NEET Chemistry Notes Surface Chemistry – Factors Affecting Adsorption of Gases on Solids
Factors Affecting Adsorption of Gases on Solids
Factors Affecting Adsorption of Gases on Solids
Adsorption depends upon following number of factors:
- Nature of Adsorbent
A gas is adsorbed in different amounts on different adsorbents. Hydrogen is strongly adsorbed on nickel surface while it is weakly adsorbed on alumina surface under identical conditions.
The adsorption of gases on the surface of metals is called occlusion.
- Nature of Adsorbate
Generally, the more liquefiable a gas is, the more readily will it be adsorbed.
Easily liquefiable gases such as NH3, HCl, Cl2 , S02, etc., are readily adsorbed than permanent gases, such as H2, N2 etc.
- Specific Area of Adsorbent
It is the surface area of adsorbent available for adsorption per gram of the adsorbate. The greater the surface area of the solid, the larger would be its adsorbing capacity.
- Pressure of Gas
At a given temperature, the extent of adsorption-twill increase with the increase of pressure of the gas.
The extent of adsorption is measured as x/m, where m is the mass of adsorbent and x that of adsorbate. At low pressure, x/m varies linearly with p
- Temperature
Low temperature value favours the physical adsorption, but on increasing temperature physical adsorption decreases.
Whereas in case of chemisorption, x/m, initially increases with temperature and then decreases.
The initial increase is due to the fact that chemisorption requires activation energy.
The graph between extent of adsorption x/m temperature T is called adsorption isobar.
Adsorption from Solutions
Solids adsorb dissolved substances from solutions, e.g. adsorption of colour impurities by activated carbon in decolourising of solutions.
Freundlich adsorption isotherm and Langmuir adsorption isotherm are also applicable to adsorptions from solutions.
Langmuir adsorption isotherm is applicable only to chemisorption.
Catalysis
The catalyst change the rate of reaction by providing an alternate path of different activation energy. This phenomenon is known as catalysis.
Types of Catalysis
- Homogeneous Catalysis
Reactants and catalyst should be in same phase
- Heterogeneous Catalysis
Reactants and catalyst are in different phase.
- Autocatalysis
The phenomenon in which one of the products formed during the reaction acts as catalyst for the reaction.
- Induced Catalysis
In this type of catalysis, one reaction influences the rate of other reaction which does not occur ordinary condition, e.g. the reduction of HgCl2 by oxalic acid is slow but becomes faster if reduction is made in mixture of KMnO 4 and HgCl2; where both are reduced. Reduction of KMn04 thus induces the reduction of HgCl2.