Problem 25 - Chemistry in 30 days

Question

A colloidal solution is subjected to an electrical field. The particles move towards anode. The coagulation of same sol is studied using \(\mathrm{NaCl}, \mathrm{BaCl}_{2}\) and \(\mathrm{AICl}_{3}\) solutions, Their coagulating power should be (a) \(\mathrm{NaCl}>\mathrm{BaCl}_{2}>\mathrm{AlCl}_{3}\) (b) \(\mathrm{BaCl}_{2}>\mathrm{AlCl}_{3}>\mathrm{NaCl}\) (c) \(\mathrm{A} \mathrm{Cl}_{3}>\mathrm{BaCl}_{2}>\mathrm{NaCl}\) (d) \(\mathrm{BaCl}_{2}>\mathrm{NaCl}>\mathrm{AlCl}_{3}\)

Step-by-Step Solution

Answer

The coagulating power is AlCl3 > BaCl2 > NaCl, option (c).

Step 1: Understanding the Problem

We need to determine the coagulating power of different salts on a colloidal solution whose particles move towards the anode under an electric field. This tells us that the colloidal particles are negatively charged and need positive ions for coagulation.

Step 2: Identifying the Ion Charge

The coagulating power of a salt is determined by the charge of the ion that neutralizes the colloidal particles. According to the Hardy-Schulze rule, the coagulating power is greater for ions with higher valence. In this case, the positive ions are: Na^+, Ba^{2+}, and Al^{3+}.

Step 3: Ranking the Coagulating Power

Based on the ion charges: Na^+ has a charge of +1, Ba^{2+} has a charge of +2, and Al^{3+} has a charge of +3. According to the Hardy-Schulze rule, the coagulating power will be in the order of the charge of the positive ions: Al^{3+} > Ba^{2+} > Na^+.

Step 4: Selecting the Correct Answer

Compare our order with the given options. The correct order AlCl3 > BaCl_2 > NaCl matches option (c).

Key Concepts

Electric Field

In the context of this exercise, an electric field is applied to a colloidal solution. Colloidal particles are tiny dispersed particles that can be influenced by external forces, including electric fields. When such a field is applied, it creates a force that causes the colloidal particles to move in a specific direction. This movement towards a pole is due to the fact that colloidal particles often carry an electrical charge. In this case, the particles move towards the anode. The direction of movement indicates that the particles are negatively charged since they are attracted to the positive charge of the anode.

• The application of an electric field is a common technique to study the behavior of colloids and their stability.
• This helps in identifying the charge of the colloidal particles.

The movement of these particles can influence various processes, including coagulation, which is crucial for understanding the interactions within the solution.

Anode

The term 'anode' refers to the electrode towards which negatively charged particles move in an electric field. In this context, since the colloidal particles are moving towards the anode, it confirms that these particles are negatively charged. To visualize, think of the anode as a positive terminal, which attracts negatively charged particles due to the inherent opposite charge nature. In electrochemical cells, anodes are critical as they determine the direction of charge movement:

• The direction of movement is always from negative to positive.
• This helps verify the charge on the particles in a colloidal solution.

This interaction is central to understanding which ions are necessary for processes like coagulation.

Coagulating Power

Coagulating power is a measure of how effectively different ions can cause the colloidal particles to aggregate and settle out of the solution. This is a vital property when trying to destabilize a colloid. The principle operating here is that the ions with higher charge (valence) have a stronger effect in neutralizing the charge of the colloidal particles. When the charge is neutralized, particles no longer repel each other and start clumping together, eventually settling down. Key points include:

• Ions such as Al^{3+} with higher charges have greater coagulating power.
• The neutralization of charge reduces repulsion between particles.
• The higher the charge, the quicker the coagulation process.

This explains why AlCl_{3}, which contains Al^{3+} ions, has the strongest coagulating power among the given salts.

Hardy-Schulze Rule

The Hardy-Schulze rule is a fundamental concept in colloidal chemistry. It states that the coagulating power of an electrolyte depends primarily on the valence of the ion opposite to the charge on the colloidal particles. In simpler terms, the more charged an ion is, the more effectively it can neutralize the charge of the colloidal particles and cause them to settle. For example:

• Al^{3+} ions have three positive charges, making them highly effective in neutralizing negatively charged colloidal particles.
• Ba^{2+} and Na^+ follow in descending order of coagulating power, supporting the observed order: Al^{3+} > Ba^{2+} > Na^+.

This rule underscores the importance of charge over quantity when it comes to the effectiveness of ions in processes like coagulation. It is a crucial guideline for predicting outcomes in various chemical processes involving colloids.