Adsorption
(1) Definition : The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting in to higher concentration of the molecules on the surface is called adsorption.
(2) Causes of adsorption : Unbalanced forces of attraction or free valencies which is present at the solid or liquid surface, have the property to attract and retain the molecules of a gas or a dissolved substance on to their surfaces with which they come in contact.
Example : Ammonia gas placed in contact with charcoal gets adsorbed on the charcoal whereas ammonia gas placed in contact with water gets absorbed into water,
Table : 14.1 Some basic terms used in adsorption
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Interface : Any surface is a plane which separates any two phases in contact with each other. The plane that separates any two phase is generally called an interface between the two phases.
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Adsorbate and Adsorbent : These are the substance which gets adsorbed on any surface is called adsorbate for example, if a gas gets adsorbed on to the surface of a solid, then the gas is termed as the adsorbate.
These substance on surface of which adsorption takes place is called adsorbent.
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Desorption : The removal of the adsorbed substance from a surface is called desorption.
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Absorption : When the molecules of a substance are uniformly distributed throughout the body of a solid or liquid. This process is called absorption.
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Sorption : The process in which adsorption and absorption occur simultaneously is called sorption.
Mc. Bain introduced a general term sorption describeing both the processes, however adsorption is instantaneous which a fast process is while absorption is a slow process.
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Occlusion : When adsorption of gases occur on the surface of metals this is called occlusion.
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(3) Difference between adsorption and absorption
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Adsorption
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Absorption
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It is a surface phenomenon.
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It concerns with the whole mass of the absorbent.
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In it, the substance is only retained on the surface and does not go into the bulk or interior of the solid or liquid.
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It implies that a substance is evenly distributed, through the body of the solid or liquid.
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In it the concentration of the adsorbed molecules is always greater at the free phase.
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In it the concentration is low.
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It is rapid in the beginning and slows down near the equilibrium.
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It occurs at the uniform rate.
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Examples : (i) Water vapours adsorbed by silica gel.
(ii) NH3 is adsorbed by charcoal.
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Examples : (i) Water vapours absorbed by anhydrous CaCl2
(ii) NH3 is absorbed in water forming NH4OH
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(4) Surface forces : Only the surface atoms of an adsorbent play an active role in adsorption. These atoms posses unbalanced forces of various types such as, Vander Waal's forces and chemical bond forces.
Thus, the residual force-field on a free surface which is responsible for adsorption is produced. For example, when a solid substance is broken into two pieces, two new surfaces are formed and therefore, the number of unbalanced forces becomes more. As a result the tendency for adsorption become large.
(5) Reversible and Irreversible adsorption : The adsorption is reversible, if adsorbate can be simply removed from the surface of the adsorbent by physical methods. If the adsorbate cannot be easily removed from the surface of the adsorbent is called irreversible adsorption.
Example for reversible adsorption: A gas adsorbed on a solid surface can be completely removed in vacuum.
Example for irreversible adsorption: Adsorption of O2 on tungusten adsorbent.
(6) Characteristics of adsorption
(i) Adsorption process refers to the existence of a higher concentration of any particular component at the surface of a liquid or a solid phase.
(ii) Adsorption process is accompanied by decrease in the ΔG (free energy change) of the system when ΔG=0, adsorption equilibrium is said to be established.
(iii) Adsorption is invariably accompanied by evolution of heat that is it is an exothermic process. Or we can say that ΔH of adsorption is always negative in nature.
(iv) When a gas is adsorbed, the freedom of movement of its molecules is restricted. On account of it decrease in the entropy of the gas after adsorption, that is ΔS is negative.
(v) For a process to be spontaneous, thermodynamic need is that ΔG must be negative, which means there is decrease in free energy. On the basis of the Gibb's Helmholtz equation ΔG = ΔH - TΔS, ΔG can be negative if the ΔH has sufficiently high negative value and TΔS has positive value.
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