Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the correct sequence of the increasing effectiveness of the following electrolyte for the coagulation of ferric hydroxide sol? 1\. \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) 2\. \(\mathrm{AlPO}_{4}\) 3\. \(\mathrm{CaSO}_{4}\) 4\. \(\mathrm{SnCl}_{4}\) Select the correct answer using the codes given below: (a) \(4,2,1,3\) (b) \(4,3,2,1\) (c) \(1,2,3,4\) (d) \(3,4,2,1\)

Step-by-Step Solution

Answer

Option (c) 1, 2, 3, 4 is the correct sequence based on increasing coagulating power.

Step 1: Understanding the Coagulation Process

Coagulation refers to the neutralization of the charge on colloidal particles, leading to their aggregation and sedimentation. The effectiveness of an electrolyte in coagulation depends on the charge of the ions: the higher the charge, the more effective the coagulation.

Step 2: Analyzing Coagulation of Ferric Hydroxide

Ferric hydroxide ( Fe(OH)_3 ) forms a positively charged colloid in solution. According to the Schulze-Hardy rule, the coagulating power of an ion is roughly proportional to the sixth power of its valency. Hence, negatively charged ions with a higher charge will be more effective in coagulating ferric hydroxide.

Step 3: Identify the Ions and Their Charges

1. K_4[Fe(CN)_6] releases 4K^+ and [Fe(CN)_6]^{4-} , where Fe(CN)_6]^{4-} provides a negative charge of 4. 2. AlPO_4 can dissociate to give PO_4^{3-} with a charge of 3. 3. CaSO_4 releases SO_4^{2-} with a charge of 2. 4. SnCl_4 provides 4Cl^- with charges of -1 each, but acts collectively more effectively with multivalent cations.

Step 4: Arrange by Increasing Coagulation Effectiveness

Based on the charges of the effective ions that can cause coagulation, we arrange the electrolytes in increasing order of effectiveness: - Least effective: CaSO_4(SO_4^{2-}) - Next: AlPO_4(PO_4^{3-}) - More effective: K_4[Fe(CN)_6([Fe(CN)_6]^{4-}) - Most effective: SnCl_4 , which uses multivalent cations for effective coagulation. Thus, the correct order from least to most effective is: 3(CaSO_4), 2(AlPO_4), 1(K_4[Fe(CN)_6]), 4(SnCl_4).

Step 5: Select the Correct Answer Based on the Order

After arranging 3-2-1-4 in increasing order of coagulating power, match this sequence with the given options: 4, 2, 1, 3; 4, 3, 2, 1; 1, 2, 3, 4; or 3, 4, 2, 1. The correct answer is option (c) as it matches the sequence 3, 2, 1, 4 in increasing effectiveness.

Key Concepts

Schulze-Hardy Rule

The Schulze-Hardy rule plays a vital role in understanding coagulation, especially in the context of colloids like ferric hydroxide sol. This rule essentially states that the effectiveness of an electrolyte in causing the coagulation of a colloid is highly influenced by the valency of the ions it releases. Specifically, the coagulating power is proportional to the sixth power of the ionic charge.

In simpler terms, ions with higher charges tend to cause particles in a colloid to clump together more efficiently, neutralizing charges and prompting sedimentation. This is crucial when dealing with positively charged colloids like ferric hydroxide, where the presence of negatively charged ions is needed for effective coagulation. Therefore, when you're looking at a list of electrolytes and trying to determine which will be most effective, pay close attention to the charge of the anions they produce.

Electrolytes in Chemistry

Electrolytes are substances that dissolve in water to produce a solution that can conduct electricity. In the world of chemistry and colloids, they play a pivotal role in processes like coagulation. When evaluating different electrolytes for their coagulating power, you should look at the dissociation products of these compounds.

Examples of Common Electrolytes and Their Effects:

• Potassium ferrocyanide ( K_4[Fe(CN)_6] ): This electrolyte dissociates to give ions such as [Fe(CN)_6]^{4-} , which with a charge of 4, can efficiently neutralize the positive charges of ferric hydroxide particles.
• Aluminium phosphate ( AlPO_4 ): Upon dissolving, it releases PO_4^{3-} ions, not as powerful as [Fe(CN)_6]^{4-} but still more effective than those with lower charges.
• Calcium sulfate ( CaSO_4 ): Produces SO_4^{2-} ions, but their lower charge makes them the least effective in this specific coagulation process.

Therefore, based on the valency and the charges generated, the effectiveness of these electrolytes can be understood and correctly ordered.

Ferric Hydroxide Sol

Ferric hydroxide sol, chemically represented as Fe(OH)_3 , is a type of positively charged colloid that frequently arises in various chemical and environmental processes. This colloid is stable due to the repulsion between the similarly charged particles suspended in a solution, which prevents them from combining and settling.

Why Coagulation Is Needed:

• Coagulation is necessary to neutralize these charges, allowing particles to come together and settle out of the solution.
• This is often a desired outcome in water treatment and other processes where clarification is needed.

In the context of coagulation using electrolytes, negatively charged ions are of paramount importance to suppress the positive charges of the ferric hydroxide sol. As highlighted by the Schulze-Hardy rule, ions with higher negative charges will cause more effective coagulation.