Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has maximum flocculation value for a negatively charge sol? (a) \(\mathrm{BaCl}_{2}\) (b) \(\mathrm{NaCl}\) (c) \(\mathrm{AlCl}_{3}\) (d) \(\mathrm{MgCl}_{2}\)

Step-by-Step Solution

Answer

The maximum flocculation value is for \\( \text{AlCl}_3 \\) (option c).

Step 1: Understanding Flocculation Value

Flocculation value refers to the concentration of an electrolyte required to cause the coagulation (or flocculation) of a sol. In the case of a negatively charged sol, the flocculating power is higher for electrolytes with cations having higher valency.

Step 2: Analyzing Valency of Cations

List the cations present in each option: (a) \[ ext{Ba}^{2+}\](b) \[ ext{Na}^{+}\](c) \[ ext{Al}^{3+}\](d) \[ ext{Mg}^{2+}\]The valency of these cations is 2 for \[ ext{Ba}^{2+}\] and \[ ext{Mg}^{2+}\], 1 for \[ ext{Na}^{+}\], and 3 for \[ ext{Al}^{3+}\].

Step 3: Applying Hardy-Schulze Rule

According to the Hardy-Schulze rule, the coagulation (flocculating) power of an electrolyte is directly proportional to the valency of the oppositely charged ion. Therefore, \[ ext{Al}^{3+}\] (option c) has the highest flocculating power due to its highest valency of 3.

Key Concepts

Hardy-Schulze Rule

The Hardy-Schulze Rule plays a fundamental role in understanding how electrolytes affect colloidal stability. This rule states that the coagulating power of ions increases with their ionic charge or valency. Simply put, the higher the valency of the ions, the more effective they are in neutralizing and destabilizing the charges on colloidal particles. This effect results from the increased ability of multivalent ions to neutralize a greater number of opposite charges on the colloidal particles.

For example, in a negatively charged colloidal solution, cations with higher positive charges will be more efficient at reducing the repulsive forces between the particles. As they reduce these forces, it makes the particles more prone to coming together, resulting in coagulation. In the context of the exercise, this means that a cation with a higher charge, like Al³⁺, will have a stronger influence in causing the sol particles to flocculate compared to cations with lower charges.

Valency of Cations

Cations are positively charged ions, and their valency is a measure of their ability to combine with other ions. The valency of a cation plays a crucial role in determining its effectiveness in causing the coagulation of sols. Specifically, the greater the number of charges or valency of a cation, the more potent it is in stabilizing negatively charged colloidal particles.

Let's consider the individual cations from the exercise options:

• Barium (\(\text{Ba}^{2+}\)) has a valency of 2.
• Sodium (\(\text{Na}^{+}\)) has a valency of 1.
• Aluminum (\(\text{Al}^{3+}\)) has a valency of 3.
• Magnesium (\(\text{Mg}^{2+}\)) also has a valency of 2.

The aluminum ion, with a valency of 3, is the most effective in causing coagulation due to its higher charge. This higher valency allows it to neutralize a greater amount of the negative charge on the sol particles, facilitating their aggregation.

Coagulation of Sols

Coagulation is a process where particles in a sol come together to form larger aggregates, leading to the settling out of these particles from the dispersion medium. For negatively charged sols, cations are important in the coagulation process as they neutralize the negative charges.

The efficiency of different cations in coagulating sols can be predicted by understanding their valency. According to the principles of coagulation, multivalent ions are significantly more effective than monovalent ions. This is because the increased charge of multivalent ions enhances their ability to neutralize opposing charges over a larger area of colloidal particles.

To summarize, when we consider ions like \(\text{Al}^{3+}\), with a valency of 3, they exhibit strong coagulating power due to their ability to effectively neutralize the surface charge of a sol, promoting particle aggregation and thus flocculation. This is why in an exercise comparing different cationic coagulants, \(\text{Al}^{3+}\) would typically be the most effective.