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 \(\mathrm{B}\) (c) blue colour is formed on both the sides side (d) no blue colouration

Step-by-Step Solution

Answer

Blue colour is formed towards side A.

Step 1: Understanding Osmosis

Osmosis is the movement of solvent molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. This process continues till equilibrium is reached.

Step 2: Analyze the Reactants

Identify the nature of the chemical reaction: - \(\mathrm{FeCl}_{3}\) is a source of \(\mathrm{Fe}^{3+}\) ions.- \(\mathrm{K}_{4} \mathrm{Fe(CN)}_{6}\) is a source of \(\mathrm{Fe(CN)}_{6}^{4-}\) ions.They can react to form a complex compound known as Prussian blue that appears blue.

Step 3: Semipermeable Membrane's Role

The semipermeable membrane allows only solvent (water) and small ions to pass through it but not large molecules like the resultant Prussian blue complex. Thus, the movement will depend largely on the concentration gradient of \(\mathrm{FeCl}_{3}\) and \(\mathrm{K}_{4} \mathrm{Fe(CN)}_{6}\) on each side.

Step 4: Determine the Direction of Solvent Flow

Identify where each component is placed in the experimental setup:- Assume side A contains \(\mathrm{FeCl}_{3}\) and side B contains \(\mathrm{K}_{4} \mathrm{Fe(CN)}_{6}\).- The solvent will move from side B to side A because \(\mathrm{K}_{4} \mathrm{Fe(CN)}_{6}\) will dissociate into more ions, resulting in a higher concentration of solute.

Step 5: Prediction of Blue Colour Formation

Once the water moves from side B to side A, the ions will collide and react on the side where more water flows into, which is side A. Hence, the blue color, indicating the formation of Prussian blue, will appear on side A.

Key Concepts

Semipermeable Membrane

A semipermeable membrane is a type of barrier that selectively allows certain substances to pass through while blocking others. This unique property plays a crucial role in processes like osmosis. Imagine a screen door that lets fresh air in but keeps bugs out. Similarly, a semipermeable membrane allows molecules like water (solvent) to move across but restricts larger molecules or specific ions. In the context of chemical reactions, such as the one involving ferric chloride (6FeCl_39) and potassium hexacyanoferrate(II) (6K_4Fe(CN)_69), this selective permeability becomes very significant. It dictates which ions can move to interact and react, thereby controlling where the resultant substances are formed. For students, it's pivotal to understand that the restriction and passage of ions or molecules affect how solutions achieve balance and where products, like Prussian blue in this exercise, will appear.

FeCl3 and K4Fe(CN)6 Reaction

When ferric chloride (6FeCl_39) and potassium hexacyanoferrate(II) (6K_4Fe(CN)_69) are in an aqueous solution, they react to form a distinctive blue compound known as Prussian blue. This reaction is a classic example of a complex ion interaction. Here's a simple breakdown of the process:

• 6FeCl_39 dissociates into 6Fe^{3+}9 ions.
• 6K_4Fe(CN)_69 dissociates into 6Fe(CN)_6^{4-}9 ions.
• These ions can then interact as they come into contact in solution, forming the Prussian blue complex.

The simplicity of the reaction hides the elegance behind ion interactions, where positive 6Fe^{3+}9 attracts the negative 6Fe(CN)_6^{4-}9 ions, leading to a visually striking result. Understanding this reaction helps demystify why certain compounds exhibit specific colors.

Prussian Blue Formation

Prussian blue is not just a captivating color; it's a product of specific chemical interactions. When 6FeCl_39 and 6K_4Fe(CN)_69 are involved, they create a compound that has long intrigued both scientists and artists due to its deep blue hue. The formation of Prussian blue occurs when the positive ferric ions (6Fe^{3+}9) react with the negative hexacyanoferrate ions (6Fe(CN)_6^{4-}9). This reaction doesn't happen indiscriminately throughout a solution but specifically where the ions meet in appropriate concentrations. In scenarios like the one depicted in the exercise, knowing where these interactions occur can determine where the blue color forms. It's about understanding the movement and interaction opportunities within the solution. A clear view of this environment helps in predicting where the vibrant Prussian blue will emerge, primarily in regions of sufficient reactant mixing.

Equilibrium in Osmosis

Equilibrium in osmosis is reached when the driving force for osmotic flow is balanced. This balance happens when the solute concentration on either side of a semipermeable membrane equalizes, or can no longer cause net movement of solvent molecules. The example of 6FeCl_39 and 6K_4Fe(CN)_69 in the exercise illustrates this. Both solutions are separated by a semipermeable membrane. As water moves from the side with lower solute concentration to the one with higher solute concentration, osmotic pressure is applied to reach equilibrium. Here’s how it works:

• The solvent, usually water, moves toward the side with more solute.
• This movement continues till the concentration of solutes on both sides becomes balanced, or until other limiting factors prevent further flow.
• It ensures substances like ions can come together to form end products at specific points in the setup, such as Prussian blue in this case.

For students, grasping equilibrium in osmosis is key to predicting the behavior of solutions separated by barriers and the resultant chemical behaviors.