Problem 174 - A Complete Resource Book in Chemistry for JEE Main

Question

Assertion: When \(\mathrm{AgNO}_{3}\) is treated with excess of potassium iodide, colloidal particles gets attracted towards anode. Reason: Colloidal particles adsorb common ions and thus become charged.

Step-by-Step Solution

Answer

Both the assertion and reason are correct, with the reason explaining the assertion.

Step 1: Understand the Assertion

The assertion states that when silver nitrate (\(\mathrm{AgNO}_{3}\)) is mixed with excess potassium iodide (\(\mathrm{KI}\)), the formed colloidal particles are attracted towards the anode. In a colloidal system, these particles gain charge and move towards an electrode of opposite charge.

Step 2: Examine the Reason

The reason provided is that colloidal particles adsorb ions from the surrounding solution. The adsorption of common ions results in colloidal particles gaining a charge, thereby causing them to move towards the opposite electrode.

Step 3: Relate the Assertion to the Reason

When \(\mathrm{AgNO}_{3}\) is treated with \(\mathrm{KI}\), silver iodide (\(\mathrm{AgI}\)) is formed, which is a typical sol. Excess \(\mathrm{I}^{-}\) ions are present, so \(\mathrm{I}^{-}\) ions adsorb onto \(\mathrm{AgI}\) particles, giving them a negative charge. Therefore, these particles move towards the positively charged anode, supporting the assertion.

Step 4: Conclusion

The assertion is correct, and the reason correctly explains that ion adsorption leads to charged colloidal particles. The process involves the adsorption of \(\mathrm{I}^{-}\) ions onto \(\mathrm{AgI}\), confirming both the assertion and reason.

Key Concepts

Ion Adsorption

Ion adsorption is a vital concept in colloidal chemistry. It refers to the process where ions from a solution adhere to the surface of colloidal particles. This phenomenon is crucial because it contributes to the stability and behavior of colloidal systems. When ions are adsorbed onto colloidal particles, they impart a charge, helping prevent the particles from clumping together. This is because particles with the same charge repel each other.

In the context of silver iodide (\(\mathrm{AgI}\)) formation, ion adsorption occurs when iodide ions (\(\mathrm{I}^{-}\)) adhere to silver iodide particles. These negatively charged ions create a shield around the particles, attracting positive ions from the surrounding, creating a stable colloidal dispersion. This adsorption significantly influences the movement of particles in an electric field, as charged particles move towards electrodes with the opposite charge.

Ion adsorption ensures the persistence of colloidal properties, defining their interaction with other substances in the solution. It is integral to technologies that rely on the manipulation of colloid stability, such as in water purification processes. Understanding ion adsorption mechanisms enables chemists to predict and control the behavior of colloidal systems, which is essential in both industrial and laboratory settings.

Silver Iodide Formation

The formation of silver iodide (\(\mathrm{AgI}\)) is a classic example of a reaction producing a colloidal sol. This occurs when solutions of silver nitrate (\(\mathrm{AgNO}_{3}\)) and potassium iodide (\(\mathrm{KI}\)) are mixed. Silver iodide, an insoluble salt, precipitates out as a colloidal suspension. This process is a prime illustration of a double displacement reaction, where the silver ions (\(\mathrm{Ag}^{+}\)) react with iodide ions (\(\mathrm{I}^{-}\)) to form a solid product.

The \(\mathrm{AgI}\) particles formed have distinctive properties due to their small size, allowing them to remain suspended throughout the liquid medium. In this sol form, the particles are large enough to scatter light (termed the Tyndall effect) but small enough to remain evenly dispersed without settling out.

This formation is of great importance due to silver iodide's unique capability to act as a weather modification agent in cloud seeding. When dispersed into clouds, \(\mathrm{AgI}\) can initiate or enhance precipitation, demonstrating the significant applications of colloidal chemistry beyond fundamental science.

Electrophoresis in Colloids

Electrophoresis is the process of moving dispersed particles through a fluid under the influence of a charged field. This phenomenon is essential in understanding the behavior of colloids in electrochemical environments. In colloidal chemistry, electrophoresis helps identify the charge properties of colloidal particles.

When colloidal particles, such as silver iodide (\(\mathrm{AgI}\)) in a solution with excess iodide ions (\(\mathrm{I}^{-}\)), gain a negative charge, they migrate towards the anode (positive electrode) when an electric field is applied. This movement affirms their charged nature due to ion adsorption, as particles migrate toward oppositely charged electrodes.

The technique is widely used for the separation and analysis of complex mixtures in biochemistry and molecular biology, such as in DNA analysis, blood sample processing, and quality control of pharmaceuticals.

• It provides insights into the size, charge, and purity of colloidal solutions.
• A useful tool for controlling separation processes in laboratories and industrial applications.

Understanding electrophoresis is crucial for advanced studies in colloid science and its practical applications in numerous scientific fields.