Problem 56 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

A freshly prepared \(\mathrm{Fe}(\mathrm{OH})_{3}\) precipitate is peptized by adding \(\mathrm{FeCl}_{3}\) solution. The charge on the colloidal particle is due to preferential absorption of (a) \(\mathrm{Cl}\) ions (b) \(\mathrm{Fe}^{\text {th }}\) ions (c) \(\mathrm{OH}^{-}\) ions (d) \(\mathrm{H}^{+}\) ions

Step-by-Step Solution

Answer

The charge on the colloidal particle of \(\mathrm{Fe}(\mathrm{OH})_{3}\) after peptization by \(\mathrm{FeCl}_{3}\) solution is due to the preferential absorption of \(\mathrm{Cl}^{-}\) ions.

Step 1: Understanding the Concept of Peptization

Peptization is the process of converting a precipitate into a colloidal solution by adding an electrolyte. The precipitate adsorbs ions from the electrolyte, which imparts a charge to the particles of the precipitate, causing them to disperse into a colloidal state due to repulsion between similarly charged particles.

Step 2: Determining the Nature of the Colloid

In the given exercise, a freshly prepared \(\mathrm{Fe}(\mathrm{OH})_{3}\) precipitate is peptized by adding \(\mathrm{FeCl}_{3}\) solution. The colloidal particles are formed due to the preferential adsorption of ions from \(\mathrm{FeCl}_{3}\).

Step 3: Identifying the Adsorbed Ion

Since \(\mathrm{FeCl}_{3}\) dissociates into \(\mathrm{Fe}^{3+}\) ions and \(\mathrm{Cl}^{-}\) ions, and \(\mathrm{Fe}(\mathrm{OH})_{3}\) already contains \(\mathrm{Fe}^{3+}\) and \(\mathrm{OH}^{-}\) ions, the charge on the colloidal particles will be due to the preferential absorption of the ion that is not already part of the precipitate. In this case, \(\mathrm{Cl}^{-}\) ions from \(\mathrm{FeCl}_{3}\) will be preferentially adsorbed, because the colloidal particles already contain \(\mathrm{Fe}^{3+}\) and \(\mathrm{OH}^{-}\) in their structure.

Key Concepts

Peptization Process

Peptization is a fundamental process in colloid chemistry that involves transforming a precipitate into a colloidal solution by the addition of an appropriate electrolyte. This aids in imparting a charge to the precipitate particles, which then disperse into a colloidal state.

This process is essential for creating colloids from substances that do not naturally form them. The electrolyte used for peptization should have an ion that is capable of being adsorbed on the surface of the precipitate. Through adsorption, the particles acquire an electric charge, providing repulsive forces between them, which prevents aggregation and thereby stabilizes the colloidal dispersion.

An important application of this process is in industries and scientific research, where precise control over the properties of colloids is necessary. For example, in the food industry, peptization can be used to create stable emulsions for various food products.

Colloidal Solution

A colloidal solution is a heterogeneous mixture wherein the particle size of the substance present is intermediate between true solutions and suspensions, typically in the range of 1 to 1000 nanometers.

Colloids can be found in various forms, such as sols (solid particles in a liquid), gels (solid networks within liquids), and emulsions (liquid droplets in another liquid). These particles remain evenly distributed without settling out because the colloidal particles are small enough to be buoyed by the Brownian motion of the surrounding molecules, yet large enough to be barred from passing through a semipermeable membrane.

The importance of colloidal solutions is widespread, encompassing everyday products like milk, mayonnaise, and fog, and also useful in medical diagnostics, drug delivery systems, and the production of materials with specific optical, electrical, or mechanical properties.

Preferential Ion Adsorption

Preferential ion adsorption occurs when one type of ion from an electrolyte is adsorbed more readily onto the surface of a colloidal particle than another, leading to the development of a surface charge on the particle.

This phenomenon essentially dictates the charge type that the colloidal particles will carry in a solution. It depends on factors such as the nature of the colloidal particle, the type of ions present in the solution, and the relative concentrations of the ions. Preferential adsorption results in a charge imbalance, which subsequently causes the particles to repel each other, increasing the stability of the colloidal system.

Understanding the preferential adsorption of ions is crucial in manipulating colloidal stability and behavior, which is significant in water treatment, ceramics manufacturing, and pharmaceutical formulation.

Electrolytes in Colloids

Electrolytes play a pivotal role in the formation and stabilization of colloidal solutions. They provide ions that can be adsorbed onto the surface of colloidal particles. The presence of electrolytes affects the electrical double layer around the colloidal particle and therefore its stability.

A proper balance of electrolytes can prevent coagulation, which is the aggregation of colloidal particles leading to their eventual settling out of the solution. On the other hand, excessive amounts of electrolytes can lead to coagulation due to the compression of the electrical double layer and reduced repulsive forces.

It's also worth noting that electrolytes are vital in various applications, such as in the purification of water, where they assist in the coagulation and subsequent removal of colloidal impurities, ensuring the treated water's quality and safety.