Problem 50 - A Complete Resource Book in Chemistry for JEE Main

Question

Perssian blue is formed when (a) ammonium sulphate reacts with \(\mathrm{FeCl}_{3}\) (b) ferrous sulphate reacts with \(\mathrm{FeCl}_{3}\) (c) ferrous ammonium sulphate reacts with \(\mathrm{FeCl}_{3}\) (d) ferric sulphate reacts with \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)

Step-by-Step Solution

Answer

Persian blue is formed when ferric sulphate reacts with K_4[Fe(CN)_6] .

Step 1: Identify What is Persian Blue

Persian blue, also known as Prussian blue, is a dark blue pigment used in paints and dyes. It is chemically composed of iron ( Fe ) and cyanide ions.

Step 2: Analyze Reaction Involving FeCl_3

To form Prussian blue, Fe^{3+} ions from FeCl_3 must react with [Fe(CN)_6]^{4-} ions, as these reactions typically involve these ions.

Step 3: Examine Option (a): Ammonium Sulphate and FeCl_3

Ammonium sulphate does not provide the necessary [Fe(CN)_6]^{4-} ions to form Prussian blue. This option is incorrect.

Step 4: Examine Option (b): Ferrous Sulphate and FeCl_3

Ferrous sulphate does not contain the cyanide ions needed for the formation of Prussian blue. This option is incorrect.

Step 5: Examine Option (c): Ferrous Ammonium Sulphate and FeCl_3

Ferrous ammonium sulphate also lacks the [Fe(CN)_6]^{4-} ion component necessary for the formation of Prussian blue. This option is incorrect.

Step 6: Examine Option (d): Ferric Sulphate and K_4[Fe(CN)_6]

This reaction involves Fe^{3+} ions from ferric sulphate reacting with [Fe(CN)_6]^{4-} ions from K_4[Fe(CN)_6] , which are the necessary ions to form Prussian blue. This option is correct.

Key Concepts

Iron Complexes

Iron complexes are fascinating chemical compounds formed by the combination of iron atoms with various ligands, which are ions or molecules that can donate electron pairs to the metal. These complexes are quite common in the field of chemistry, particularly when discussing transition metals like iron.
In the formation of Prussian blue, the iron present is in two oxidation states: iron(II) or ferrous ( Fe^{2+} ) and iron(III) or ferric ( Fe^{3+} ). The transition between these states is crucial for the formation of iron complexes. The unique combinations of oxidation states allow iron to form the network structure of Prussian blue, giving it that distinct dark blue color.

• Fe forms stable Fe^{3+} complexes which play a key role in creating the Prussian blue pigment.
• These complexes are often involved in the transfer of electrons, contributing to important chemical properties like magnetism and color.

Cyanide Ion

The cyanide ion ( CN^{-} ) might be small, but it has a large impact in the world of chemistry, particularly in the formation of Prussian blue. In Prussian blue, CN^{-} acts as a ligand, bonding with iron to form complex structures like [Fe(CN)_6]^{4-} .
This coordination of cyanide ions with iron leads to the formation of the stable structures needed for the pigment.

• Cyanide ions bond strongly to the metal, due to their ability to donate electron pairs to the iron atoms.
• This strong bond is crucial for forming the robust three-dimensional structures observed in Prussian blue.
Despite its association with poisons, in chemistry, the CN^{-} ion is valuable for its strong and stable metal coordination properties.

Chemical Reaction Analysis

Analyzing chemical reactions involves understanding how different substances interact to form new products. In the case of Prussian blue formation, the reaction requires a specific combination of reactants.
This involves ferric ions ( Fe^{3+} ) reacting with hexacyanoferrate(II) ions ( K_4[Fe(CN)_6] ).

• The reaction is a fine example of a redox process where iron exchanges electrons, impacting its oxidation state and enabling the complex formation.
• Choosing the correct reactants is critical so that all necessary ions are present for the reaction to occur.
• Option (d) from the original exercise highlights this precise combination, where ferric sulfate provides the Fe^{3+} ions and potassium hexacyanoferrate(II) provides the [Fe(CN)_6]^{4-} ions.

Understanding these reactions requires breaking down the transfer of electrons and the way ions interact, which is at the heart of chemical reaction analysis.