Surface chemistry- Unit 5

Surface chemistry deals with phenomena that occur at surfaces or interfaces.

Adsorption : The accumulation of molecules species at the surface rather than in bulk of solid or liquid is termed as adsorption.

Adsorbate: The molecular species or substance, which concentrates or accumulates at the surface is termed as adsorbate.

Adsorbents: The material on the surface of which the adsorption takes place is called adsorbent.

Eg: Charcoal, Silica Gel, aluminum gel, clay, colloids, metals in finely divided state

Desorption: The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

Difference between adsorption and absorption:

Adsorption	Absorption
Molecules collect on the surface	Molecules penetrate into the bulk
Concentration is high at the surface	Concentration is same throught the the bulk of solid
Highly selective	Not selective
Always exothermic	May be exothermic or endothermic

Types of adsorption :There are two types of adsorption of gases on solids.

Physical adsorption or physisorption :

Accumulation of gas on the surface of solid occurs in account of weak van der waal’s forces , the adsorption is termed as physical adsorption or physisorption.

Chemical adsorption or chemisorption :

When the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed as chemical adsorption or chemisorption. Sometimes both the adsorptions take place simultaneously.

For examples :

Dihydrogen first adsorbed by nickel by vander waal forces

Molecules of hydrogen then disassociates to form hydrogen toms which are held on the surface.

Characteristics of physisorption:

1. Lack of specificity : A given surface of an adsorbent does not show any preference for particular gas as the vander waals’ forces are universe.
2. Nature of adsorbate : The amount of gas adsorbed by a solid depends on the nature of gas. Easily liquefiable gases are readily adsorbed.
3. Reversible nature : Physical adsorption of gas by solid is generally reversible.
4. Surface of adsorbent :The extent of adsorption increases with the increase of surface area of adsorbent.
5. Enthalpy of adsorption : Physical adsorption is an exothermic process.

Characteristics of chemisorption :

1. High specificity : Chemisorption is highly specific and it will only occur if there is some possibility of chemical bonding between adsorbent and adsorbate.
2. Irreversibility : Chemisorption involves compound formation. So this is usually irreversible in nature.
3. Surface area: Chemisorption increases with increase of surface area of adsorbent.
4. Enthalpy of adsorption : Enthalpy of chemisorption is high (80-240 kJ/mol) as it involves chemical bond formation.

Comparison of physisorption and chemisorption

Physisorption	Chemisorption
Arises because of Vander waal’s forces	Formed by chemical bond formation
Not specific to nature	Highly specific to nature
Reversible in nature	Irreversible
Depends on nature of gas. Liquifiable gases are adsorbed readily.	This also depends on the nature of gas, which can react with adsorbent show chemisorption.
Enthalpy of adsorption is low (20-40 kJ/mol)	Enthalpy of adsorption is high(80-240 kJ/mol) in this case.
Low temperature is favourable.	High temperature is favourable
Not much activation energy required	High activation energy is required
It depends on surface area , increases with surface area.	This also increases with surface area.
It results in multi molecules layer on adsorbent surface under high pressure.	This results in unimolecular layer.