Adsorption

Adsorption:

The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.

· The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate

· The material on the surface of which the adsorption takes place is called adsorbent

· Adsorption is essentially a surface phenomenon

Difference between Adsorption and Absorption:

Adsorption:

The substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent

Absorption:

The substance is uniformly distributed throughout the bulk of the solid

Adsorption and Absorption

Observation	Absorption	Adsorption
Phenomenon	Bulk phenomenon	Surface phenomenon
Heat exchange	Endothermic process	Exothermic process
Temperature	It is not affected by temperature	It is favoured by low temperature

Mechanism of Adsorption:

· Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk.

· The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure.

· Adsorption is invariably an exothermic process

· Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system

· ∆G must be negative

Types of Adsorption:

1. Physisorption

2. Chemisorption

Physisorption:

It is the accumulation of gas on the surface of a solid which occurs on account of weak van der Waals’ forces.

· A physical adsorption at low temperature may pass into chemisorption as the temperature is increased.

Example:

· Di hydrogen is first adsorbed on nickel by van der Waals’ forces.

· Molecules of hydrogen then dissociate to form hydrogen atoms which are held on the surface by chemisorption.

Characteristics of Physisorption:

Lack of specificity:

· A given surface of an adsorbent does not show any preference for a particular gas

Reversible nature:

· Physical adsorption of a gas by a solid is generally reversible

Surface area of adsorbent:

· The extent of adsorption increases with the increase of surface area of the adsorben

Enthalpy of adsorption:

· Enthalpy of chemisorption is high

Chemisorption:

Chemisorption is when the gas molecules or atoms are held to the solid surface by chemical bonds.

· The chemical bonds may be covalent or ionic in nature.

· Chemisorption involves a high energy of activation

Characteristics of Chemisorption:

High Specificity:

· Chemisorption is highly specific and it will only occur if there is some possibility of chemical bonding between adsorbent and adsorbate

Irreversibility:

· Chemisorption is also an exothermic process but the process is very slow at low temperatures

Surface Area:

· chemisorption also increases with increase of surface area of the adsorbent

Enthalpy of Adsorption:

· Enthalpy of chemisorption is high

Comparison of Physisorption and Chemisorption:

Physisorption	Chemisorption
Only van der Waals force are present between adsorbate and surface of adsorbent	Chemical bonds are formed between adsorbate and surface of adsorbent
Low enthalpy of adsorption ie, in the order of 20 kjmol^-1.	High enthalpy of adsorption i.e, order of 200 kjmol^-1.
Reversible	Irreversible
It usually takes place at low temperature and does not require any activation energy.	It takes place at high temperature and require activation energy.
Multi molecular layer of adsorbate are formed on the surface	Only monomolecular layers are formed.
Not specific.	Highly specific.

Adsorption Isotherms:

Adsorption isotherm is the variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve.

Freundlich adsorption isotherm:

· Freundlich, in 1909, gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature

= k.p ^{1/ n} (n > 1)

Adsorption from Solution Phase:

Solids can adsorb solutes from solutions also.

Example:

· The precipitate of Mg (OH) ₂ attains blue colour when precipitated in presence of magneson reagent.

· The colour is due to adsorption of magneson.

Observation of Adsorption in Solution Phase:

· The extent of adsorption decreases with an increase in temperature

· The extent of adsorption increases with an increase of surface area of the adsorbent

· The extent of adsorption depends on the concentration of the solute in solution

· The precise mechanism of adsorption from solution is not k

Application of Adsorption:

Production of High Vacuum:

The remaining traces of air can be adsorbed by charcoal from a vessel evacuated by a vacuum pump to give a very high vacuum.

Gas Masks:

Used for breathing in coal mines to adsorb poisonous gases.

Control of Humidity:

Silica and aluminium gels are used as adsorbents for removing moisture and controlling humidity.

Separation of Inert Gases:

A mixture of noble gases can be separated by adsorption on coconut charcoal at different temperatures.

Froth Floatation Process:

A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method using pine oil and frothing agent.

Chromatographic Analysis:

Chromatographic analysis based on the phenomenon of adsorption finds a number of applications in analytical and industrial fields.