subject: Physical Adsorption [print this page] Adsorption is the adhesion or attachment of molecules to a surface. The molecules attaching to the surface may be solids, liquids or gases. The molecules which are accumulating on the surface are collectively called as adsorbate and the surface on which these molecules are accumulated is called adsorbent.
The adsorption is mainly of two types based on the forces of attraction existing between the adsorbent and the adsorbate. They are:
1. Physical adsorption or Physisorption
2. Chemical adsorption or Chemisorption
If the attractive forces between the adsorbent and the adsorbate are weak, it comes under Physical adsorption and if the attractive forces between them are very strong (such as bond formation between adsorbate and adsorbent) then it is classified as Chemical adsorption
Properties of Physical Adsorption
In Physical adsorption, the forces of attraction between the adsorbent and the adsorbate are weak forces such as the Vander Waals forces. As the forces of attraction are weak, it is very easy to reverse the process by using simple techniques such as heating or pressure removal etc., The reverse process of Adsorption is called as desorption.
Some important properties of Physical Adsorption are
Involves weak forces of attraction such as Vander waals forces.The Physical adsorption is nonspecific.Multilayers of Adsorbate can be formed by this adsorptionIt can be performed at low temperatures and the enthalpy of adsorption is also low.Physical adsorption is a reversible process.
Chemical adsorption:
Adsorption is a process through which the concentration of a molecule is increased in the interface due to operation of surface forces. Generally adsorption is dealing with attachment of gas molecules over solid surface. It is a surface phenomenon. Generally the gas molecules are called as adsorbent and the metal surface is known as adsorbate. Depending upon the nature of interaction between adsorbent and adsorbate the adsorption may be classified as
Chemical adsorption
Physical adsorption
In chemical adsorption the adsorbents are attached with adsorbate through strong chemical bonds
Characterestics of chemical adsorption:
Chemical adsorption has the following characteristics.
- Due to strong chemical bond formed between adsorbent and adsorbate the heat of adsorption is generally high.
- As there is selective reaction between adsorbent and adsorbate the adsorption is specific.
- As an increase in temperature favours chemical reaction and thus bond formation, the adsorption is increased with temperature.
- As there is direct chemical bonding between adsorbent and adsorbate only single layer adsorption is possible.
- Increase in pressure has no effect as only monolayer adsorption is only possible.
- The process is completely irreversible.
Theories of chemical adsorption:
Langmuir Isotherm
Langmuir made significant contributions to physics and chemistry. He was awarded noble prize for this
Theory on surface chemistry.
According to his theory the rate of adsorption is directly related with the free area available for adsorption. In the initial stages of adsorption as there are so many vacant sites available for adsorption the rate of adsorption is high. But as time passes away the sites are filled with adsorbents hence the rate of adsorption decreases.
This equation is called as Langmuir adsorption isotherm. It relates the rate of adsorption to the pressure of gas over the surface P .
Here is the Langmuir constant for the metal surface. P is the pressure of gas. is the fraction of surface covered
Few difference between adsorption and absorption
In absorption the substance is uniformly distributed through out the bulk of the solid.In absorption sometimes substance in one state changes to other state. If we pass gaseous carbon dioxide in water it completely dissolves forming a homogeneous liquid.Both absorption and adsorption can take place simultaneously.
There are two kinds of absorption as physical absorption and chemical absorption. Ammonia dissolving in water and carbon dioxide dissolving in water are examples. Absorption of gases in a liquid depends on the solubility of the gas, Pressure and temperature.
Chemical and physical absorption:
Here it involves chemical reaction. In the production of ammonia the out coming hydrogen sulphide gas is removed by passing the mixture of the above gases over zinc oxide.
Absorption may be reversible or irreversible. Oxygen is sparingly soluble in water. If water is heated the dissolved oxygen gas escapes out. If Carbon dioxide dissolves in water it cannot be reversed.
The surface particles of the absorber are not surrounded by atoms or molecules of their kinds on all sides. They posses unbalanced attractive forces. These forces are responsible for attracting the adsorbate particles.
The extent of adsorption increases with increase of surface area. The extent of adsorption increases with increase of surface area unit mass of the adsorbent at a given temperature and pressure.
There are two kinds of adsorption
Chemical adsorption and a physical adsorption.
Chemical adsorption is highly specific. It will occur only if a chemical bonding will occur between the adsorbent and adsorbate. Formation of metallic oxide on a metal is a chemical adsorption Gases which can react with the adsorbent show chemisorptions. Chemisorptions is irreversible. High activation energy and high temperature are needed.
Physio adsorption is reversible. More of the gas is adsorbed when the pressure is increased and the gas can be removed by decreasing pressure.
Application of adsorption:
Production of high vacuum (Charcoal is used to remove the residual air from evacuated vessel)
Gas masks in coal mines,
To control humidity (the gels of silica and aluminium)
Removal of colouring matter from solutions,
Heterogeneous catalysts in the manufacture of ammonia and sulphuric acid,
In the manufacture of ammonia iron is used as catalyst and in sulphuric acid
Vanadium pent oxide is used as a catalyst. Nickel is used in hydrogenation of oil.
Separation of inert gases, to concentrate the ore by froth floatation process and in chromatographic analysis.
Examples and few Applications of Physical Adsorption
- Charcoal is used in the adsorption of many gases as it has more surface area.- Small particles of Platinum adsorb hydrogen gas.- Bone black and charcoal adsorb colors.
Applications:
Physical adsorption has many applications in various industries. Few of them are
- In the gas masks, where charcoal is used to adsorb poisonous gases.
- In the preparation of ammonia and sulphric acid, where platinum is used as a adsorbent.
- Removal of colors from different solutions where charcoal and bone black are used.
- In air conditioning and other processes, adsorption chillers are used.
