Question

Which of the following has minimum flocculation value for positively charged sol? (A) \(\mathrm{Cl}^{-}\) (b) \(\mathrm{SO}_{4}^{2-}\) (C) \(\mathrm{PO}_{4}^{3-}\) (d) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\)

Short answer

Option (d) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\) has the minimum flocculation value for positively charged sol.

Step-by-step solution

Understand the Concept of Flocculation Value

The flocculation value of an ion is the minimum concentration of that ion required to cause the flocculation or coagulation of a sol. For a positively charged sol, anions with a higher charge will have a lower flocculation value, due to the stronger electrostatic attraction.

Determine the Charge of Each Anion

Identify the charges of the given anions. Chloride (Cl-) has a charge of -1, sulfate (SO4^2-) has a charge of -2, phosphate (PO4^3-) has a charge of -3, and hexacyanoferrate ([Fe(CN)6]^4-) has a charge of -4.

Compare the Charges to Find the Minimum Flocculation Value

The anion with the highest negative charge will have the lowest flocculation value for a positively charged sol. In this case, hexacyanoferrate ([Fe(CN)6]^4-) has the highest charge of -4.

Choose the Correct Option

Based on the charges of the anions, hexacyanoferrate ([Fe(CN)6]^4-) with a charge of -4 will have the minimum flocculation value. Therefore, the correct answer is option (d) [Fe(CN)6]^4-.

Key concepts

Electrostatic Attraction in Colloids

Understanding electrostatic attraction in colloids is foundational to comprehending various phenomena in physical chemistry, including flocculation. Colloids are mixtures where one substance is dispersed evenly throughout another, and the particles of the dispersed phase, known as colloidal particles, carry an electric charge. These charges can be either positive or negative, leading to a complex balance of forces.

Electrostatic attraction refers to the force that holds oppositely charged particles together. In the context of colloids, it plays a crucial role in stabilizing the colloidal system. Positively charged particles will attract negatively charged ions (anions), and vice versa. This attraction can prevent the particles from settling and help maintain the dispersion. However, when excess ions with a charge opposite to that of the colloidal particles are introduced into the system, this balance is disrupted. It leads to neutralization of the charge and causes aggregation or flocculation of the colloidal particles, which then may precipitate out of the suspension.

Ion Charge and Coagulation

The process of coagulation, or the aggregation of particles, is intimately linked with the charge of ions present in a colloidal system. Coagulation can occur when the ions' charges neutralize the charges on the colloidal particles, effectively reducing the repulsive forces that keep them apart.

The flocculation value is an important concept that quantifies the tendency of a colloid to aggregate. It represents the minimum concentration of an ion necessary to induce the sol to coagulate. Generally, ions with a higher charge (higher valency) will have a lower flocculation value because they can neutralize the charge on the colloidal particles more efficiently. This is highly relevant in water treatment, medicine, and materials science where controlled flocculation is often required. A detailed understanding of the relationship between ion charge and coagulation is fundamental for students preparing for competitive exams such as the Joint Entrance Examination (JEE) in physical chemistry.

Physical Chemistry JEE

Physical Chemistry forms a significant part of the Joint Entrance Examination (JEE), which assesses the aptitude of students for engineering and other scientific fields. The curriculum includes a deep dive into the behaviour of matter on a molecular and atomic level, such as the principles governing colloid chemistry.

Topics like electrostatic attraction and ion charge, which are essential for understanding processes like coagulation and flocculation, are critical aspects of this section. Students are expected to not only memorize the concepts but also apply them to solve complex problems efficiently. The JEE puts a strong emphasis on conceptual clarity and problem-solving skills, which are cultivated through practice and a strong understanding of fundamental theories such as those discussed in the exercise on flocculation values.