Adsorption


?Adsorption is the process of accumulation of molecules at the surface rather than in the bulk of a solid or liquid which results into the higher concentration of molecules at the surface.?

The substance which is absorbed at the surface is called ?adsorbate? and the substance on which it is adsorbed is called ?absorbent?. By applying heat or by reducing pressure, the reverse process, i.e. removal of the adsorbed substance from the surface occurs, called ?desorption?. The adsorption of gases on the surface of metals is called ?occlusion?.

Generally, we confuse between the two terms adsorption and absorption, so following are the point differences between the two terms:

Adsorption

Absorption

1. It is a surface phenomenon in which substance is con-centrated only at the surface of adsorbent.

1. It is a bulk phenomenon where the substance is uniformly distributed through- out the bulk of the solid.

2. Rate of adsorption is initially high and then decreases till the equilibrium is attained.

2. Rate of absorption is uniform throughout the process.


Most common example which explains the distinction between the adsorption and absorption process is the interaction of water vapour with silica gel and anhydrous CaCl2, as illustrated in the following figure:

It is found that solids, in finelly divided state act as good absorbents because of large surface area. Some examples of good absorbents are charcoal, alumina gel, silica gel, finely divided metals, clay, colloids, etc.

(i) Mechanism of Adsorption

From the above figure, it is clear that inside the adsorbent or bulk, all the forces acting between the particles are mutually balanced. But on the surface the particles experience only the residual attractive forces by the atoms or molecules from one side i.e., from bulk side. These forces of adsorbent are responsible for attracting the adsorbate particles on its surface.

During adsorption the residual forces on the surface decreases. As a result surface energy decreases which appears as heat. Thus adsorption is an exothermic process (i.e., ΔH is always negative). When a gas is adsorbed, its entropy decreases due to restriction of freedom of movement of its molecules (i.e., ΔS is also negative).

As per the thermodynamic requirement, at constant pressure and temperature, ΔH = (ΔH - TΔS) must be negative, so the value of ΔH must have sufficiently high negative for a process to be spontaneous. As the adsorption proceeds, ΔH becomes less and less negative and ultimately becomes equal to TΔS, so that ΔG becomes zero and the equilibrium state is obtained.

During adsorption, ΔG = ΔH - TΔS = negative

At equilibrium, ΔG = 0, or ΔH = TΔS

(ii) Types of Adsorption

On the basis of the nature of bond between the adsorbate and adsorbent, adsorption may be of two types ? physical adsorption or physisorption and chemical adsorption or chemisorption.

Following table describes the comparison between the two types of adsorption.

Physisorption

Chemisorption

1. It involves weak van der Waal?s forces between the adsorbate and adsorbent.

1. If involves the chemical bond (covalent or ionic) between adsorbate and the adsorbent.

2. In this case, enthalpy of adsorption is low (20 - 40 kJ mol-1)

2. In this case, enthalpy of adsorption is high  (80 - 240 kJ mol-1).

3. No appreciable activation energy is required.

3. High activation energy is required, thus also called activated adsorption.

4. Not specific in nature as the van der Waal?s forces are same for all types of adsorbate.

4. Highly specific in nature because it occurs only if there is possibility of chemical bond between the adsorbate and adsorbent.

5. Reversible in nature, i.e.,

5. Irreversible in nature.

6. It depends on the physical nature of gas, i.e., the gases which are more liquefiable can be easily adsorbed.

6. It depends on the chemical nature of gas, i.e., gases which can react with the adsorbent show chemisorptions.

7. It results into multi-molecular layers on adsorbent surface under high pressure.

7. It results into unimolecular layer on adsorbent.

 

(iii) Effect of Pressure and Temperature on Adsorption

In both types of adsorption, i.e., physisorption and chemisorption, when the pressure is increased, more of gas is adsorbed because the volume of the gas decreases (Le-chatelier?s principle).

As the adsorption is an exothermic process, it readily occurs at low temperature and decreases with the rise in temperature according to Le-chatelier?s principle. But the chemisorption process is very slow at low temperature because of high activation energy, therefore it increases with the rise in temperature like most chemical changes.

It is observed that physisorption of a gas adsorbed at low temperature may change into chemisorption at high temperature.

(iv) Adsorption Isotherm

A curve which shows the expression of the variation in the amount of gas (x) adsorbed by a definite mass of adsorbent (m) with the pressure (p) at constant temperature, is known as adsorption isotherm.

Freundlich Adsorption Isotherm

Given by Freundlich in 1909, according to which:

Where, k, p and n are constants which depend on the nature of adsorbent and the gas at a particular temperature.

Taking log of both sides,

Following curve is the representation of Freundlich adsorption isotherm which indicates that at a fixed pressure, there is a decrease in physical adsorption with increase is temperature.

Fig. Plot of against p

(Adsorption isotherm)

Fig. Plot of against log p
(Freundlich isotherm)

 

The equation corresponds to straight line equation (y = mx + C) So, the Freundlich isotherm is valid only when a straight line is obtained in the above plot otherwise not valid.

 

Practically the value of ranges between 0 to 1 (Probable range 0.1 to 0.5)

Freundlich isotherm is not applicable at high pressure because it doesn?t explain why the experimental isotherms always seem to approach saturation at high pressure.

 

(v) Adsorption of Solution by a Solid :

Solids can also adsorb solutes from solution like they adsorb gases. For example, the litmus solution becomes colourless when shaken with charcoal. In the presence of magneson reagent, the precipitate of acquires blue colour which is due to adsorption of magneson.

Freundlich?s equation describes the behaviour of adsorption from solution. In this case, instead of pressure, concentration of solution (C) is taken into account. The equation is:

C = equilibrium concentration i.e., when adsorption is complete.

When a plot of against log C is drawn, a straight line is obtained that proves the validity of Freundilich isotherm.

 

 

 

 

(vi) Some Applications of Adsorption :

 

(a) Silica and aluminium gels are used as adsorbents to remove the moisture and control humidity.

(b) Gas mask which consists of activated charcoals or mixture of adsorbents, is used for breathing in coal mines to adsorb poisonous gases.

(c) Adsorbent removes the colouring matter from solutions. For example, animal charcoal removes colours from solutions by adsorbing coloured impurities.

(d) Adsorption helps in creating high vacuum. Charcoal removes the traces of air by its adsorption from a vessel evacuated by a vacuum pump to give a very high vacuum.

(e) A number of drugs are used to kill the germs by getting adsorbed on them.

(f) Chromatography has a number of applications in analytical and industrial fields. All chromatographic processes are based on the phenomenon of adsorption.

(g) A mixture of noble gases can be separated by adsorption of gases on coconut charcoal at different temperatures due to the difference in degree of adsorption of gases by charcoal.