Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{FeCl}_{3}\) reacts with \(\mathrm{K}_{4} \mathrm{Fe}(\mathrm{CN})_{6}\) in aqueous solution to give blue colour if these two are separated by a semipermeable membrance (as shown in the figure), then due to osmosis (a) blue colour is formed towards side \(\mathrm{A}\) (b) blue colour is formed towards side B (c) blue colour is formed on both the sides side (d) no blue colouration

Step-by-Step Solution

Answer

(a) Blue color is formed towards side A.

Step 1: Understanding Osmosis

Osmosis is the movement of solvent molecules through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration until equilibrium is reached.

Step 2: Analyze Ion Molarity

In this reaction, semipermeable membranes allow only solvents to pass but not solutes like \(\mathrm{FeCl}_{3}\) and \(\mathrm{K}_{4} \mathrm{Fe} (\mathrm{CN})_{6}\). Side B contains a solution with more iron and potassium ions compared to side A which represents a concentration gradient.

Step 3: Predict Osmosis Direction

As osmosis directs solvent from low to high solute concentration, water from side A (less concentrated) will move towards side B (more concentrated). This dilutes side B, where the blue color formation will occur.

Step 4: Reaction Conditions

For the formation of the blue color (Prussian blue), \(\mathrm{FeCl}_3\) in side A must react with \(\mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6\) in side B. This forms \(\mathrm{Fe}_4\mathrm{[Fe}\mathrm{(CN)}_6]_3\) which is insoluble and appears blue.

Key Concepts

Osmosis

Osmosis is a fascinating natural phenomenon that involves the movement of solvent molecules through a semi-permeable membrane. This membrane allows only certain molecules to pass through while blocking others. Think of osmosis as water attempting to balance its concentration across a barrier.
When you have a solution on two sides of this membrane, solvent particles (like water molecules) will move from an area with a lower concentration of solutes (dissolved substances) to an area with a higher concentration of solutes. This movement continues until the concentration on both sides becomes balanced.
For example, imagine our situation with \ \ \( \mathrm{FeCl}_3 \) and \( \mathrm{K}_4 \mathrm{Fe} ( \mathrm{CN} )_6 \) separated by a membrane. Osmosis would drive the solvent from side A to side B since the latter has more ions dissolved in it. This movement results in what we call equilibrium - a state where the concentration gradient of the solution no longer exists between the two sides.

Iron Complexes

Iron complexes are unique compounds comprised of iron atoms combined with other atoms or molecules, often through coordination bonds. These complexes play a critical role in color changes during chemical reactions.
In the context of our experiment, iron from \( \mathrm{FeCl}_3 \) can form complex ions when it interacts with other components like cyanide ions from \( \mathrm{K}_4 \mathrm{Fe} ( \mathrm{CN} )_6 \). The iron in these complexes often exists in different oxidation states, such as +2 (ferrous) and +3 (ferric), which can influence their reactivity and the color they exhibit.
Overall, these complexes are pivotal for creating Prussian Blue, as they contribute to the solid structure with intense coloration.

Prussian Blue Formation

The formation of Prussian Blue is a classic demonstration of chemical reactions leading to color changes. In chemical terms, Prussian Blue forms from the reaction between ferric ions \( ( \mathrm{Fe}^{3+} ) \) in \( \mathrm{FeCl}_3 \) and ferrocyanide ions \( ( \mathrm{Fe} (\mathrm{CN})_6^{4-} ) \) in \( \mathrm{K}_4 \mathrm{Fe} ( \mathrm{CN} )_6 \).
When osmosis brings the solutions into contact, the ferric and ferrocyanide ions interact, producing an insoluble compound called ferric ferrocyanide, or more commonly known as Prussian Blue. This compound gives rise to a distinct blue color due to its unique structure, where it forms a framework trapping the iron ions in a lattice.
Primarily, Prussian Blue's intense color makes it useful in pigments and dyes. However, it is the result of a relatively straightforward reaction involving the interplay of osmosis and iron complex formation in a controlled environment.