Factors Affecting Adsorption
Adsorption occurs on the surface of almost all solids. However, the extent of adsorption of a gas on the surface of a solid depends upon the following factors :
(i) Nature and surface area of the adsorbent
(ii) Nature of the adsorbed gas
(iv) Pressure of the gas
Let us now discuss these factors briefly.
Different solids would adsorb different amounts of the same gas even under similar conditions. Substances like charcoal and silica gel are excellent adsorbents. The substances that are porous in nature and have rough surfaces are better adsorbents.
The extent of adsorption also depends upon the surface area of the solid. Greater the surface area, more is the surface available for adsorption and greater is the adsorption. The surface area depends upon the particle size of the substance. A cube of each side equal to 1cm has six faces. Each of them is a square with surface area of 1cm2. Thus, the total surface area of this cube is 6 cm2 Fig. 17.3 (a). If its each side is divided into two equal halves, ½ cm long, and the cube is divided into two equal halves, ½ cm long, and the cube is cut along the lines indicated in the Fig (b), the cube would be divided into 8 smaller cubes with each side 0.5 cm long [Fig. 17.3 (b)]. Surface area of each small cube would be (6 ´ 0.5 ´ 0.5) = 1.5 cm2 and the total surface area of all the 8 smaller cubes would be 12 cm2 which is double the surface area of the original cube. If it is subdivided into smaller cubes, each of side equal to 1 ´ 10–6 cm the surface area will increase to 6 ´ 106 cm2 or 600 m2. The increase in surface area would result in greater adsorption.
Now we can explain why the solids that are porous in nature and have rough surfaces are better adsorbents. It is so because each of these features increases the surface area.
The extent of adsorption also depends upon the nature of the gas. The gases which are more easily liquifiable or are more soluble in water are more readily adsorbed than others. For example, under similar conditions, the amount of SO2 or NH3 adsorbed by charcoal is much more than that of H2 or O2 gases. It is because the intermolecular forces are stronger in more easily liquifiable gases, therefore, they get adsorbed more strongly.
The extent of adsorption decreases with rise in temperature. For example, under one atmosphere pressure, one gram of charcoal adsorbs about 10 cm3 of N2 gas at 272 K, 20 cm3 at 248 K and 45 cm3 at 195 K.
Adsorption is an exothermic process. The change in enthalpy when one mole of a substance is adsorbed, is called enthalpy of adsorption. The adsorption process is similar to the condensation process. The reverse process is called desorption and is endothermic in nature. It is similar to the evaporation process. When a gas is kept in contact with a solid adsorbent in a closed container, a dynamic equilibrium is established in due course of time.
gas adsorbate + solid adsorbent ↔ gas adsorbed on the solid + heat
Since the forward process (adsorption) is exothermic in nature, according to the Le Chatelier's principle, it would be favoured at low temperature. Therefore, the extent of adsorption would increase on decreasing the temperature and would decrease on increasing the temperature.
At a constant temperature the extent of adsorption increases with increase in the pressure of the gas (adsorbate). We shall study the relation between the two in detail a little later.