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:

  1. Nature and surface area of the adsorbent
  2. Nature of the adsorbed gas
  3. Temperature
  4. Pressure of the gas

Nature and Surface Area of the Adsorbent

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.

Nature of the Adsorbed 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 inter-molecular forces are stronger in more easily liquifiable gases, therefore, they get adsorbed more strongly.

Temperature

The extent of adsorption decreases with rise in temperature. 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 favored at low temperature. Therefore, the extent of adsorption would increase on decreasing the temperature and would decrease on increasing the temperature.

Pressure of the gas

At a constant temperature the extent of adsorption increases with increase in the pressure of the gas (adsorbate).