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

(c) \( \text{AlCl}_3 \) has the maximum flocculation value.

Step 1: Understanding Flocculation Value

A flocculation value is the minimum concentration of an electrolyte required to cause the precipitation or coagulation of a sol. According to Hardy–Schulze rule, the greater the valency of the oppositely charged ion, the greater is its ability to cause coagulation. We need to identify the electrolyte with the highest valency to coagulate the negatively charged sol.

Step 2: Analyzing Valencies

Let's determine the valency of each cation from the given options: - Ba in BaCl2 has a valency of +2. - Na in NaCl has a valency of +1. - Al in AlCl3 has a valency of +3. - Mg in MgCl2 has a valency of +2. The one with the highest valency will have the maximum flocculation value.

Step 3: Comparing and Finding the Maximum

The cation with the highest valency among the given options is Al with a valency of +3 in AlCl3. Hence, AlCl3 will have the maximum flocculation value for the negatively charged sol.

Key Concepts

Hardy–Schulze Rule

The Hardy–Schulze Rule is pivotal when studying the coagulation of sols. It provides insight into how different electrolytes affect sol particles. In simple terms, this rule states that the power of an electrolyte to cause the coagulation of a sol is primarily dependent on the valency of the ion that is opposite in charge to the sol particles. In the problem at hand, the rule helps us understand why certain electrolytes are more effective than others at inducing flocculation.

According to the Hardy–Schulze Rule:

• Higher valency ions have greater coagulating power for sols with an opposite charge.
• The more charges the ion carries, the more effectively it brings about coagulation.

For instance, a trivalent ion such as \({\text{Al}^{3+}}\) is more effective than a divalent ion like \({\text{Ba}^{2+}}\) or \({\text{Mg}^{2+}}\) when it comes to coagulating a negatively charged sol. The rule essentially emphasizes the crucial role of charge interactions in the stability and destabilization of colloidal systems.

Electrolyte Valency

Valency is a critical concept in understanding how electrolytes interact with colloidal particles. It represents the combining capacity of an ion and is a central factor in determining an electrolyte's coagulation ability according to the Hardy–Schulze Rule.

When evaluating electrolyte valency:

• The higher the valency, the more significant the effect on the negatively charged sol.
• Each unit increase in valency results in a disproportionately higher ability to coagulate or flocculate a sol.

Given the options in the exercise:- \({\text{NaCl}}\) features Na with a valency of +1.- \({\text{BaCl}_{2}}\) and \({\text{MgCl}_{2}}\) have Ba and Mg ions of valency +2.- \({\text{AlCl}_{3}}\) comes with Al having a valency of +3.The greater the valency, the more effective the electrolyte in achieving coagulation of the negatively charged sol. Thus, AlCl3 is the most suited for this purpose due to its highest valency.

Coagulation of Sols

Coagulation refers to the process where sol particles, initially stable due to charge repulsion, come together to form larger aggregates or flocs. This process is largely influenced by electrolytes, which contain ions capable of neutralizing the charges on sol particles.

Key aspects of coagulation:

• It results in the settling of dispersed sol particles.
• The effectiveness is highly reliant on ion valency, as per the Hardy–Schulze Rule.
• Achieved through the addition of oppositely charged electrolytes that reduce repulsion among particles.

In the context of our problem, observing which electrolyte has the highest capacity for causing coagulation is essential. AlCl3, carrying the \(\text{Al}^{3+}\) ion, effectively neutralizes the charge on a negatively charged sol, facilitating coagulation. Understanding this concept allows students to see the role of different ionic compounds in scientific and industrial applications where coagulation is necessary, such as in water treatment processes.