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

The assertion is false; the reason is true.

Step 1: Understand the Assertion

The assertion states that when silver nitrate (\(\mathrm{AgNO}_3\)) is treated with excess potassium iodide (\(\mathrm{KI}\)), colloidal particles are attracted towards the anode. This suggests that the colloidal particles have a net positive charge.

Step 2: Examine the Chemical Reaction

The reaction between \(\mathrm{AgNO}_3\) and \(\mathrm{KI}\) produces silver iodide (\(\mathrm{AgI}\)) and potassium nitrate (\(\mathrm{KNO}_3\)). In the presence of excess \(\mathrm{KI}\), \(\mathrm{AgI}\) particles will adsorb \(\mathrm{I}^-\) ions from the solution.

Step 3: Analyze the Charge on Colloidal Particles

Since \(\mathrm{AgI}\) colloidal particles adsorb iodide ions (\(\mathrm{I}^-\)), they gain a negative charge. However, the assertion mentions that particles are attracted towards the anode, indicating a net positive charge, which contradicts the behavior when excess \(\mathrm{KI}\) is present.

Step 4: Evaluate the Reason

The reason given states that colloidal particles adsorb common ions and thus become charged. This understanding is correct; however, in the context of the assertion, we need to determine if this explains why the colloidal particles would have a positive charge.

Step 5: Determine the Truth of Assertion and Reason

The assertion is false if the particles are attracted to the anode under excess \(\mathrm{KI}\), as these particles should actually be negatively charged. However, the reason is true about particle charge being due to adsorption.

Key Concepts

Charge on Colloids

In the world of colloidal chemistry, understanding the charge on colloids is crucial. Colloidal particles, though too small to settle out by gravity, possess a fascinating ability to carry an electric charge. This charge plays an essential role in the stability of colloids. Without it, the particles would clump together, leading to precipitation.
Colloids can be either positively or negatively charged, and their charge depends on the type of ions they adsorb from the surrounding solution. For example, if a colloidal particle adsorbs anions, it becomes negatively charged. Conversely, adsorbing cations renders it positively charged. This charge characteristic affects how colloids interact with electric fields, such as in electrophoresis experiments where charged particles move in response to an electric field.
The overall charge on a colloidal system can determine its behavior during reactions. For example, colloids with like charges repel each other, aiding in their suspension within the solution. This phenomenon is essential in various applications, from drug delivery systems to the food industry.

• Charge stabilizes colloids and prevents aggregation.
• The type of charge depends on the ions adsorbed from the solution.
• Colloids move in specific directions in electric fields, depending on their charge.

Adsorption in Colloids

Adsorption is the process by which atoms, ions, or molecules adhere to a surface. In the context of colloids, adsorption refers to the accumulation of particles on the surface of colloidal particles. This adsorption is a key factor in determining the charge and stability of colloids.
When a colloid particle is exposed to a solution, it can attract and hold various ions on its surface. For example, \[\mathrm{AgI}\] colloids can adsorb \(\mathrm{I}^-\) ions, which influences their net charge. Adsorption helps colloids maintain their stability by preventing coagulation, where particles clump together and settle out of the solution.
Understanding adsorption processes sheds light on why certain substances enhance or reduce colloid stability. This knowledge benefits industries like pharmaceuticals and food technology. For instance, controlled adsorption of specific ions can modify the behavior of colloids in drug delivery, ensuring they reach the intended target within the body effectively.

• Adsorption determines colloidal charge and stability.
• Colloid surfaces attract ions from the surrounding solution.
• It is crucial for preventing unwanted particle coagulation.

Colloid Formation and Behavior

Colloid formation involves dispersing a substance (dispersed phase) in another (dispersion medium). To understand colloid behavior, one must grasp the concept of phases within the medium — one dispersed and the other continuous.
Colloids differ from true solutions in that their particles are larger, resulting in unique behaviors such as light scattering, also known as the Tyndall effect. This property is due to the particles being large enough to scatter light, making colloids visibly distinct.
Upon formation, the stability of colloids can be influenced by external conditions, such as temperature and the presence of electrolytes. These can affect how the dispersed particles interact with each other and the medium. For instance, the addition of excess electrolytes can lead to coagulation if they neutralize the charge on the particles.

• Colloids form from a dispersed and a continuous phase.
• Colloidal size causes light scattering (Tyndall effect).
• Stability is affected by factors like temperature and electrolyte presence.

Understanding these concepts of colloid formation and behavior is essential in fields where manipulating colloidal properties is critical, from industrial applications to biomedical technologies.