Absorption and Adsorption

SURFACE CHEMISTRY
DEFINITIONS
Adsorption
It is a surface phenomenon. In this, concentration of any molecular species is greater at the surface than in the bulk. The phenomenon of accumulation of gas or liquid molecules on the surface of a solid or liquid is called adsorption.
The adsorption of gas on a solid is sometimes called occlusion.
Ex : Adsorption of ammonia (adsorbate) in charcoal (adsorbent)
Adsorption of water vapours (adsorbate) on silice gel (adsorbent)
Adsorption of dye (adsorbate) by charcoal (adsorbent)

Absorption
It is a bulk phenomenon. It is a process in which, a substance is not only retained on the surface but uniformly distributed throughout the body of the solid or liquid
Ex : Absorption of ammonia by water
Absorption of water vapours by only CaCl2
Absorption of water by sponge
Adsorbate
The substance on the surface of the solid is called the adsorbate.
Ex : Various gases (NH3, CO2, N2, O2 etc.)

Adsorbent
The substance to which the adsorbate is attached is called adsorbent.
Ex : Silica gel, charcoal, alumina gel, clay, Pd, Pt, Ni

Sorption
The process in which both adsorption and absorption takes place simultaneously.

Characteristics of adsorption
Adsorption is a physical phenomenon but accompanied by a chemical change.
It is a spontaneous process
It is a selective process
It is always accompanied by evolution of heat
It is specific
The rate of adsorption depends on temperature.
It depends on the nature of adsorbent and adsorbate.

Difference between adsorption and absorption
Adsorption
- Surface phenomenon
- Rapid process
- Equilibrium is attained easily
- Concentration of molecule are more on the surface and less in the bulk
- NH3 adsorbed on charcoal

Absorption
- Bulk Phenomenon
- Slow process
- Equilibrium is slowly , step by a step
- Distribution is uniform
- NH3 absorbed by water

Types of adsorption
Adsorption is classified into two types
1. Physical adsorption (or) vander waals adsorption (or) physisorption.
2. chemical adsorption (or) activated adsorption (or) chemisorption.

Physical adsorption (or) physisorption
It is the one in which the adsorbed molecules are held on the surface of the adsorbent by weak physical or Vander waal’s forces of attraction.
Ex. Adsorption of H2 (or) O2 on charcoal

Chemical adsorption (or) chemisorption
It is the one in which the adsorbed molecules are held on the surface of the adsorbent by chemical bonds (covalent bond (or) ionic bond)
Ex. Adsorption of H2 on NI.

Difference between physisorption and chemisorption

Physisorption
- Due to inter molecules vander waal’s forces
- Low heat of adsorption (o-40 K.cal/mol/)
- Easy and fast process
-Reversible
-Decreases with increase in temperature
-The rate of adsorption increase with increases of pressure and concentration
-Multi layer adsorption occurs
-Not specific
-No surface compound form action takes place.
-Equilibrium is established rapidly
-Involves very small activation energy

Chemisorption
-Due to story chemical bond formation.
- High heat of adsorption(40 – 400 k.cal/mol)
-Difficult and slow process
-Irreversible
-Increases with increases in temperature.
-The rate of adsorption decreases with increase of pressure and concentration.
-Monolayer adsorption occurs.
- Highly specific
-Surface compound formation takes place.
-Requires time
-Involves appreciable activate energy