Adsorption Chemistry Notes
→ We know that the surfaces of solids and liquids have unbalanced attractive forces. Hence, the surface of solids and liquids attracts the molecules of the phase with which it comes into contact. These attracted molecules remain accumulated at the surface and do not penetrate into the bulk. This leads to a relatively higher concentration of one of the phases at the surface or interface. This is called adsorption.
→ Thus, adsorption is a phenomenon of attracting and retaining the molecules of a substance at the surface or interface of a solid or a liquid resulting into higher concentration of the molecules on the surface than in the bulk. For example : When activated charcoal is added to coloured solution then it decolourises due to adsorption. Similarly, on addition of silica gel in closed vessel then the pressure of the gas in vessel decreases. It is due to the adsorption of gases on the surface of silica gel.. Some important terms related to adsorption are given below :
The substance which gets adsorbed at any surface is called adsorbate. For example, coloured dye gets adsorbed at the surface of charcoal then coloured dye is termed as the adsorbate.
The solid or liquid substance at which surface adsorption of a molecule, atom or ionic species takes place is termed as adsorbent. For example: powdered or activated charcoal, powdered metal, silica gel, alumina etc. are some common adsorbents.
The removal of adsorbate from the surface of adsorbent is termed as desorption. On decreasing the pressure and increasing the temperature, desorption occurs.
When both adsorption and absorption occur simultaneously simply the term sorption is used. It means that a particular substance gets distributed throughout the bulk of some other substance but at the same time its concentration also increases at the surface (i.e..concentration is higher at surface than in bulk). It is quite difficult to measure how much of a substance is adsorbed or how much is absorbed then it is termed as sorption. Example :
- Adsorption of hydrogen gas at the surface of charcoal but after sometime gas is absorbed in the bulk of charcoal
- Dyes get adsorbed as well as absorbed in the cotton fibre.
- Zeolite takes up a gas by sorption.
The surface of adsorbent, at which adsorbate is adsorbed or concentrated is known as interface.
Active Centre :
Generally the surface of adsorbent has some free valencies. Adsorbate is adsorbed on these free valencies, These free valencies are not distributed uniformly throughout the surface of adsorbent. The place where the surface is rough, the number of free valencies are higher there. These free valencies are termed as active centres.
The sorption of gases at metal surface is called occulsion.
Positive Adsorption :
When the concentration of adsorbate is higher at the surface relative to its concentration in the bulk then this type of adsorption is called positive adsorption. For example: adsorption of gases on powdered metal.
Negative Adsorption :
When the concentration of adsorbate is lower at the surface relative to its concentration in the bulk then this type of adsorption is called negative adsorption. For example : in case of some liquid solutions, the concentration of solute is less at the surface than in the bulk of the solution.
Heat of Adsorption :
It is the amount of heat released during the adsorption of one mole of adsorbate at the surface of adsorbent.
Adsorption and Absorption :
There are two similar sounding terms adsorption and absorption but these two phenomena are quite different to each other. The adsorption occurs only at surface of solid or liquid while absorption occurs throughout the bulk of the solid or liquid
For example :
- NH3 gas is adsorbed by charcoal while it is absorbed by water to form NH4 OH
- Water vapours are adsorbed by silica gel while it is absorbed by anhydrous CaCl2.
- Diagramatically, it can be represented as:
→ Adsorption in Different Action In order to illustrate the process of adsorption, let us consider some examples of it:
- Aqueous solution of raw sugar which is brownish in colour when passed through animal charcoal then it is decolourised because the colouring substance is adsorbed by animal charcoal.
- Silica gel is used to remove moisture because water molecules get concentrated or adsorbed on the surface of silica gel.
- The gases like O2, H2, CO, Cl2, NH3, SO2, O2 etc. if present in vessel, then they create pressure. It is observed that the pressure of the gases in closed vessel decreases when activated charcoal is placed in vessel.
- It is due to the adsorption of gases on the surface of charcoal.
- In a solution of an organic dye like methylene blue or blue litmus, when activated charcoal or animal charcoal is added and the whole solution is shaken well then it is absorbed.
- Then filtered solution is colourless as the molecules of the dye get adsorbed on the surface of charcoal.
- Hydrogen gas is adsorbed at the surface of nickel and palladium metal.