Adsorption of gases on solids | Chemistry Optional Notes for UPSC PDF Download

Introduction

• Whenever a clean surface is exposed to a gas, the gas molecules get adsorbed on the free surface. However, if the surface is already having a weakly held adsorbate on it, the same is displaced by the substance which has a tendency to get adsorbed more strongly. 
• When the activated charcoal used in the gas masks is already exposed to the atmospheric air, the gases and the water vapors in the air are adsorbed on its surface. But when it is exposed to chlorine atmosphere, these gases are displaced by chlorine, Thus we find that the substances which get strongly adsorbed can easily displace the weakly adsorbed adsorbate.
• Almost all solids adsorb gases to some extent.
• Charcoal adsorbs many gases. It even adsorbs polluting gases present in air in small concentration. 
• Gases such as H₂, N₂, O₂ and CO are adsorbed by finely divided transition metal such as Ni, Pt, Pd, Fe, Co, etc.

Factors which affect the extent of adsorption on solid surface

The following are the factors which affect the adsorption of gases on solid surface. 

• Nature of the adsorbate (gas) and adsorbent (solid)
  (i) In general, easily liquefiable gases e.g., CO₂, NH₃, Cl₂ and SO₂ etc. are adsorbed to a greater extent than the elemental gases e.g. H₂, O₂, N₂, He etc. (while chemisorption is specific in nature.)
  (ii) Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks. 
• Surface area of the solid adsorbent
  (i) The extent of adsorption depends directly upon the surface area of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the extent of adsorption.
  (ii) Surface area of a powdered solid adsorbent depends upon its particle size. Smaller the particle size, greater is its surface area.
  (iii) The surface area per gram of the adsorbent is called specific surface area of the adsorbent.
• Effect of pressure on the adsorbate gas
  (i) An increase in the pressure of the adsorbate gas increases the extent of adsorption.
  (ii) At low temperature, the extent of adsorption increases rapidly with pressure.
  (iii) Small range of pressure, the extent of adsorption is found to be directly proportional to the pressure.
  (iv) At high pressure (closer to the saturation vapor pressure of the gas), the adsorption tends to achieve a limiting value. 
• Effect of temperature 
  (i) As adsorption is accompanied by evolution of heat, so according to the Le-Chatelier’s principle, the magnitude of adsorption should decrease with rise in temperature.
  (ii) The relationship between the extent of adsorption and temperature at any constant pressure is called adsorption isobar.
  (iii) A physical adsorption isobar shows a decrease in x/m (where ‘m’ is the mass of the adsorbent and ‘x’ that of adsorbate) as the temperature rises.
  (iv) The isobar of chemisorption show an increase in the beginning and then decrease as the temperature rises.
• Activation of adsorbent
  (i) Activation of an adsorbent means, increase in the adsorbing power of the adsorbent.
  (ii) This can be achieved by increasing the surface area of the adsorbent.
  (iii) This can be done by making the surface of adsorbent rough or by breaking it into small pieces.
  (iv) If the particle are made of then interparticle space will be too small hence the extent of adsorption may decrease.
  (v) The active sites (clear surface) can be actually free from the adsorbed gases by heating in very high vacuum ( 10 ^-10 or 10 ^-11 mm Hg).

Adsorption isotherms

• A mathematical equation which describes the relationship between pressure (p) of the gaseous adsorbate and the extent of adsorption at any fixed temperature is called adsorption isotherms.
• The extent of adsorption is expressed as mass of the adsorbate adsorbed on one unit mass of the adsorbent.
• Thus, if x g of an adsorbate is adsorbed on m g of the adsorbent, then
  

Various adsorption isotherms are commonly employed in describing the adsorption data:

Freundlich adsorption isotherm 

• Freundlich adsorption isotherm is obeyed by the adsorptions where the adsorbate forms a monomolecular layer on the surface of the adsorbent.  (Freundlich adsorption isotherm) or where, x is the weight of the gas adsorbed by m gm of the adsorbent at a pressure p, thus x/m represents the amount of gas adsorbed by the adsorbents per gm (unit mass), k and n are constant at a particular temperature and for a particular adsorbent and adsorbate (gas), n is always greater than one, indicating that the amount of the gas adsorbed does not increase as rapidly as the pressure.
• At low pressure, the extent of adsorption varies linearly with pressure.
• At high pressure, it becomes independent of pressure.
• At moderate pressure m/x depends upon pressure raised to powers.
  

Note: Equationsimilar to the equation of a straight line

Therefore, the plot of log (x/m) against log p should be a straight line with an intercept equal to log k and slope 1/n.

The Langmuir - adsorption isotherms

One of the drawbacks of Freundlich adsorption isotherm is that it fails at high pressure of the gas. Irving Langmuir in 1916 derived a simple adsorption isotherm, on theoretical considerations based on kinetic theory of gases. This is named as Langmuir adsorption isotherm.
The main points of Langmuir’s theory of adsorption are as follows:

• Adsorption takes place on the surface of the solid only till the whole of the surface is completely covered with a unimolecular layer of the adsorbed gas.
• Adsorption consists of two opposing processes, namely Condensation of the gas molecules on the solid surface and Evaporation(desorption)of the gas molecules from the surface back into the gaseous phase.
• The rate of condensation depends upon the uncovered (bare) surface of the adsorbent available for condensation. Naturally, at start when whole of the surface is uncovered the rate of condensation is very high and as the surface is covered more and more, the rate of condensation progressively decreases. On the contrary, the rate of evaporation depends upon the covered surface and hence increases as more and more of the surface is covered ultimately an equilibrium will be set up at a stage when the rate of condensation becomes equal to the rate of evaporation (adsorption equilibrium).
• The rate of condensation also depends upon the pressure of the gas since according the kinetic theory of gases, the number of molecules striking per unit area is proportional to the pressure. Mathematically, where a and b are constants and their value depends upon the nature of gas (adsorbate), nature of the solid adsorbent and the temperature. Their values can be determined from the experimental data.

Limitation of Langmuir theory 

• Langmuir’s theory of unimolecular adsorption is valid only at low pressures and high temperatures.
• When the pressure is increased or temperature is lowered, additional layers are formed. This has led to the modern concept of multilayer adsorption.

Note: The Langmuir adsorption isotherm is restricted to the formation of unimolecular layer of gas molecules on the surface of solids. However, it was suggested that there is possibility of multimolecularlayer of gas molecules on the surface of the solids rather than single layer. On this basis, Brunauer, Emmett and Teller proposed a new theory known as B.E.T theory.