Problem 81 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following colloidal solution is prepared when very dilute solution of ferric chloride reacts with potassium ferrocyanide? (a) Prussian blue (b) Arsenic sulphide (c) Ferric hydroxide (d) Extrinsic colloid

Step-by-Step Solution

Answer

Prussian blue (a)

Step 1: Identify Reactants and Reaction

The problem provides us with the reactants: a very dilute solution of ferric chloride (FeCl₃) and potassium ferrocyanide (K₄[Fe(CN)₆]). We need to determine the product formed when these reactants combine.

Step 2: Recall Reaction Outcome

When ferric chloride reacts with potassium ferrocyanide, one of the well-known reactions leads to the formation of Prussian blue. This is due to the formation of an insoluble precipitate of iron ferrocyanide.

Step 3: Determine Product as a Colloid

Prussian blue is a well-established example of a colloidal solution or a colloid. It is the result of the reaction between FeCl₃ and K₄[Fe(CN)₆] forming a colloidal suspension of iron(III) ferrocyanide.

Step 4: Verify the Correct Choice

The product that matches the description of a colloidal solution formed from these reactants is Prussian blue, which matches option (a).

Key Concepts

Prussian Blue

Prussian Blue is a fascinating substance known for its striking deep blue color. It was first discovered in the early 18th century and has since been used in various applications such as pigments for paints, dyes, and as a medicine to treat metal poisoning. Prussian Blue is scientifically recognized as iron(III) ferrocyanide. The vivid color results from its unique chemical structure and the way it absorbs light.

This compound forms during specific chemical reactions and is a classic example of a colloidal solution. Its ability to form from dissolved iron and cyanide complexes in water makes it a pertinent subject in inorganic chemistry and colloidal studies. Prussian Blue is not just a pretty color; it is a reminder of the delicate dance of chemistry between iron ions and the cyanide group that results in something both helpful and beautiful.

Iron Ferrocyanide Reaction

The reaction between ferric chloride (FeCl₃) and potassium ferrocyanide (K₄[Fe(CN)₆]) is an excellent showcase of the formation of complex compounds. When these two substances are combined in solution, they undergo a chemical process known as the iron ferrocyanide reaction. This results in the formation of iron(III) ferrocyanide, or Prussian Blue.

The reaction can be understood through the interaction of the ferric ions from FeCl₃ and the ferrocyanide ions from K₄[Fe(CN)₆]. When they come together in solution, they form a complex precipitate that is insoluble in water. This insolubility is key to Prussian Blue's formation, as it allows the compound to remain suspended in the solution, rendering it a colloid. This reaction is sensitive to the concentration of the reactants, and a very dilute mixture is ideal for observing the signature deep blue.

• Reactants: Ferric chloride and potassium ferrocyanide.
• Product: Iron(III) ferrocyanide precipitate.

Insoluble Precipitate

An insoluble precipitate is a solid product that forms out of a solution during a chemical reaction and does not dissolve in the solvent. In the case of Prussian Blue, the reaction between ferric chloride and potassium ferrocyanide forms this insoluble solid. When these components meet in solution, they quickly bind together to create a solid that falls out, or precipitates, from the solution.

This characteristic of forming a solid is important for creating many types of materials in chemistry, including colloids. Insoluble precipitates are often employed in purification and analytical processes to isolate or identify specific ions or compounds. The strong color and stability of Prussian Blue as an insoluble precipitate are what set it apart in the formation of colloids.

Colloid Formation

Colloids are mixtures where fine particles are dispersed within a continuous medium without settling out or dissolving. Prussian Blue serves as a prime example of colloid formation. When the iron ferrocyanide reaction takes place, the product does not fully settle due to its finely dispersed particles, instead, forming a colloidal suspension.

In colloid chemistry, the size of the dispersed particles is crucial – they must be large enough to scatter light, which is why Prussian Blue can appear so deeply colored. However, these particles are also small enough that they don't just settle out of the suspension. This balance creates a stable mixture often observed in colloids. Colloids are found in various industries and everyday products, from paints to food products, highlighting their versatility and importance in both practical and scientific contexts.

• Stable dispersion without settling.
• Characteristic intense deep blue of Prussian Blue.
• Visible light scattering due to particle size.