Chemistry Adsorption and It's Mechanism

Topics Covered :

● Introduction
● Adsorption
● Distinction between Adsorption and Absorption
● Mechanism of Adsorption

Introduction :

`=>` Surface chemistry deals with phenomena that occur at the surfaces or interfaces.

`=>` The interface or surface is represented by separating the bulk phases by a hyphen or a slash.

`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
Surface chemistry is that branch of chemistry which deals with the study of the bulk phenomena occurring at the surface or interface,i.e., at the boundary separating two bulk phenomena.

`=>` For example, the interface between a solid and a gas may be represented by solid-gas or solid/gas.

`=>` Due to complete miscibility, there is no interface between the gases.

`=>` The bulk phases that we come across in surface chemistry may be pure compounds or solutions.

`=>` The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases.

`=>` Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallisation occur at interfaces.

`=>` The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.

`=>` To accomplish surface studies meticulously, it becomes imperative to have a really clean surface.

`=>` Under very high vacuum of the order of `10^(-8)` to `10^(-9)` pascal, it is now possible to obtain ultra clean surface of the metals.

`=>` Solid materials with such clean surfaces need to be stored in vacuum otherwise these will be covered by molecules of the major components of air namely dioxygen and dinitrogen.

Adsorption :

`=>` The surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact.

`=>` These molecules remain only at the surface and do not go deeper into the bulk.

1. `color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
`text(Adsorption) :` The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.

`text(Adsorbate) :` The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate.

`text(Adsorbent) :` The material on the surface of which the adsorption takes place is called adsorbent.

`=>` Adsorption is essentially a surface phenomenon.

`=>` Solids, particularly in finely divided state, have large surface area and therefore, charcoal, silica gel, alumina gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.

`text(Adsorption in action) :`

(i) If a gas like `O_2`, `H_2`, `CO`, `Cl_2`, `NH_3` or `SO_2` is taken in a closed vessel containing powdered charcoal, it is observed that the pressure of the gas in the enclosed vessel decreases. The gas molecules concentrate at the surface of the charcoal, i.e., gases are adsorbed at the surface.

(ii) In a solution of an organic dye, say methylene blue, when animal charcoal is added and the solution is well shaken, it is observed
that the filtrate turns colourless. The molecules of the dye, thus, accumulate on the surface of charcoal, i.e., are adsorbed.

(iii) Aqueous solution of raw sugar, when passed over beds of animal charcoal, becomes colourless as the colouring substances are adsorbed by the charcoal.

(iv) The air becomes dry in the presence of silica gel because the water molecules get adsorbed on the surface of the gel.

`=>` It is now clear that solid surfaces can hold the gas or liquid molecules by virtue of adsorption.

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

Distinction between Adsorption and Absorption :

`=>` In adsorption, the substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent, while in absorption, the substance is uniformly distributed throughout the bulk of the solid.

`text(Example) :` ● When a chalk stick is dipped in ink, the surface retains the colour of the ink due to adsorption of coloured molecules while the solvent of the ink goes deeper into the stick due to absorption. On breaking the chalk stick, it is found to be white from inside.

● A distinction can be made between absorption and adsorption by taking an example of water vapour. Water vapours are absorbed by anhydrous calcium chloride but adsorbed by silica gel.

`=>` In other words, in adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid.

`text(Sorption) :` Both adsorption and absorption can take place simultaneously also. The term sorption is used to describe both the processes.

Mechanism of Adsorption :

`=>` Adsorption arises because the surface particles of the adsorbent are not in the same environment as the particles inside the bulk.

`=>` Inside the adsorbent all the forces acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms or molecules of their kind on all sides, and hence they possess unbalanced or residual attractive forces. These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface.

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

`text(Heat of Adsorption) :`

● Another important factor featuring adsorption is the heat of adsorption.

● During adsorption, there is always a decrease in residual forces of the surface, i.e., there is decrease in surface energy which appears as heat.

● Adsorption, therefore, is invariably an exothermic process.

● In other words, `ΔH` of adsorption is always negative.

`text(Freedom of Movement or Entropy of Molecules) :`

● When a gas is adsorbed, the freedom of movement of its molecules become restricted.

● This amounts to decrease in the entropy of the gas after adsorption, i.e., `ΔS` is negative.

`=>` Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system.

`text(Gibbs Free Energy of Adsorption) :`

● For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, `ΔG` must be negative, i.e., there is a decrease in Gibbs energy.

● On the basis of equation, `ΔG = ΔH – TΔS`, `ΔG` can be negative if `ΔH` has sufficiently high negative value as `- TΔS` is positive.

● Thus, in an adsorption process, which is spontaneous, a combination of these two factors makes `ΔG` negative.

● As the adsorption proceeds, `ΔH` becomes less and less negative ultimately `ΔH` becomes equal to `TΔS` and `ΔG` becomes zero. At this state equilibrium is attained