Question

Arsenous sulphide sol is prepared by passing \(\mathrm{H}_{2} \mathrm{~S}\) through arsenous oxide solution. The charge developed on the particies is due to adsorption of: (a) \(\mathrm{H}^{+}\) (b) \(\mathrm{S}^{2-}\) (c) \(\mathrm{OH}^{-}\) (d) \(\mathrm{O}^{2-}\)

Short answer

The charge on the arsenous sulphide particles is due to the adsorption of \(\mathrm{H}^{+}\) ions.

Step-by-step solution

Understand the concept of sol formation

Sol is a type of colloidal solution where solid particles are dispersed in a liquid. When hydrogen sulfide, \(\mathrm{H}_{2} \mathrm{S}\), is passed through a solution of arsenous oxide, a sol of arsenous sulfide is formed. The charge on the particles of a sol is due to the preferential adsorption of certain ions from the solution.

Determine which ions would be adsorbed

In the preparation of arsenous sulfide sol from arsenous oxide, it's important to understand that arsenous oxide would likely react with water present in the solution to form arsenous acid:\(\mathrm{As}_{2}\mathrm{O}_{3} + 3\mathrm{H}_{2}\mathrm{O} \rightarrow 2\mathrm{H}_{3}\mathrm{AsO}_{3}\) Arsenous acid can ionize, releasing hydrogen ions into the solution, and subsequently, arsenous sulfide sol is formed by the addition of \(\mathrm{H}_{2}\mathrm{S}\).

Key concepts

Sol Formation

Sol formation is a fascinating process in colloidal chemistry where tiny solid particles, often too small to be seen individually with the naked eye, are evenly dispersed throughout a liquid, creating what we call a colloidal sol. Think of this like very fine dust specks floating in air, but in this case, the 'air' is actually a liquid.

During the formation of an arsenous sulphide sol, as in our exercise, hydrogen sulfide (H2S) gas is passed through an aqueous solution of arsenous oxide. This results in a chemical reaction where the arsenous oxide reacts with water to create arsenous acid, which then combines with hydrogen sulfide to form the colloidal particles of arsenous sulfide. These newly formed particles do not dissolve but instead remain suspended, creating a murky mixture.

Understanding the process of sol formation is crucial, as it determines the physical and chemical properties of the colloid, including its stability, its behaviour in electric fields, and how it interacts with other substances. Different methods can be used to prepare sols, and each method can affect the size, charge, and concentration of the particles formed.

Adsorption in Colloids

Adsorption in colloids is a key phenomenon where ions or molecules adhere to the surface of the colloidal particles. It is this adsorption that imparts a charge to these particles. Unlike absorption, where substances are taken up internally, adsorption strictly involves the surface. Picture it like magnets sticking to a refrigerator, only in this case, the fridge is the colloidal particle, and the magnets are ions.

In the context of our exercise, when arsenous sulfide colloidal particles form, they show a preference for certain types of ions. Depending on which ions are in excess, these will be the ones that get 'picked' by the colloidal particles. In a way, these particles are like picky eaters selecting their preferred seasoning from a table full of spices.

This selective adsorption can lead to a net positive or negative charge on the particles, which is crucial for the stability of the colloidal system. The charge helps to keep the particles from clumping together and settling out of the solution, thus maintaining the colloidal state. Understanding which ions get adsorbed on the particle surfaces is a crucial step in predicting the behaviour of the colloid and for processes like coagulation or flocculation.

Ionization in Colloids

Ionization in colloids plays a pivotal role in the overall charge and stability of the colloidal particles. This refers to the tendency of the dispersed substance, or the dispersion medium, to release ions into the solution. Now, imagine you're squeezing a lemon into water; some of the lemon juice's particles break away and disperse into the liquid. Similarly, when a substance ionizes, it releases ions into the surrounding medium.

In our case with arsenous sulfide sol, the ionization pertains to arsenous acid releasing hydrogen ions (H+) into the solution. Once the hydrogen sulfide gas reacts and forms arsenous sulfide, these hydrogen ions do not just disappear; they play a significant role by attaching to the colloidal particles due to electrostatic forces, thereby giving the particles their charge.

The degree to which a substance ionizes in the colloidal state can have profound implications, affecting the colloid's electrical conductivity, reactivity, and interaction with external fields. It's a delicate balance where the extent of ionization must be just right; too much or too little can lead to the breakdown of the colloidal state, causing the particles to either repel each other too much or to aggregate and settle out of the solution.