Animated Solution for Chemistry - Surface Chemistry: The nature of charge on resulting colloidal particles when ${\text{FeCl}}_3$ is added to excess of hot water is

A Chemistry Lab Scenario

Imagine you are standing in a chemistry laboratory. In front of you is a beaker filled with an excess of hot water. You take a dropper and carefully add a few drops of a yellowish-brown solution—ferric chloride (${\text{FeCl}}_3$).

What happens next is not merely a physical mixing of two liquids. It is the birth of a fascinating surface chemistry phenomenon that leads to the formation of a colloidal sol. Let's dive into the microscopic world to understand the exact sequence of events.

As soon as the ${\text{FeCl}}_3$ drops hit the hot water, a rapid chemical reaction takes place. The thermal energy provided by the hot water accelerates the hydrolysis of ferric chloride.

This reaction produces insoluble particles of hydrated ferric oxide, represented chemically as ${\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$.

These newly formed particles are not your average precipitates. They are of colloidal dimensions—large enough to scatter a beam of light (Tyndall effect) but small enough to remain suspended in the dispersion medium without settling down.

Preferential Adsorption

Now, we arrive at the most crucial part of the process. Colloidal particles are highly active entities. They possess a strong tendency to selectively attract and hold onto specific ions from the surrounding electrolyte solution. This phenomenon is known as preferential adsorption.

Think of the colloidal particle as a highly selective magnet. It prefers to adsorb ions that are common to its own crystal lattice. In our scenario, we added ${\text{FeCl}}_3$ to the water. Therefore, the common ion available in the solution that matches the lattice of the hydrated ferric oxide particle is the ferric ion (${\text{Fe}}^{3+}$).

The positively charged ${\text{Fe}}^{3+}$ ions rush towards the surface of the ${\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$ particles and get firmly adsorbed. This creates a tightly bound, fixed positive layer around the colloid.

But what about the chloride ions (${\text{Cl}}^{-}$)? They do not vanish. They remain in the surrounding dispersion medium, forming a diffused negative layer to balance the overall electrical neutrality of the system. However, the primary charge of the colloidal particle itself is determined solely by the firmly adsorbed inner layer.

Because the hydrated ferric oxide particles preferentially adsorb the positively charged ${\text{Fe}}^{3+}$ ions, the resulting colloidal sol acquires a positive charge.

This is a classic and highly tested concept in competitive exams. The golden rule to remember is: always look for the common ion between the colloidal particle and the added electrolyte to determine the charge of the sol!