Question

A sol is prepared by addition of excess of \(\mathrm{AgNO}_{3}\) solution in KI solution. The charge likely to develop on colloidal particles is: (a) positive (b) negative (c) no charge (d) both charges

Short answer

The charge likely to develop on colloidal particles is positive.

Step-by-step solution

Identify the Type of Reaction

Determine the chemical reaction taking place between \(\mathrm{AgNO}_3\) and \(\mathrm{KI}\). This is a double displacement reaction where \(\mathrm{Ag}^+\) ions from \(\mathrm{AgNO}_3\) will react with \(\mathrm{I}^-\) ions from \(\mathrm{KI}\) to form \(\mathrm{AgI}\), which is a sparingly soluble compound and will precipitate out. The remaining \(\mathrm{K}^+\) and \(\mathrm{NO}_3^-\) remain in the solution.

Predict the Precipitate Formation

Recognize that \(\mathrm{AgI}\) forms a precipitate. In this reaction, when excess \(\mathrm{AgNO}_3\) is added to \(\mathrm{KI}\), more \(\mathrm{Ag}^+\) ions are present resulting in the formation of a colloidal dispersion of \(\mathrm{AgI}\).

Determine the Charge on the Colloidal Particles

Since \(\mathrm{Ag}^+\) is in excess and it is what combines with \(\mathrm{I}^-\) to form the colloidal particles of \(\mathrm{AgI}\), the remaining unreacted \(\mathrm{Ag}^+\) ions will adsorb onto the surface of the colloidal \(\mathrm{AgI}\) particles. This results in the colloidal particles carrying a positive charge.

Key concepts

Double Displacement Reaction

Understanding a double displacement reaction is central to predicting the outcomes of many chemical processes. In such a reaction, two compounds exchange ions or elements to form two new compounds. The general formula for this type of reaction can be represented as \(AB + CD \rightarrow AD + CB\).

In the given exercise, when \(\mathrm{AgNO}_3\) is mixed with \(\mathrm{KI}\), the exchange of ions occurs. Here, the \(\mathrm{Ag}^+\) ion from \(\mathrm{AgNO}_3\) switches places with the \(\mathrm{K}^+\) ion from \(\mathrm{KI}\), leading to the formation of \(\mathrm{AgI}\) and \(\mathrm{KNO}_3\). Acquainting oneself with the solubility rules would help predict that \(\mathrm{AgI}\) is sparingly soluble and is likely to form a precipitate.

Precipitate Formation

Precipitate formation is a key phenomenon in many chemical reactions, particularly those involving aqueous solutions. A precipitate is a solid that forms when the product of a chemical reaction is insoluble in water. The exercise highlights the reaction between \(\mathrm{AgNO}_3\) and \(\mathrm{KI}\), leading to the formation of \(\mathrm{AgI}\), a compound that is not soluble enough to remain dissolved in water.

The formation of the precipitate is a visible indicator of the chemical change occurring during the reaction. The appearance of this solid is utilized in various applications, such as purifying water by removing unwanted ions and in qualitative analysis to detect the presence of specific ions.

Adsorption of Ions

Adsorption of ions onto the surface of colloidal particles is fundamental in explaining the charge of colloids. Colloidal particles, due to their small size and large surface area, provide an excellent platform for ions to cling onto—this process is termed as adsorption.

In the described exercise, after the formation of \(\mathrm{AgI}\) colloidal particles, they become surrounded by an excess of \(\mathrm{Ag}^+\) ions. This happens because \(\mathrm{AgNO}_3\) was added in excess, and not all of the \(\mathrm{Ag}^+\) finds an \(\mathrm{I}^-\) to bond with, leading to free \(\mathrm{Ag}^+\) ions in the solution. These free ions tend to adsorb onto the surfaces of the \(\mathrm{AgI}\) colloidal particles, imparting a positive charge to the particles. Understanding this process is crucial in fields such as nanotechnology, medicine, and water treatment, where the manipulation of colloidal charges can affect the stability and behavior of colloids.