Tag Archives: Physical Chemistry

Colloids: A Brief Note

Colloids are the particles having size between 1nm to 1000nm such as H20, Alcohol, Benzene are assumed to be aggregate of many small molecules which have high molecular mass. They have the ability to scatter light and have low osmotic pressure.

Colloidal systems are divided into two parts
i. Lyophilic Colloids
ii. Lyophobic Colloids

The Lyophobic Colloids are polymeric molecules and Lyophilic Colloids are proteins or starch in water, rubber in benzene.

i. Lyophobic Colloids:
image

They are solvent hating colloids. When the dispersed phase has less affinity for dispersion medium, the system is called Lyophobic Colloids.

When dispersion medium is water, they are given the name Hydrophobic. If they are separated, they can not be made by simply adding with dispersion medium.

ii. Lyophilic Colloids:

image

When the dispersed phase has the highest affinity for dispersion medium, then it is called Lyophilic Colloids.

When the dispersed phase is separated from dispersion medium, they can be again made by simply remixing with the dispersion medium.

Advertisements

Assumptions of Langmuir- Hinshelwood Mechanism: A Brief Review

Langmuir-Hinshelwood has proposed a mechanism for adsorption of gases on the surface of solid. The surface reaction is assumed to occur in the following steps:
1. Diffusion of the reactants on the surface of the solid adsorbent.
2. Adsorption of the reactants at the surface.
3. Chemical reaction at the surface.
4. Desorption of the products from the surface.
5. Diffusion of the products from the surface of the adsorbent.

Step 1 & 5 are generally very fast and step 2 & 4 are generally faster than 3. Thus step 3 is the rate determining step in the surface reaction, i.e., chemisorption of the reactants on the surface of the solid adsorbent.

Two feature of this type of surface reaction:
1. Chemisorption plays a very important role in such type of adsorption.
2. The reaction rate per unit surface area is proportional to the fraction of total surface covered with adsorbed gas molecule.

Thanks for reading 🙂