Problem 190 - A Complete Resource Book in Chemistry for JEE Main

Question

The disperse phase in colloidal iron (III) hydroxide and colloidal gold is positively and negatively charged, respectively. which of the following statements is incorrect? (a) magnesium chloride solution coagulates, the gold sol more readily than the iron (III) hydroxide sol (b) sodium sulphate solution causes coagulation in both sols (c) mixing of the sols has no effect (d) coagulation in both sols can be brought about by electrophoresis

Step-by-Step Solution

Answer

(c) is incorrect because mixing charged sols leads to coagulation.

Step 1: Understanding Colloidal Charges

Colloidal iron (III) hydroxide carries a positive charge, and colloidal gold carries a negative charge. When considering coagulation, it is important to remember that oppositely charged ions neutralize charges on colloids, causing them to coagulate.

Step 2: Analyzing the Coagulating Agents

Magnesium chloride ( MgCl_2 ) contains Mg^{2+} ions, which are divalent cations. Sodium sulfate ( Na_2SO_4 ) contains sulfate ions ( SO_4^{2-} ), which are divalent anions. Both can coagulate the oppositely charged gold and iron (III) hydroxide sols but generally require different efficiencies based on charge and valence (Schulze-Hardy Rule).

Step 3: Evaluating Sodium Sulfate's Effect

Sodium sulfate can cause coagulation of colloidal gold because of the SO_4^{2-} ions that neutralize the charge on the gold sol. It can also cause coagulation of positively charged iron (III) hydroxide sol, adhering to the characteristic of causing coagulation in both sols.

Step 4: Examining Electrophoresis and Mixing Effects

Electrophoresis can cause coagulation in both systems by moving and eventually settling the colloidal particles at the electrodes, thus causing aggregation. Mixing positively and negatively charged sols results in mutual neutralization and consequently precipitation, which is contrary to statement (c) that mentions no effect.

Step 5: Identifying the Incorrect Statement

Based on the steps above: (a) and (b) are true since oppositely charged ions indeed cause coagulation. (d) is also true, given electrophoresis works in this manner. However, (c) is incorrect as mixing results in mutual neutralization and coagulation, not no effect.

Key Concepts

Coagulation

Coagulation in colloids refers to the process where colloidal particles come together to form larger aggregates, leading them to settle out from the dispersion medium. Imagine it like how milk curdles or small dust particles clump together in the air.
The main element influencing coagulation is the charge on the colloidal particles. Since opposite charges attract, introducing ions or particles of opposite charge can neutralize the electric charge on the colloids.

• For example, adding magnesium chloride to a negatively charged colloid like gold causes coagulation due to the positive charge of magnesium ions neutralizing the negative charge.
• Similarly, a positive colloid like iron (III) hydroxide is coagulated by negatively charged ions like sulfate ions present in sodium sulfate.

Coagulation is different for each sol, and it depends on factors such as the valency of the ions involved. According to the Schulze-Hardy Rule, ions with higher valency are usually more effective in neutralizing the charge, leading to faster coagulation.

Electrophoresis

Electrophoresis is a fascinating technique used to separate charged particles using an electric field. It's like watching tiny racers on a track, moving toward electrodes carrying the opposite charge.
In the case of colloids, electrophoresis can cause coagulation by driving charged particles to electrodes. Once there, the particles lose their stability due to charge neutralization and aggregate with each other to form a settled mass.
Here's how it works:

• Colloidal particles move based on their charge; positive to negative electrodes and vice versa.
• Eventually, these particles either lose their charge or gain excessive opposite charges, leading to their aggregation and settling.
• This method is effective for coagulation of both positive and negative colloids, making them settle out of their dispersion medium.

Electrophoresis is not just a laboratory technique; it also mirrors many natural processes where charged particles are moved under natural electric fields, playing a key role in water purification and various industrial processes.

Colloidal Charges

The charge on colloidal particles is central to their stability and behavior in sols. The way these tiny particles remain dispersed in a medium, without settling, is due to the charge they possess. This charge imparts a mutual repulsion among the particles, maintaining their state of suspension.
Think of it like a bustling crowd of people, where everyone has a personal bubble—keeping them apart and preventing any settlements.
Charges on colloids arise from a few key processes:

• Adsorption of ions: Colloids can adsorb ions from their surrounding medium leading to a net charge.
• Ionization: Certain colloids ionize in water, gaining a charge.
• Ion dissolution: Genetically charged ions are released, which leads to a charge on particles.

This charged nature of colloids makes them allow processes like coagulation and electrophoresis to be controlled deliberately. Thus, understanding colloidal charges helps in effectively manipulating and using colloids in various scientific and industrial applications.