Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Ferric iron forms a prussian blue coloured precipitate with \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\), due to the formation of (a) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (b) \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (d) \(\mathrm{Fe}(\mathrm{CNS})_{3}\)

Step-by-Step Solution

Answer

The correct answer is (a) \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \).

Step 1: Understanding the Reaction Components

In the given reaction, ferric iron (Fe³⁺) is reacting with potassium ferrocyanide (\( \mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)). Ferric iron (Fe³⁺) is in the +3 oxidation state.

Step 2: Determining the Product Formation

When ferric ion (Fe³⁺) reacts with potassium ferrocyanide \( \mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\), a complex ferric ferrocyanide precipitate is formed. This compound is the famous 'Prussian blue'. It has the formula \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\).

Step 3: Confirming the Characteristic Color

The formation of Prussian blue is indicated by the blue color of the precipitate formed in the reaction. This color change is characteristic of the compound \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \).

Step 4: Concluding the Correct Option

From the given choices, option (a) \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \) matches the description of the prussian blue colored precipitate formed due to the reaction of ferric iron with potassium ferrocyanide.

Key Concepts

Ferric Iron Reaction

Ferric iron, denoted as Fe³⁺, plays a crucial role in a variety of chemical reactions due to its ability to combine with other ions and ligands. In the Prussian blue formation, ferric iron reacts with potassium ferrocyanide in a specific way. During this process, Fe³⁺ is in its +3 oxidation state, meaning it has lost three electrons and thus has a positive charge.
Understanding this state is important because the charge influences how Fe³⁺ interacts with potassium ferrocyanide, contributing to the formation of new compounds. Ferric iron's reactivity is key to the development of complex ions and precipitates like Prussian blue. This reactivity leads to the blue coloration, which signals the completion of the reaction.

Potassium Ferrocyanide

Potassium ferrocyanide, known chemically as \( \text{K}_{4}\left[\text{Fe}(\text{CN})_{6}\right]\), is an iron-cyanide compound that forms a critical part of the reaction with ferric iron. Despite containing cyanide ions, which are toxic, potassium ferrocyanide itself is not toxic due to its stable complex form.
It acts as a ligand, providing a structure for iron to form a new compound through complexation. In prussian blue formation, the ferrocyanide ion \(\left[\text{Fe}(\text{CN})_{6}\right]^{4-}\) is essential; it interacts specifically with the ferric ions to yield the Prussian blue compound. This means that each ferrocyanide ligand can bind multiple ferric ions to form a deeply colored precipitate.

Complex Compound Formation

The formation of complex compounds is a fascinating aspect of chemistry, one that involves the combination of simpler substances to create a more intricate structure. In the case of Prussian blue, ferric iron and potassium ferrocyanide come together to form \(\text{Fe}_{4}\left[\text{Fe}(\text{CN})_{6}\right]_{3}\).
This compound is called a coordination complex because it involves a central metal ion surrounded by molecules or ions that form coordinate bonds. Each iron atom within the compound can coordinate with multiple cyanide groups, leading to a network that stabilizes the structure.
The coordination leads to the characteristic deep blue color of Prussian blue, making it not only a point of interest in chemical studies but also historically significant in art and dye industries.