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 Side A Semi-permeable Side B membrance (a) blue colour is formed towards side \(\mathrm{A}\) (b) blue colour is formed towards side \(\mathrm{B}\) (c) blue colour is formed on both the sides side (d) no blue colouration

Step-by-Step Solution

Answer

The blue color forms on side B where the solute concentration increases.

Step 1: Understand Osmosis

Osmosis is the movement of solvent molecules through a semipermeable membrane from a region of lower solute concentration to one of higher solute concentration. It results in solvent flow from the less concentrated side to the more concentrated side.

Step 2: Analyze the Reaction Between FeCl3 and K4Fe(CN)6

When \( \mathrm{FeCl}_3 \) and \( \mathrm{K}_4 \mathrm{Fe} (\mathrm{CN})_6 \) react in aqueous solution, they form a blue precipitate known as Prussian blue.

Step 3: Determine Concentration Gradient

Consider which side has higher solute concentration initially. \( \mathrm{FeCl}_3 \) and \( \mathrm{K}_4 \mathrm{Fe} (\mathrm{CN})_6 \) will not form a precipitate initially because they are separated by a semipermeable membrane that allows only solvent, not solute, to pass through.

Step 4: Predict Solvent Movement

Due to osmosis, water will move from the less concentrated side to the more concentrated side through the membrane, dragging small amounts of solute to the higher concentration side.

Step 5: Result of Osmotic Pressure

Over time, as solute and solvent molecules move, the concentration of reactants becomes sufficient on one side to allow the formation of Prussian blue.

Key Concepts

FeCl3

Ferric chloride, abbreviated as \(\mathrm{FeCl}_3\), is an iron salt consisting of iron (\( \mathrm{Fe}\)) combined with three chlorine atoms. This compound is often used in aqueous solutions where it serves as a source of ferric, or \( \mathrm{Fe}^{3+}\), ions.
In this particular reaction, \( \mathrm{FeCl}_3\) plays a crucial role by supplying the necessary ferric ions that participate in the formation of Prussian blue. It's important to note that \( \mathrm{FeCl}_3\) is typically more concentrated on one side of a setup when we consider scenarios involving a semi-permeable membrane.
This disparity in concentration initiates osmotic flow, which is vital for the reaction process, leading to the formation of the blue precipitate. \( \mathrm{FeCl}_3\) solutions are readily available and have an acidic pH, which can also influence how chemical reactions progress in aqueous environments.

K4Fe(CN)6

Potassium ferrocyanide, represented as \( \mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6\), is a compound that contains cyanide groups bound to iron. Despite the presence of cyanide, it is generally considered non-toxic in its stable state.
The role of \( \mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6\) in chemical reactions often involves its iron component, which reacts under specific conditions to form complex structures such as Prussian blue.
In our scenario, \( \mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6\) provides ferrocyanide ions (\( \mathrm{Fe}(\mathrm{CN})_6^{4-}\)) necessary to react with ferric ions from \( \mathrm{FeCl}_3\). For any reaction to take place, sufficient movement of these ions across the semipermeable membrane is essential, highlighting the importance of osmosis in this process.

Prussian blue

Prussian blue is a deep blue pigment formed when \( \mathrm{FeCl}_3\) reacts with \( \mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6\). The chemical reaction results in the formation of a complex known as ferric ferrocyanide. Prussian blue is famous not only for its color but also as an indicator in many chemical reactions.
This pigment forms when ferric ions (\( \mathrm{Fe}^{3+}\)) from \( \mathrm{FeCl}_3\) combine with ferrocyanide ions (\( \mathrm{Fe}(\mathrm{CN})_6^{4-}\)) from \( \mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6\).

• The resulting reaction can be represented by the following equation:

\[4 \mathrm{FeCl}_3 + 3 \mathrm{K}_4 \mathrm{Fe}(\mathrm{CN})_6 \rightarrow \mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3 + 12 \mathrm{KCl}\]
In settings where a semi-permeable membrane is present, Prussian blue may not form immediately. Instead, as osmotic pressure increases and more solutes cross the membrane, the concentration may eventually reach a level where this deep blue precipitate forms prominently on the side with higher solute concentration.

semi-permeable membrane

A semi-permeable membrane is a critical component in the study of osmosis. This type of membrane allows only certain molecules or ions to pass through while blocking others. The most common molecules to pass through are water molecules, while larger solute particles are retained.
In reactions involving \( \mathrm{FeCl}_3\) and \( \mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6\), a semi-permeable membrane acts to separate these two reactants initially. This separation ensures that a spontaneous reaction does not occur early on, as the membrane restricts direct contact between the two solutes.
Osmosis causes water to move from a region of lower solute concentration to higher solute concentration through this membrane. As the water moves, it can carry some solute molecules along with it, slowly creating conditions favorable for reactions like the formation of Prussian blue. Over time, the accumulation of solutes on one side can alter concentrations sufficiently to drive the intended reaction.