Adsorption

Surface chemistry: Deals with phenomena that occur at the surfaces or interfaces.

Adsorption:

⇒ Surface phenomenon.

⇒ Accumulation of molecular species at the surface.

⇒ Adsorbate: the substance that accumulates at the surface.

⇒ Adsorbent: the material on the surface of which adsorption occurs.

⇒ Adsorption α Surface Area.

Desorption: The process of removing an adsorbate from the surface on which it is adsorbed.

Differences between adsorption and absorption:

	Adsorption	Absorption
Definition	Assimilation of molecular species throughout the bulk of the solid or liquid is termed as absorption.	Accumulation of the molecular species at the surface rather than in the bulk of the solid or liquid is termed as adsorption.
Nature	It is a bulk phenomenon	It is a surface phenomenon.
Heat exchange	Endothermic process	Exothermic process
Temperature	It is not affected by temperature	It is favoured by low temperature
Rate of reaction	It occurs at a uniform rate.	It steadily increases and reach to equilibrium
Concentration	It is same throughout the material.	Concentration on the surface of adsorbent is more than in the bulk

Types of adsorption:

1. Physisorption: when a gas is adsorbed on a solid by weak Van der Waal’s forces.

Characteristics:

⇒ Non-specific - no preference for a particular gas.

⇒ Reversible.

⇒ Enthalpy of adsorption is low (20 - 40 kJ/mol).

⇒ Easily liquefied gases are adsorbed easily.

⇒ Low temperature is favourable.

⇒ Increases with increase in surface area.

⇒ No appreciable activation energy is required.

⇒ Results in a multi molecular layer on adsorbent under high pressure.

2. Chemisorption: when a gas is adsorbed on a solid by chemical bonds (covalent or ionic).

Characteristics:

⇒ Highly specific - occurs only when there is a possibility of forming a covalent bond between the adsorbate and the adsorbent.

⇒ Irreversible.

⇒ Enthalpy of adsorption is high (80-240 kJ/mol).

⇒ High temperature is favourable.

⇒ High activation energy needed sometimes.

⇒ Increases with increase in surface area.

⇒ Results in unimolecular layer.