Irving Langmuir derived an expression for the variation in the extent of an adsorption on a solid with pressure, which is based on the following assumptions:
1. Surface of solid contains a fixed number of adsorption site, only where gaseous molecules can bind (gets adsorbed).
2. Each site can hold only one molecule and the process involves a constant heat of adsorption. This is identical for all adsorption site.
3. The adsorption is mono-layer.
4. The gaseous molecules adsorbed different adsorption site do not interact.
5. The process of adsorption involves a dynamic equilibrium.
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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.
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