Problem 195 - A Complete Resource Book in Chemistry for JEE Main

Question

The coagulating power of electrolytes having ions \(\mathrm{Na}^{+}, \mathrm{Al}^{3+}\) and \(\mathrm{Ba}^{2+}\) for arsenic sulphide sol increases in the order: (a) \(\mathrm{Ba}^{2+} \mathrm{Na}^{+}<\mathrm{Al}^{3+}\) (b) \(\mathrm{Al}^{3+}<\mathrm{Na}^{+} \mathrm{Ba}^{2+}\) (c) \(\mathrm{Al}^{3+}<\mathrm{Ba}^{2+}<\mathrm{Na}^{+}\) (d) \(\mathrm{Na}^{+}<\mathrm{Ba}^{2+}<\mathrm{Al}^{3}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{Na}^{+}<\mathrm{Ba}^{2+}<\mathrm{Al}^{3+}\)

Step 1: Understand Coagulation and Charge

Coagulation is the process in which particles in a colloid come together to form a mass. According to Hardy-Schulze rule, the greater the valency (charge) of the coagulating ion, the more effective it is in causing coagulation.

Step 2: Analyze the Charges of Ions

Examine the charges of the given ions: \(\mathrm{Na}^{+}\) has a +1 charge, \(\mathrm{Ba}^{2+}\) has a +2 charge, and \(\mathrm{Al}^{3+}\) has a +3 charge.

Step 3: Evaluate Coagulating Power

Apply the principle that a higher charge leads to faster coagulation. Therefore, the coagulating power should be in increasing order of \(\mathrm{Na}^{+} (1) < \mathrm{Ba}^{2+} (2) < \mathrm{Al}^{3+} (3)\).

Step 4: Choose the Correct Option

Given the analysis of charges and coagulating power, select the option where coagulating power increases as \(\mathrm{Na}^{+} < \mathrm{Ba}^{2+} < \mathrm{Al}^{3+}\). The correct answer is (d).

Key Concepts

Hardy-Schulze Rule

The Hardy-Schulze Rule plays a crucial role in understanding how colloids behave in the presence of different electrolytes. This classical rule states that the effectiveness of an ion in coagulating a colloid depends primarily on the charge of the ion. The higher the valency (or charge) of the ion, the stronger its ability to neutralize the charges of the colloidal particles.
It essentially means that multivalent ions are more effective in bringing about coagulation than monovalent ones. When the charged ions neutralize the oppositely charged colloidal particles, the particles start to cluster together, leading to coagulation.
By applying this rule, we can predict how a particular electrolyte will affect the stability of a colloid. For example, when considering ions such as \(\mathrm{Na}^{+}, \mathrm{Ba}^{2+},\) and \(\mathrm{Al}^{3+},\) the Hardy-Schulze Rule would predict that \(\mathrm{Al}^{3+}\) would be the most effective because it has the highest charge, and \(\mathrm{Na}^{+}\) would be the least effective since it has the smallest charge.
Remember, this rule doesn't depend on the size of the ion, but solely on its charge, making it a simple yet powerful tool in predicting coagulation behaviors in colloids.

Valency and Coagulation

Valency refers to the ability of an element or ion to combine with other ions, which in the context of coagulation, is all about charge. The higher the valency, the stronger the capability of the ion to bring about coagulation. In terms of colloids, coagulation is when the colloidal particles aggregate to form larger clusters, which eventually settle out of the solution.
For example, when ions are introduced to a colloid such as an arsenic sulfide sol, the difference in their valency plays a monumental role in their coagulating power:

• \(\mathrm{Na}^{+}:\) With a valency of +1, these ions are less effective because they provide minimal charge to neutralize the particles.
• \(\mathrm{Ba}^{2+}:\) Having a valency of +2, these ions are more effective than sodium ions, doubling the charge they can neutralize.
• \(\mathrm{Al}^{3+}:\) With the highest valency of +3, these ions offer the utmost coagulating effectiveness by providing the greatest charge for neutralization.

The significance of valency lies in its direct impact on how efficiently an ion can destabilize the colloidal particles, causing them to clump together and settle. Thus, in practical applications, knowing the valency can guide the choice of electrolyte used to achieve the desired coagulation in various industrial and laboratory settings.

Coagulating Power of Ions

The coagulating power of ions is fundamentally based on the charge that these ions possess. The more charge an ion carries, the more powerful it is at causing the colloidal particles to coagulate. This is because charged ions effectively neutralize the surface charges of the colloidal particles, reducing their stability and leading to coagulation.
The effectiveness of the ions can be clearly understood by considering the following examples:

• \(\mathrm{Na}^{+}:\) As a monovalent ion, it has the least coagulating power among the considered ions because it can only neutralize a limited amount of charge.
• \(\mathrm{Ba}^{2+}:\) With a divalent nature, this ion has a notable increase in coagulating power compared to \(\mathrm{Na}^{+},\) making it more effective at inducing colloidal instability.
• \(\mathrm{Al}^{3+}:\) Possessing a trivalent charge, \(\mathrm{Al}^{3+}\) is significantly more powerful and efficient in coagulating colloidal particles, rendering it the most effective among the ions listed.

Understanding the coagulating power of ions is essential for fields such as water purification, pharmaceuticals, and the food industry, where controlling the stability of colloids is critical to process outcomes and product quality.
In summary, when evaluating the coagulating power, always consider the charge strength of the ions first and foremost, as this is the key factor driving the coagulation process.