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

(b)  ext{Fe}_4[ ext{Fe(CN)}_6]_3 is the compound that forms Prussian blue.

Step 1: Understanding the Problem

The exercise asks which compound forms a Prussian blue colored precipitate when a ferric ion is involved. Recognize that Prussian blue is a well-known pigment formed by a specific iron compound.

Step 2: Recall Chemistry Knowledge

Prussian blue is commonly formed when iron(III) ions ( ext{Fe}^{3+}) combine with hexacyanoferrate(II) ions ( ext{Fe(CN)}_6^{4-}) to produce the compound  ext{Fe}_4[ ext{Fe(CN)}_6]_3.

Step 3: Evaluate the Options

Look at each option to assess whether it can form Prussian blue: (a)  ext{Fe(OH)}_3 is iron hydroxide, not related to cyanide. (b)  ext{Fe}_4[ ext{Fe(CN)}_6]_3 matches the formula for Prussian blue. (c)  ext{KMnO}_4 is potassium permanganate, unrelated to cyanide. (d)  ext{K}_4[ ext{Fe(CN)}_6] is similar but refers to the hexacyanoferrate(II) ion that combines with ferric ions, not the final precipitate.

Step 4: Conclude the Correct Option

Based on the evaluation, option (b)  ext{Fe}_4[ ext{Fe(CN)}_6]_3 is the compound that forms the Prussian blue precipitate with ferric ions.

Key Concepts

Ferric Ion

The ferric ion, represented as \(\text{Fe}^{3+}\), is a form of iron that has lost three electrons. This results in a positive charge, which makes the ion highly reactive. Ferric ions are crucial in many chemical reactions, especially those involving transition metals. These ions are often found in iron compounds and play a significant role in forming various colored precipitates, including Prussian blue. Understanding how ferric ions interact with other chemical species is vital in predicting the outcomes of reactions that involve iron.

The ability of ferric ions to form intricate compounds contributes to their significance in fields like chemistry and materials science. These ions readily combine with negatively charged species, which leads to the formation of complex structures such as Prussian blue. This versatility and reactivity highlight the importance of ferric ions in chemical reactions.

Hexacyanoferrate

Hexacyanoferrate is a compound featuring the ferrate ion \(\text{Fe(CN)}_6^{4-}\). This ion consists of iron surrounded by six cyanide ligands. The negative charge signifies an excess of electrons, making the hexacyanoferrate ion a potent agent in precipitation reactions. This ion comes in two common forms: hexacyanoferrate(II) and hexacyanoferrate(III). The distinguishing feature between these is the oxidation state of the central iron atom.

Hexacyanoferrate(II), \(\text{K}_4[\text{Fe(CN)}_6]\), reacts particularly well with ferric ions to form insoluble compounds, like Prussian blue. This formation occurs when three \(\text{Fe}^{3+}\) ions link with four hexacyanoferrate(II) ions to create \(\text{Fe}_4[\text{Fe(CN)}_6]_3\). The strong affinity between the ferric ions and hexacyanoferrate ions underscores their role in precipitate formation and pigment creation.

Precipitate Reaction

A precipitate reaction is a process where soluble reactants create an insoluble product. In chemistry, this often results in a solid forming from a solution. Such reactions are crucial for identifying compounds and purifying substances. Prussian blue formation is a classic example of a precipitate reaction, where ferric ions \(\text{Fe}^{3+}\) interact with hexacyanoferrate(II) ions \(\text{Fe(CN)}_6^{4-}\) to produce a blue solid.

Many precipitate reactions rely on precise conditions, such as pH and concentration, to occur. The distinctive colors of various precipitates enable chemists to visually confirm the presence of certain ions. Precipitate reactions are not only important in academic labs but also in industrial processes like wastewater treatment and mineral extraction.

Iron Compounds

Iron compounds encompass a diverse range of chemical species that include iron in varying oxidation states. They are integral to numerous biological and chemical processes. Ferric and ferrous compounds differ in the number of electrons iron atoms have donated or accepted. Ferric compounds involve \(\text{Fe}^{3+}\), while ferrous compounds relate to \(\text{Fe}^{2+}\).

Each type of iron compound has unique properties and applications. For instance, ferrous sulfate is utilized in treating blood disorders, whereas ferric chloride is used in water treatment. Understanding these compounds’ chemical behavior is crucial because they form the basis for products like pigments (including Prussian blue) and various industrial materials. Their versatility makes them invaluable in both organic and inorganic chemistry.