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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What is Electro-Organic Synthesis?
In electro-organic synthesis, the electrodes act as “courier” of electrons. The anode which is electron deficient abstracts electron from a substrate, i.e., oxidises it and this is called Anodic oxidation, whereas the cathode which has excess of electrons can transfer an electron to the substrate i.e., reduces it and is called Cathodic reduction.
(i) Such synthesis has a green perspective as no reagent is necessary for the redox reactions- the electrons are directly exchanged between the substrate and the inert electrodes like Platinum.
(ii) Such synthesis helps in bringing about “umpolung” in the substrate and allows synthesis of compound from single substrate.
(iii) Since in electrosynthesis, the reactive species react on the electrode surface their “effective volume” is very high for which rates of the reactions are also high.
It Also Has Following Disadvantages:
(i) Multiplicities of the product are the main disadvantage.
(ii) Organic compounds are non electrolytes, i.e., non-conducting. This is overcome by adding supporting electrolytes like Li-salts or tetraalkyl ammonium salts which are appreciably soluble in organic solvents.
The reactions are usually carried out in two ways-
(i) By Potentiostatic technique (i.e. constant energy process)
In this technique the reactions become more chemo-selective and multiplicity of the product is decreased.
(ii) By Galvanostatic technique (i.e. constant current process)
This is easy to carry out but chemo-selectivity is less.
Note: Scientists claim, there is a huge potential in this field to get a Nobel Prize. Using no reagents and no chemical wastage is the main reason for this.
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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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