The tendency of accumulation of molecular species at the surface 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 and the material on whose surface on whose surface the adsorption has taken place is called adsorbent.
DISTINCTION BETWEEN ADSORPTION AND ABSORPTION
In adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentrations is uniform throughout the bulk of the solid.
Adsorption is due to the face that the surface particles of the adsorbent are different state than the particles inside the bulk. Inside the adsorbent all the force acting between the particles are mutuall y 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 particle on its surface.
TYPES OF ADSORPTION
There are two main types of adsorption of gases on solids.
If accumulation of gas on the surface of a solid occurs on account of weak van der Walls’ force, the adsorption is termed as physical adsorption or physic-sorption. When t he gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption. The chemical bonds may be covalent or ionic in nature. Chemisorption has a rather high energy of activation and is, therefore, often referred to as activated adsorption. Sometimes these two processes occur simultaneously and it is not easy to ascertain the type of adsorption. A physical adsorption at low temperature may pass into chemisorption as the temperature is increased. For example, hydrogen is first adsorbed on nickel by van der Walls’ force. Molecules of hydrogen then dissociate and hydrogen atoms are held on the surface by chemisorption.
COMPARISON OF PHYSI-SORPTION AND CHEMISORPTION
|S. No.||Physical adsorption||Chemical adsorption|
|1.||It is caused b y intermolecular van der Waals’ forces.||It is caused by chemical bond formation|
|2.||It is not specific||It is highly specific|
|3.||It is reversible||It is irreversible|
|4.||It depends on the nature of gas. More easily liquefiable gases are adsorbed readily.||It depends on the nature of gas. Gases which form compounds with the adsorbent exhibit chemisorption.|
|5.||Heat of adsorption is low.||Heat of adsorption is high|
|6.||Low temperature is favourable. It decreases with increases of temperature.||High temperature is favourable. It increases with increase of temperature.|
|7.||No appreciable activation energy is involved.||High activation energy is involved.|
|8.||High pressure is favourable. Decrease of pressure causes desorption.||High pressure is favourable. Decrease of pressure does not cause desorption.|
|9.||It depends on the surface area. It increases with increase of surface area.||It also depends on the surface area. It increases with increase of surface area.|
|10.||If forms multila yers on adsorbent surface under high pressure.||It forms unimolecular layer.|
ADSOPTION OF GASES ON SOLIDS
Following are the factors which influence the extent of adsorption of gases on solid surface:
(a) Nature of adsorbent:
(b) Nature of adsorbate: Easily liquefiable gases (e.g., HCl, NH3, CO2) are adsorbed to greater extent than others (e.g., H2, N2, O2). The adsorption of gases on finely divided metal surface is also called occlusion.
(c) Surface area of adsorbent: Larger is surface area of adsorbent more is adsorption. The surface area per gram of adsorbent is called specific surface area of adsorbent. That is why a porous or finely divided form of adsorbents adsorb larger quantities of adsorbate.
(d) Activation of solid adsorbent: The activation of surface for adsorption can be made by rubbing or by passing in colloidal state or by passing super heated steam, etc. Activated charcoal is commonly used for the adsorption of easily liquefiable gases. Also the active sites on adsorbent can be increased by heating the adsorbent in vacuum.
(e) Temperature: An increase in temperature decreases physical adsorption (Figure 1) whereas, chemisorption first increases and then decreases. Usually adsorption being exothermic and thus effect of temperature is in accordance with Le-Chatelier principle. However in chemisorption, some activation energy is needed to cross the energy barrier of adsorption like ordinary chemical reactions. The plots of extent of adsorption (x/m) vs. temperature at constant pressure, are called adsorption isobars.
(f) Pressure: The plots of extent of adsorption (x/m) vs. pressure at constant temperature are called adsorption isotherms, ‘x’ is the amount of adsorbate on m gram of adsorption. Freundlich studied the influence of pressure on adsorption of gases at different temperature and obtained Figure. 2.
FREUNDLICH ADSORPTIN 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. The relationship can be expressed by the following equation.
Where ‘x’ is the mass of the gas adsorbed on a mass ‘m’ of the adsorbent at a pressure P.k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature.
Langmuir Adsorption Isotherm : The Langmuir adsorption isotherm is represented as
(where a and are two Langmuir parameters) At very high pressure, the above isotherm acquires the limiting form;
(K1p >>> 1 ∴ bp >>> 1)
And at very low pressure it is reduced to
(K1P <<<1 ∴bp <<< 1)
The parameters a and b, can be determined by following equation
A plot of m/x against 1/P would give a straight line with slope and intercept equal to 1/a and b/a respectively.