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

Assertion is true and reason correctly explains it; colloidal particles adsorb ions to become charged.

Step 1: Analyze the Given Assertion

The assertion states that when silver nitrate (\(\mathrm{AgNO}_3\)) is treated with excess potassium iodide (KI), the resulting colloidal particles are attracted towards the anode. This implies that the colloidal particles acquire a positive charge, as they are moving towards the negatively charged anode.

Step 2: Break Down the Reason provided

The reason suggests that the colloidal particles become charged by adsorbing common ions from the solution. This process is known as ion adsorption, which helps them acquire a net charge. In this case, we need to understand how the ions present in the solution influence the charge of the colloid.

Step 3: Application of Common Ion Adsorption

When \(\mathrm{AgNO}_3\) is mixed with excess \(\mathrm{KI}\), initially, \(\mathrm{Ag}^+\) ions react with \(\mathrm{I}^-\) ions to form \(\mathrm{AgI}\), a sparingly soluble salt. With excess \(\mathrm{KI}\), the surface of \(\mathrm{AgI}\) particles can adsorb \(\mathrm{I}^-\) ions, giving the particles a net negative charge. However, if there is excess \(\mathrm{AgNO}_3\), \(\mathrm{Ag}^+\) ions may adsorb onto the \(\mathrm{AgI}\) surface, making them positively charged and moving towards the anode.

Step 4: Verification of Colloidal Charge

Since the assertion points towards an attraction to the anode, we conclude that in this setup, the colloidal particles are positively charged. This aligns with the situation where excess \(\mathrm{AgNO}_3\) causes \(\mathrm{Ag}^+\) ions to adsorb onto \(\mathrm{AgI}\), resulting in a positive charge.

Step 5: Evaluate the Relationship Between Assertion and Reason

The reason provided does correctly explain why colloidal particles become charged: by adsorbing ions from their surrounding medium. However, the outcome of the assertion being true in terms of which ions are adsorbed depends on whether excess \(\mathrm{AgNO}_3\) or \(\mathrm{KI}\) controls the environment. Since particles are attracted towards the anode, they have adsorbed \(\mathrm{Ag}^+\) ions in this scenario.

Key Concepts

Ion Adsorption

When substances dissolve in water, they break into ions that interact with surrounding molecules and particles. This interaction leads to the phenomenon known as ion adsorption. Ion adsorption is a process where ions from a solution are attracted and adhere to the surface of particles. In the context of colloids, this often involves tiny particles like silver iodide ( AgI ) in a solution with other salts. These particles can attract and bind ions from the dissolved salts in their surroundings to their surface.

The behavior of these ions can drastically affect the net charge on the colloidal particles. For instance:

• If AgNO_3 is in excess in a solution with KI, the Ag^+ ions could be more present at the AgI particle surface, due to their positive charge.
• This causes the colloidal particles to acquire a positive charge.

This mechanism of ion adsorption helps explain many physical behaviors of colloids, including their movement in an electric field.

Colloidal Charge

Colloids consist of tiny, insoluble particles dispersed throughout a liquid. These particles can carry an electric charge due to processes like ion adsorption. The resulting colloidal charge is pivotal in determining how these particles interact with external fields and other particles.

A colloidal particle's charge depends on the ions adsorbed on its surface:

• In a setup where AgI is in a medium of excess AgNO_3 , the adsorption of Ag^+ ions makes these particles positive.
• Alternatively, in a different scenario, surplus I^- ions could lead to negatively charged particles.

These charges lead to interactions that are crucial for applications in chemistry and industry. For example, positively charged colloids suspended in a liquid will tend to migrate towards a negatively charged electrode.

Electrophoresis

Electrophoresis is a technique used to study the movement of charged particles within an electric field. When an external electric field is applied, charged colloidal particles move towards the opposite charged electrode.

In the case of silver iodide ( AgI ) particles:

• If these particles become positively charged due to excess AgNO_3 , they would head towards the cathode, a negatively charged electrode.
• The specific page setup in our exercise shows attraction towards an anode, indicating that under certain conditions, these particles carry a positive charge.

Electrophoresis is significant as it not only helps confirm the nature of charges present on colloidal particles, but it also allows scientists to separate and analyze constituents based on their charge and size. This method is foundational in both chemical and biological research, particularly in separating proteins and nucleic acids.