3+} \). Ferric ions are quite reactive and play a significant role in various chemical processes. These ions possess a higher oxidation state compared to ferrous ions, which carry a +2 charge. This higher oxidation state makes ferric ions particularly potent oxidizing agents.

A primary feature of ferric ions is their ability to form complex compounds. In the specific reaction forming Prussian blue, ferric ions react with ferrocyanide ions. As a result, a deep blue compound is formed. This makes them essential for the development of Prussian blue pigments.

By understanding the behavior of ferric ions, we can better understand their role in broader chemical processes, including corrosion and the treatment of wastewater where they help in removing contaminants.

Ferrocyanide

Ferrocyanide is a coordination compound that has fascinating properties. The ferrocyanide ion is denoted as \([\mathrm{Fe} (\mathrm{CN})_{6}]^{4-}\). It arises when six cyanide ions coordinate themselves around a central iron atom in its +2 oxidation state, forming a stable and symmetrical structure.

The uniqueness of ferrocyanide lies in its reactivity with ferric ions to produce Prussian blue. This reaction is both a classic and practical demonstration of coordination chemistry in creating deeply colored compounds. When ferrocyanide ions encounter ferric ions in solution, they combine in a specific ratio to form the compound known as ferric ferrocyanide, or simply Prussian blue.

Beyond their ability to form colorful compounds, ferrocyanide ions are significant in various industrial applications. They serve as anti-caking agents in salt and are also used in the separation of heavy metals from solutions.

Chemical Reactions

Chemical reactions are the process by which substances interact to form new products. They are central to chemistry and are involved in every process that constitutes life. In the case of Prussian blue formation, a specific chemical reaction occurs between ferric ions \((\mathrm{Fe}^{3+})\) and ferrocyanide ions \([\mathrm{Fe} (\mathrm{CN})_{6}]^{4-}\).

This reaction can be written as: \[3 \mathrm{Fe}^{3+} + 4 [\mathrm{Fe} (\mathrm{CN})_{6}]^{4-} \rightarrow \mathrm{Fe}_{4} [\mathrm{Fe} (\mathrm{CN})_{6}]_{3}\]It demonstrates the principle of conservation of mass, where the total number of atoms of each element remains constant throughout the reaction.

Understanding this specific chemical reaction allows students to appreciate the intricate balance and beauty of chemistry. It exemplifies how simple ionic interactions can lead to the creation of complex compounds with significant practical applications, such as in pigments and dyes. This highlights the broader theme of chemical reactivity in crafting useful and novel materials.