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) \(\mathrm{K}_{3}[\mathrm{Fe}(\mathrm{CN})]\) (d) \(\mathrm{Fe}(\mathrm{CNS})_{3}\)

Step-by-Step Solution

Answer

The prussian blue precipitate is \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\), which is option (a).

Step 1: Understanding the Reaction

The reaction involves ferric iron (Fe³⁺) reacting with potassium ferrocyanide (\(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)) to form a Prussian blue precipitate. We need to identify the correct chemical formula of this precipitate.

Step 2: Identifying the Chemical Precipitate

Prussian blue is a well-known coordination compound formed when ferric ions react with ferrocyanide ions. The chemical name for Prussian blue is iron(III) hexacyanoferrate(II), and its formula is \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\).

Step 3: Confirming the Correct Option

We match the identified compound \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) with the options given in the question. Option (a) corresponds to this formula.

Key Concepts

Ferric Iron Reactions

Ferric iron, denoted as Fe³⁺, is a form of iron in a +3 oxidation state. This is a common form of iron in natural environments and in many chemical reactions. When ferric iron participates in reactions, its positive charge means that it is always seeking electrons from other molecules or compounds to achieve a stable electron configuration.

In coordination chemistry, ferric iron can form complexes by binding to various ligands. Ligands are ions or molecules that can donate pairs of electrons to the metal ions. These complexes are called coordination compounds. The nature of the resulting compound depends on the ligands and the specific reaction conditions.

In reactions involving ferric iron, it may associate with negative ions or electron-donating molecules to form solid precipitates. These precipitates can have distinct colors depending on the ligands involved, such as the characteristic deep blue seen in Prussian blue when reacting with ferrocyanides.

Prussian Blue Formation

Prussian blue is one of the most well-known coordination compounds formed when ferric ions react with ferrocyanide ions. The iconic blue color results from a particular arrangement of iron and cyano groups in the chemical structure. This compound is scientifically referred to as iron(III) hexacyanoferrate(II), with the formula \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\).

The formation of Prussian blue involves a reaction where Fe³⁺ ions interact with \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\), which provides the ferrocyanide ions \([\mathrm{Fe}(\mathrm{CN})_{6}]^{4-}\). The resulting compound has a complex lattice structure, giving it a stable and vibrant hue. This color and its formation make it a valuable pigment and a significant subject of study for its chemical properties.

In laboratory settings, the production of Prussian blue is a fascinating illustration of how coordination chemistry can lead to compounds with distinct visual characteristics. It also shows how the interaction between metal ions and ligands can result in new materials with unique properties.

Potassium Ferrocyanide

Potassium ferrocyanide, chemically represented as \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\), plays a crucial role in forming Prussian blue. As a coordination compound, it consists of a ferrocyanide ion \([\mathrm{Fe}(\mathrm{CN})_{6}]^{4-}\) combined with potassium ions (K⁺). Each ferrocyanide ion serves as a multi-dentate ligand, meaning it can attach to central metal ions at multiple points.

Despite its name suggesting cyanide content, potassium ferrocyanide is quite safe to handle in non-acidic conditions. This is because the cyanide ions are tightly bound to the iron, making them non-reactive under normal circumstances.

When used in reactions with ferric ions, the potassium ions dissociate, and the remaining ferrocyanide ions interact with the ferric ions to form the Prussian blue precipitate. This transformation is a classical example of coordination chemistry in action, highlighting how metal ions and complex ligands interact to create compounds with new properties.